Professional Traffic Operations Engineer

Certification Program Refresher Course

Student Supplement

Module 3
Traffic Safety

© 2022 - Institute of Transportation Engineers

1627 Eye Street, NW, Suite 550 | Washington, DC 20006
Introduction to Refresher Course
This Refresher Course provides an overview of topics, key references and independent study materials by
topic for practicing engineers who intend to take the PTOE certification examination. The suite of modules
includes six (6) webinar recordings on traffic operations analysis, operational effects of geometric designs,
traffic safety, traffic control devices, traffic engineering studies, and social, environmental, and
institutional issues, each accompanied by a student supplement.
This 2022 version of the course and student supplement is an update and expansion to a July 2016 course
managed by Robert K. Seyfried, P.E., PTOE. Contributors to that course were:
• Robert K. Seyfried, P.E., PTOE; President, R. K. Seyfried and Associates, Inc.; Evanston, IL
• Jerome Hall, Ph.D., P.E., Professor Emeritus, Civil Engineering, University of New Mexico,
Albuquerque, NM
• Pat Noyes, Principal, Pat Noyes and Associates, Boulder, CO
• Eric T. Donnell, Ph.D., P.E., Assistant Professor, Department of Civil and Environmental
Engineering, The Pennsylvania State University, State College, PA
• John M. Mason, Jr., Ph.D., P.E., Associate Dean of Graduate Studies, Research, and Outreach and
Professor of Civil Engineering, The Pennsylvania State University, State College, PA
• Martin E. Lipinski, Ph.D., P.E., PTOE, Professor, Department of Civil Engineering, University of
Memphis; Memphis, TN
This 2022 version was updated by:
• Peter J. Yauch, P.E., PTOE, RSP2i, Vice President, Iteris, Inc., Tampa, FL
Much appreciation is given to Stephen J. Manhart, P.E., PTOE, PTP, RSP1, Project Manager for Traffic
Engineering, Michael Baker International, Minneapolis, MN, for his review of the student supplements on
behalf of the Transportation Professional Certification Board.

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Contents
Introduction .................................................................................................................................................. 1
Road Safety Concepts ................................................................................................................................... 1
Visibility And Conspicuity .......................................................................................................................... 4
Signage And Pavement Marking Usage .................................................................................................... 4
Roadside, Medians, And Side Slopes ........................................................................................................ 4
Clear Zones And Recovery Areas .............................................................................................................. 5
Crash Safety Devices ................................................................................................................................. 5
Access Management ................................................................................................................................. 5
Human Factors .......................................................................................................................................... 6
Sight Distance............................................................................................................................................ 6
Speed Management .................................................................................................................................. 7
Environmental ........................................................................................................................................... 7
Conflict Points ........................................................................................................................................... 7
Safety Analysis And Evaluation ..................................................................................................................... 8
Crash Record and Data Systems ............................................................................................................... 8
Hazard Identification................................................................................................................................. 9
Crash Frequency, Rate, and Severity ........................................................................................................ 9
Spot And Systemic Analyses ................................................................................................................... 13
Collision And Condition Diagrams........................................................................................................... 14
Select Countermeasures ......................................................................................................................... 14
Predictive Models ................................................................................................................................... 15
Safety Performance Functions ................................................................................................................ 17
Crash Modification Factors ..................................................................................................................... 17
Economic Appraisal of Proposed Improvements.................................................................................... 17
Prioritization of Projects ......................................................................................................................... 19
Safety Effectiveness Evaluation of Implemented Improvements........................................................... 19
Road Safety Audits .................................................................................................................................. 20
Safety Countermeasures............................................................................................................................. 22
Traffic Control Devices ............................................................................................................................ 23
Pavement Treatments ............................................................................................................................ 24
Roadside Barriers .................................................................................................................................... 24
Improve Sight Distance ........................................................................................................................... 24

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 Speed Control ......................................................................................................................................... 25
Roadway Lighting .................................................................................................................................... 25
Sidewalks And Crosswalks ...................................................................................................................... 25
School Zones ........................................................................................................................................... 26
Removal, Relocation, Or Protection Of Fixed Objects ............................................................................ 26
Geometric Design Improvements ........................................................................................................... 26
Enforcement ........................................................................................................................................... 26
Traffic Calming ........................................................................................................................................ 26
Regulations ............................................................................................................................................. 27
Visibility ................................................................................................................................................... 27
Safe System Approach ................................................................................................................................ 27
Principles ................................................................................................................................................. 27
Elements ................................................................................................................................................. 28
Kinetic Energy Management ................................................................................................................... 29
Road Safety Management....................................................................................................................... 30
REFERENCES ................................................................................................................................................ 31

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 Professional Traffic Operations Engineer
Certification Program Refresher Course

Module 3- Traffic Safety

Introduction
It is the goal of the traffic engineering profession to provide a transportation system that maximizes safety
for all users – motorists, pedestrians, bicyclists, and others. While it is well documented that geometric
design and traffic operations improvements can make roads safer,
reducing crashes and their severity is more than an engineering
problem. It is said that “human error” is a contributing cause of 80 to
90 percent of all crashes. An effective safety program involves
consideration of the human element, the vehicle, the roadway, and
the environment. A comprehensive approach requires the use of all
the safety E’s: Engineering, Education, Enforcement, Emergency
Medical Services, Environment, Economics, Evaluation, and Everyone.
In 2019, there were a total of 6,756,084 reported traffic crashes in the
United States; in 2020, because of the pandemic on travel patterns,
that number dropped to 5,250,837. However, the number of persons
killed in crashes increased from 36,355 in 2019 to 38,824 in 2020, and,
alarmingly, to an estimated 42,915 deaths in 2021. In 2020, there
were over 2.1 million emergency room visits for traffic crash injuries.
Based on National Highway Traffic Safety Administration (NHTSA)
analyses, the cost to society for all crashes is now over one trillion
dollars per year.
Traffic crashes are the leading cause of death for people of ages 3 to
33. This excessively high toll is considered to be one of the nation’s
primary public health issues.
For years, we had seen relatively static crashes and fatalities and a generally decreasing fatality rate – 1.58
deaths per 100 million vehicle-miles of travel in 2000 to 1.20 in 2019. However, that number jumped to
1.46 deaths per 100 million vehicle-miles in 2020.

Road Safety Concepts

Safety, in itself, is a somewhat abstract term. What is more to the point is that we, as traffic professionals,
strive to make our roadway systems and their elements safer.
It is not sufficient to categorize a roadway as safe just because it meets
all engineering standards. By satisfying all applicable standards one
provides nominal safety. This concept does not consider the actual
crash history, or their potential, at a site. We are more concerned with
substantive safety – the actual safety performance of a site. We should
focus on providing substantive safety and consider all factors that are
contributing to the safety record at the location being reviewed.

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The concept of nominal safety is a consideration of whether a roadway, design alternative, or design
element meets minimum design criteria. According to this concept, a highway or proposed design is
considered to have nominal safety if its design features (such as lane width, shoulder width, alignment,
sight distance, etc.) meet the minimum values or ranges. The measure of nominal safety is simply a
comparison of design element dimensions to the adopted design criteria.
As an example, the criterion for lane width on an Interstate highway is 12 feet. A design alternative that
proposes 12-foot lane widths suggests a nominally safe design, whereas an alternative that proposes 11-
foot lane widths would not.
In actuality, the safety effects of incremental differences in a given design dimension can be expected to
produce an incremental, not absolute, change in safety. The nominal safety concept is limited in that it
does not examine or express the actual or expected safety performance of a highway. This second
dimension of safety is critical to making good decisions regarding design exceptions.
Substantive safety is defined as the actual long term or expected safety performance of a roadway. This
would be determined by its crash experience measured over a long enough time period to provide a high
level of confidence that the observed crash experience is a true representation of the expected safety
characteristics of that location or highway. Quantitative measures of substantive safety include:
• Crash frequency (number of crashes per mile (km) or location over a specified time period).
• Crash type (run-off-road, intersection, pedestrian, etc.).
• Crash severity (fatality, injury, property damage).
Expected safety performance will vary based on inherent differences among highway types and contexts.
For example, the frequency and other characteristics of crashes differ for a two-lane road in rolling rural
terrain versus a multi-lane urban arterial versus a freeway interchange.
Understanding a location’s substantive safety and making judgments about whether it meets expectations
should involve formal comparison of its crash profile with aggregate data for facilities with similar
characteristics—traffic volume, location (urban, rural, suburban), functional classification, facility type
(two-lane, multi-lane divided, etc.), and terrain. There are well–established methods for characterizing a
location’s substantive safety. This generally includes applying statistical models of crash experience from
broader data bases (safety performance functions and crash modification factor analysis). It should be
based on models and data from the same jurisdiction of the site being studied.
Safety is not an automatic byproduct of the application of established design and traffic operations
standards and practices. Rather, safety must be “designed into” the roadway, explicitly considering the
safety implications of design and operational decisions.
Highway features affect safety by:
• Influencing the ability of the driver to maintain vehicle control
and identify hazards. Significant features include lane width,
alignment, sight distance, superelevation, and pavement
surface characteristics.
• Influencing the number and types of opportunities that exist
for conflicts with other vehicles, pedestrians, bicycles, and other road users. Significant features
include access control, intersection design, railroad grade crossings, traffic control devices,
number of lanes, and medians.

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• Affecting the consequences of an out-of-control vehicle leaving the traveled way. Significant
features include shoulder width and condition, roadside slopes, fixed objects, and hazards,
roadside barriers, and appurtenances.
• Affecting the behavior and attentiveness of the driver, particularly the choice of travel speed and
path. Driver behavior is affected by virtually all elements of the roadway environment.
In many cases, safety and/or operational problems can be traced to a historical tendency to base highway
designs on minimum standards, rather than adopt an optimum design. The acceptance of minimum
standards as the criteria for design too often has occurred for reasons of economy. Frequently, a more
liberal design would have cost little more over the life of the project and would have increased safety and
efficiency substantially.
The ideal highway is one with uniformly high-quality design applied consistently over a section. It avoids
discontinuities in the highway environment such as abrupt major
changes in design speeds, transitions in roadway cross section, the
introduction of a sharp curve in a series of flatter curves, change from
full to partial control of access, constrictions in roadway width by
narrow bridges or other structures, intersections without adequate
sight distances, or other failures to maintain consistency in the
roadway design and operational characteristics.
• Alignment and cross-section design have significant safety
effects, especially on rural two- lane highways. Safety issues
tend to focus on sharp horizontal curves, crest vertical curves
that limit sight distance to major hazards, and narrow lanes
and shoulders.
• Sight distance is a fundamental need of drivers to safely travel
along the highway, including sight distance ahead, across intersections, at railroad grade
crossings, and within passing zones. Because sight obstructions are a pervasive condition on
streets and highways, careful attention is needed to clear obstructions, to place compensating
traffic control devices, or to use other countermeasures. A greater sensitivity to not only the
traffic safety consequences of sight obstructions, but also the long-term safety benefits of
relatively simple, inexpensive countermeasures could have a significant impact on traffic safety.
• Intersection safety is a function of location, adjacent land use, and functional classification of the
intersecting streets. Improvements to enhance safety include the categories of geometric design,
traffic control devices, and signal operations as well as enforcement and education of drivers and
pedestrians.
• Roadsides that are flat and unobstructed provide drivers the best opportunity to recover control
of their vehicle after they leave the traveled way. Desirably, roadside slopes should be 1V:4H or
flatter, and fixed objects and other hazards should be removed from within the clear zone.
• Traffic signing and pavement marking improvements have one of the highest benefit/cost ratios
of any highway safety treatment. In particular, larger signs, larger sign legends, and improved
retroreflectivity of signs for nighttime visibility aid all drivers, but especially older drivers.
• Traffic signals are effective in reducing certain types of intersection crashes. Appropriate timing
of phase change intervals and pedestrian clearance intervals, and appropriate placement of
detectors to provide decision zone protection are critical to improved safety.

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• Pavement surface conditions, including slippery pavements, rutting, inadequate drainage, and
inadequate cross-slope, can lead to crashes, especially in wet weather.
The highway should offer no surprises to the driver in terms of either geometrics or traffic controls. The
violation of driver expectancies, the result of such a discontinuity of the highway environment, is
undoubtedly a large factor in the chain of events that leads to many crashes, and thus a situation to be
avoided.
Visibility And Conspicuity
In the driving task, the recognition of potential hazards and the
understanding of guidance and control devices in a timely manner is
critical. During daylight hours and under good weather conditions, this
is generally achievable. However, at night or during adverse weather,
drivers may have limited ability to see what they are approaching.
About half of all traffic fatalities occur at night, although only about
one quarter of all travel occurs during these same hours. While there are other factors involved (including
intoxication and fatigue), driving at night has a constraint of reduced visibility. At night, a driver travelling
at speed may not have sufficient time to recognize a hazard and react safely. To help overcome this
constraint, traffic engineers use both roadway lighting and traffic control device retroreflectivity to
improve safety.
Roadway lighting provides for improved road user visibility, benefiting not only drivers but also
pedestrians and bicyclists. Roadway lighting can be placed along roadway segments or at spot locations
based on the need for increased safety.
Retroreflectivity is the ability of a surface to return light to its source and is a key component of traffic
signs and markings. Retroreflectivity allows drivers to see signs and markings located beyond the range
of illumination of the vehicle’s headlights. Standards for traffic sign retroreflectivity were adopted by the
Federal Highway Administration in 2008; for pavement markings, standards were adopted in 2022.
Signage And Pavement Marking Usage
Signing and pavement markings provide valuable visual cues to the
driver and overall safety benefits for the road user. Traffic signs
provide regulatory, warning, and guidance information, and all can
have a major role in safety.
Pavement markings provide longitudinal guidance and travel lane
separation as well as supplementing other traffic control devices. They
provide important information to the driver while minimizing the driver’s diversion of attention from the
roadway. Pavement markings do have limitations, as they may be obscured by snow or water on the
roadway.
Roadside, Medians, And Side Slopes
Approximately thirty percent of all highway fatalities are the result of single-vehicle run-off-the- road, or
roadway departure crashes. For two-lane rural roads, about one-half of all fatalities are roadway
departure crashes.
The three types of fixed objects or roadside features most frequently involved in fatal crashes are: trees
or shrubbery, culverts/ditches/curbs, and utility poles. The forgiving roadside concept allows for errant

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vehicles leaving the roadway for whatever reason, and supports a roadside designed such that the serious
consequences of the roadway departures are minimized.
Design options for treating roadside obstacles, in order of preference, are:
• Remove the obstacle.
• Redesign the obstacle so it can be safely traversed.
• Relocate the obstacle to a point where it is less likely to be struck.
• Reduce impact severity by using an appropriate breakaway device.
• Shield the obstacle with a longitudinal traffic barrier designed for redirection or use a crash
cushion.
• Delineate the obstacle if the above alternatives are not appropriate.
Clear Zones And Recovery Areas
The clear roadside concept, as described in the AASHTO Roadside Design Guide, is applied to improve
safety by providing an unencumbered roadside recovery area that is as wide as practical on a specific
highway section. When first introduced, a value of 30 feet (9m) from the edge of the traveled way was
assumed for all roadways, regardless of roadway volume or speed. Within this clear zone, objects are
treated in accordance with the design options stated above.
However, it became apparent that this distance couldn’t be justified on low-volume, low-speed roadways.
Current procedures for determining clear zone use a design process that includes the following: backslope
or foreslope, design speed, horizontal curvature, and roadway AADT.
The foreslope extends from the edge of the shoulder to a drainage ditch or ground surface; the backslope
extends from the back edge of the drainage ditch to the ground surface. Foreslopes of 1V:4H or flatter are
considered recoverable. Foreslopes steeper than 1V:3H are considered critical where a vehicle is likely to
turn over.
For low-speed rural collector and local roads, a minimum clear zone width of 10 feet (3.0m) should be
provided.
Crash Safety Devices
There are occasions where the desired clear zone and recovery area cannot be provided, or where the
possibility of crossing into opposing traffic lanes is considerable. In these cases, crash safety devices
should be used to minimize the severity of a crash if one cannot be avoided. These can include longitudinal
barriers (guardrail, median barrier, and bridge railings), barrier terminals and crash cushions, breakaway
sign supports, luminaire supports, and a variety of work zone devices.
Access Management
Access management consists of the development and implementation of standards designed to manage
vehicular access points from adjacent parcels to the roadway. Access management promotes safety by
reducing conflict points and maintaining smoother traffic flow. Techniques used in access management
include providing for minimum spacing between signalized intersections, limitations on the number of
driveways, providing for turning lanes into the parcels, implementing medians where necessary to
regulate access, and the reservation of right-of-way for future widening and to maintain adequate sight
distance.

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Human Factors
Human factors take into account the abilities, limitations, and other characteristics of the human
population as they relate to the tasks of the driver. Driver performance is influenced by many factors,
including environmental, psychological, and vehicle design. The driving task requires three primary
elements, including control, guidance, and navigation (ranked in order of complexity).
A key human factor is the function of perception and reaction. The driver must recognize the existence
of a hazard, and then make the appropriate response. This has a defined time component known as
Perception-Reaction Time (PRT). A goal of safety design is accommodating the PRT and even to shorten
it, through signing and pavement markings, if necessary.
Driver error is a significant contributing factor in most crashes. For example, drivers can make errors in
judgment concerning closing speed, gap acceptance, curve negotiation, and appropriate speeds to
approach intersections. In-vehicle and roadway distractions, driver inattentiveness, and driver weariness
can lead to errors. A driver may also be overloaded with information processing required to carry out
multiple tasks simultaneously, which may lead to errors. Drivers are more likely to make mistakes when
their expectations are not met.
One way to accommodate human limitations is to design roadways to meet driver expectations. When
drivers can rely on past experience to assist in the driving task, errors are less likely. Drivers develop both
long- and short-term expectancies. Long-term expectancies are global in nature, and drivers anticipate
that they will be true everywhere. Examples of long- term expectancies include:
• Freeway exits will be on the right-hand side of the road
• When approaching an intersection, drivers must be in the left lane to make a left turn
• A continuous through lane will not end at an interchange or intersection
Short-term expectancies are developed over the most recent few miles or minutes of travel on a roadway.
These may include:
On a gently winding road, upcoming curves will continue to be gentle
• After traveling at high speed for some considerable distance, the road ahead will be designed to
accommodate the same speed
• A road that has the characteristics of a freeway will not have any at-grade intersections or
driveways.
Drivers can respond more quickly and correctly to conditions that they expect and are more likely to react
more slowly and are more likely to make mistakes when conditions do not meet their expectations. The
best (safest) roadway design is one that provides consistency and meets driver expectations.
Sight Distance
Sight distance refers to the length of roadway visible to a driver. Having adequate sight distance is critical
to safety as it helps to address the limitations of the PRT.
Three primary types of sight distance are utilized in traffic safety engineering:
• Intersection sight distance – A driver approaching an intersection should have an unobstructed
view of the entire intersection and an adequate view of the intersecting highway to permit control
of the vehicle to avoid a collision. Different sight distance requirements exist for intersections

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with no control, with stop control on the minor road, and with yield control on the minor road,
and apply to crossing and turning movements.
• Stopping sight distance – A driver should have sufficient sight distance to identify, react, and stop
to avoid hitting a stationary object in their path. The stopping sight distance has two primary
components:
o The distance traveled by the vehicle from the instant the driver first sees the object until
the brakes are applied is known as the Brake Reaction Distance.
o The distance needed to stop the vehicle from the time the brakes are applied is the
Braking Distance.
• Passing Sight Distance – On two lane roads where passing is allowed, drivers should have sufficient
sight distance to safely initiate and complete the passing movement before encountering an
oncoming vehicle.
Speed Management
Excessive speed is a factor in approximately one-third of all injury crashes and is an ongoing concern in
traffic safety. Speeds should be appropriate for the nature of the roadway facility; lower speeds are
particularly desirable where vulnerable road users such as pedestrians and bicyclists are present.
When designing a new facility, an appropriate design speed should be defined for the context of the
facility. To reduce speeds, design features such as roundabouts, appropriate lane widths, and other self-
enforcing techniques are useful. It is also important to have a credible speed limit – both reasonable and
safe – and to provide the appropriate level of enforcement.
Environmental
Environment considerations for safety include weather conditions and drainage. Rain and snow present
the most issues; heavy rain can reduce the available sight distance and poor drainage resulting in water
standing on the roadway can create hydroplaning conditions. In northern climates, snow and ice create
slippery conditions and extreme accumulations can impact intersection sight distance.
In some locations, wind can be a factor, particularly for large trucks which tend to overturn when hit with
very strong perpendicular winds. Fog and smoke conditions can significantly reduce the available sight
distance; many multi-vehicle crashes have occurred when motorists unexpectedly hit a fogbank.
With our changing climate, there are indications that we are and will be seeing a higher incidence of severe
weather events, such as derechos, tornadoes, hurricanes, dust storms and sandstorms, and flooding.
Resilience to these events should be built into designs as much as possible, and considerations for
evacuation and emergency response should be made.
Conflict Points
Conflict points are locations in or on the approaches to an intersection where vehicles paths merge,
diverge, or cross. Research has indicated that intersection crash patterns are related to the number of
conflict points; minor street crossing and left turn movements at a traditional intersection are generally
the most hazardous due to the high level of judgment needed to identify gaps in traffic and the relative
speeds of the vehicles on the major street.
In safety analyses, opportunities to reduce conflict points may result in significant safety improvements.

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Safety Analysis And Evaluation

An unfortunate reality of traffic safety improvement programs is that funding is limited. As a result, it is
necessary to identify locations of high crash frequency and/or severity, identify potential
countermeasures, and prioritize their improvements based on a comparison with other roadways.
An effective road safety management process consists of the following
elements:
• Collect and maintain a crash records database
• Perform a network screening to identity locations of crash
concerns
• Develop a diagnosis of sites and crash causing situations
• Select the appropriate countermeasures
• Perform an economic appraisal of the value of the improvements
• Prioritize the proposed safety improvement projects
• Perform a follow up evaluation of the effectiveness of the implemented treatments.
Crash Record and Data Systems
The data used in safety studies, to a large extent, is recorded by police
officers on report forms or electronic devices soon after a crash has
occurred. Important information includes crash location, type,
severity, environmental conditions, and driver actions. It must be
recognized that this data collection is subject to several potential
weaknesses that must be considered when using the data:
• Not all crashes are reported to the police, especially those that involve only property damage
• Different jurisdictions may have significantly different reporting thresholds, and such thresholds
may change over time making comparisons difficult
• There can be inaccuracies in reported data due to simple typographical errors, location estimates,
and incomplete reporting.
There are also inherent limitations to crash data due to natural variations in crash occurrence from year
to year. If not considered and accounted for, these limitations may introduce bias into safety analyses.
Because crashes are random events, crash frequencies naturally fluctuate over time at any given site. This
randomness means that short-term crash frequencies alone are not a reliable estimator of long-term
crash frequency. Short-term crash frequency may vary considerably from the long-term average crash
frequency. It is difficult to know whether changes in observed crash frequency are due to changes in site
conditions or are due to natural fluctuations.
When a period with comparatively high crash frequency is observed, it is statistically probable that the
following period will have a comparatively low crash frequency. This tendency is known as regression-to-
the-mean. If not accounted for, regression-to-the-mean bias can result in the over-estimation (or under-
estimation) of the effectiveness of a safety treatment. The effect of regression-to-the-mean can be
overcome by using long-term average crash frequency in selecting sites for improvement and in evaluating
effectiveness of improvements. However, this can only be done if there are no changes at the site (e.g.,

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geometrics, traffic control, traffic volumes, roadside development) during the long-term period. This is
often not the case.
Finally, it must be recognized that relying on data about past crashes, as a method of identifying sites for
safety improvements, is an inherently reactive approach. Because of the limitations in the use of crash
data, increasingly, agencies are using a more proactive approach to road safety, using the analytic tools
of the Highway Safety Manual.
Hazard Identification
Various techniques are available for identifying spot locations or
roadway segments that have experienced a higher-than-expected
frequency, rate, or severity of crash occurrence. A preliminary listing
of problem locations may be based on actual crash experience or on
the potential for crashes. For each location that has been identified as
having an existing or potential safety problem, the severity of that
problem must be quantified. In this manner, locations can be
compared in terms of relative hazard, and efforts to identify causes and candidate solutions concentrated
on the more serious problems.
The identification of hazardous locations and features is an important process. These may include spot
locations (such as a railroad grade crossing or a narrow bridge); intersections; roadway sections (perhaps
of considerable length); or a street system (such as a neighborhood street system where crashes at
individual intersections may be infrequent, but on an aggregated basis, a safety problem emerges).
In some cases, it is desirable to give more emphasis to locations that have experienced greater than
expected crash severity. However, fatal crashes should not be over-emphasized in the process of
identifying high-hazard locations. In most cases, the difference between a fatal or an injury crash is simply
a matter of chance, or even the proximity of the nearest trauma center.
Hazardous highway locations may or may not be high-crash locations. Many locations with narrow
bridges, slick pavements, numerous rigid roadside obstacles, etc., have a high crash potential but may not
yet have a history of high-crash occurrence. Early warning analysis should be conducted routinely to
identify locations that have a sudden increase in crashes or crash potential. Sudden increases in crash
potential may be noticed by observing a rash of skid marks, erratic maneuvers, dents in guardrail, or other
such indicators at a location.
Various techniques are available to identify spot locations or roadway sections that have experienced a
higher-than-expected frequency or rate of crash occurrence. The appropriate technique depends on
availability of data (e.g., traffic volumes), size and complexity of roadway system, and technical
sophistication of the analyst and decision-maker. The goal of any technique used is to select those
locations most in need of safety improvements.
Crash Frequency, Rate, and Severity
The frequency of crash occurrence (crash frequency) is the simplest
technique for identifying high hazard locations. Intersections or
roadway segments of uniform lengths are simply ranked in order of
the number of crashes that occurred during a given time period. This
technique requires data from crash reports. Although simple to
perform, reliance on crash frequency tends to bias the identification
process in favor of higher volume roadway sections and intersections.

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As a result, it may ignore severe safety problems on low-volume roads or intersections. The identification
process may be improved by categorizing roadway segments and intersections according to functional
classification (e.g., freeway, arterial, collector, local), and developing separate rankings for each category.
Crash rates are normally considered better indicators of risk than crash
frequencies alone, because they account for differences in traffic
volumes, and hence exposure. Crash rates for roadway segments are
normally expressed in terms of crashes per 100 million vehicle-miles
(100 million vehicle-kilometers) of travel (100MVM or 100MVK), using
the following equation:

where:
RSEC = crash rate for the roadway section
C = number of reported crashes
T = time period of the analysis (years)
V = annual average daily traffic volume (veh/day)
L = length of the segment (mi or km)
Crash rates for spots (such as intersections) are normally expressed in
terms of crashes per million entering vehicles (MEV), using the
following equation:

where:
RSPOT = crash rate for the spot C = number of reported crashes
T = time period of the analysis (years)
V = annual average daily traffic volume entering the spot (veh/day)
Ranking locations by crash rates requires traffic volume data for all roadway segments or spots. Because
it accounts for exposure to potential crashes, it is generally superior to crash frequency as a means of
identifying high hazard locations. However, it may result in a bias in favor of low-volume locations that
have relatively few crashes, but a high crash rate. Although such a location may be of concern, it may offer
less overall benefit in terms of crashes reduced when compared with a higher volume location with more
numbers of crashes (and hence more crashes that could be reduced.

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Number-rate methods of ranking high hazard locations attempt to

correct the biases of the crash frequency and crash rate methods. First,
all spot locations or roadway segments are ranked by crash frequency,
and those with less than a certain number of crashes removed from
further consideration. The minimum crash frequency criteria may be
established at a level that reduces the group of remaining locations to
a workable size. Then the remaining locations are re-ranked using the
crash rate. Finally, locations with less than a certain crash rate are removed from further consideration.
The remaining locations are assured of having a minimum crash frequency and crash rate.
Equivalent property damage only (EPDO) rates adjust the high hazard
identification process to give greater weight to injury and fatal crashes.
This technique compares the relative importance of crashes that result
in only property damage with that of injury and fatal crashes.
Weighting factors must be developed which reflect the relative
importance to society of crashes of different severities. For example,
one agency uses a weighting factor of 12 for fatal crashes and five for
injury crashes. The number of fatal and injury crashes are multiplied by the weighting factors, and these
“equivalent property damage only” crashes added to the actual number of property damage crashes.
Then an EPDO rate can be calculated similar to normal crash rates for intersections or roadway segments.
Establishing weighting factors is difficult. Care should be taken so that fatal and/or injury crashes are not
weighted too heavily, which can lead to biases in the ranking of high hazard locations by over-emphasizing
or locations with a few severe crashes.
The rate quality control method applies statistical concepts to avoid
pitfalls associated with random variation in crash frequency. Because
crashes are relatively rare events, the actual number of crashes at a
given location may vary considerably from year to year. The rate
quality control method determines whether the actual crash rate
varies from the average of similar locations more than could be
attributed to pure chance. The critical crash rate is calculated for each
location as follows:

where:
Rc = critical crash rate per 100 MVM (or 100 MVK) or per MEV
RA = average crash rate for locations with characteristics similar to the subject location
K = constant corresponding to the level of confidence in the findings of the analysis
V = volume of traffic at subject location (same units as used for crash rates)
If the actual crash rate for a location is greater than the critical rate, then the location is considered
hazardous. In other words, the crash rate is significantly higher than the average rate for similar locations.

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K values of 1.282, 1.645, and 2.327 correspond to levels of confidence of 90, 95, and 99 percent,
respectively. The lower the value of K, the greater the chance that a location will be identified as hazardous
when it really is not.
Empirical Bayes methods use the concept of a conditional probability,
which is the probability that something is true given the knowledge
that something else has occurred. In dealing with crashes, analysts
want to know the probability that a location is truly hazardous given
the crash history, traffic volumes, and physical characteristics of the
location. The Bayesian procedure provides a method to combine the
crash frequency predicted by a crash prediction model (NP) with the
crash frequency from site-specific crash history data (NA). The expected crash frequency considering both
the predicted and observed crash frequencies is computed as:

where:
EP = Expected crash frequency
NP = Number of crashes predicted by a crash prediction model for a specified period of time
NA = Number of crashes observed during a specified period of time
w = Weighting factor which determines the weight to be placed on the crash frequency predicted
by the crash prediction model
At railroad grade crossings, past crash history is often a poor indicator of the true hazardousness of a given
location because crashes tend to be very rare and random. The use of a crash prediction model, along
with actual crash data, can improve the ability to correctly identify high-hazard locations. The expected
crash frequency (EP) calculated using Bayesian methods can be used directly to rank locations for safety
investigation.
Potential for Safety Improvement (PSI) is another method that can be
used as a criterion for identifying high-hazard locations. PSI is the
difference between a location’s actual crash frequency and the
expected crash frequency for all locations with similar classification
(NP). The locations with the largest potential safety improvement
(crash reduction) would be ranked highest. The expected crash
frequency for similar locations can be estimated using Bayesian
methods.
Expected value analysis is intended to identify abnormal crash
patterns. The method uses a statistical test to determine whether a
crash pattern at a location is significantly higher than the same crash
pattern at similar locations within the jurisdiction. For example, this
method may be used to identify intersections where the frequency of
left turn crashes is significantly greater than the number of left turn
crashes at similar intersections. Patterns to be analyzed may be based

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on vehicle maneuvers (e.g., left turn, rear-end, run-off-the-road, etc.), time-related conditions (e.g., day
vs. night, weekday vs. weekend, seasonal, etc.), weather conditions (e.g., wet vs. dry pavement), or
severity (PDO, injury, or fatal).
Spot And Systemic Analyses
When a manageable number of locations have been identified by a
network screening method, the next step is an analysis of crash
patterns. If a repeated pattern (or patterns) of crash occurrence can
be identified, this may indicate a causal relationship between a design
or traffic operational characteristic of the site and the crashes which
are occurring.
The more crashes that can be included in this analysis, generally the easier it is to identify a pattern, if one
exists. At least three years of crash data are desirable, especially if there are relatively few crashes each
year. If there are a significant number of crashes at a location, a shorter data collection period may be
used. If data are available, additional years of data should be analyzed as long as no significant design or
traffic operational changes have occurred at the site.
One of the most useful crash analyses is a crash summary by type. This
serves as a major indicator of possible design or traffic operational
factors that are contributing to crashes. Crash types are typically
categorized as:
• Left-turn
• Right-angle
• Rear-end
• Sideswipe
• Pedestrian
• Bicycle
• Run-off-the-road
• Fixed object
• Head-on
• Parked vehicle
• Animal
• Other
Crashes may also be summarized by contributing environmental conditions, such as:
• Rain
• Snow/ice
• Fog
• Darkness
• Construction

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Collision And Condition Diagrams

The crash types are often summarized on a collision diagram. A
collision diagram is a schematic drawing that uses symbols to
represent each crash type. The collision diagram should show:
• Direction of movement and intended maneuvers prior to
impact for each vehicle or pedestrian involved in the crash
• Non-contact vehicles that were involved in the chain of events
that led to the crash, but were not contacted by any other
vehicle
• Date, day of week, and time of day of crash
• Unusual conditions such as adverse weather, construction, pavement conditions, etc.
All crashes related to the site (but only crashes related to the site)
should be included in the crash summary and/or collision diagram. In
some cases, a crash that occurred at a distance remote from the site
may be site-related (e.g., a rear-end collision that occurred several
hundred feet upstream of a congested intersection). In other cases, a
collision that occurred near the site might not be site- related because
it was a random event unrelated to site design or operational
conditions (e.g., an animal collision at an intersection).
In many cases, the only source of information on intended maneuvers and non-contact vehicles is in the
narrative or diagram portion of a police crash report form. Unless a copy of the original report form can
be accessed during the preparation of the collision diagram, this information may be lost, and the pattern
shown on the collision diagram may provide misleading information.
Additional summaries of crash characteristics may also be useful in identifying site deficiencies and
selecting appropriate countermeasures. These include:
• Crash severity
• Contributing circumstances
• Environmental conditions
• Time period
Some crash database systems can develop collision diagrams
automatically.
It is also important to understand the physical characteristics at the
site. The condition diagram is a scale drawing, as-built plan, or aerial
photograph of the location showing all roadway features, including
roadway dimensions, abutting land uses, traffic control devices and their location, geometric features
such as sight distance restrictions, and other pertinent information.
Select Countermeasures
After one or more patterns of crashes have been identified at a study location, the next step is the
generation of a list of potential countermeasures.

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Desirably, a range of alternative countermeasures should be

considered for any study location. This may include relatively high-cost
to relatively low-cost treatments. These alternative countermeasures
could potentially be implemented individually or in combination with
one another. In any such analysis, the do-nothing alternative should
always be considered.
Potential countermeasures may be identified through:
• Detailed investigations of crashes to identify causal factors
• Reviews of site plans and condition diagrams
• Site inspections
• Other traffic studies, such as spot speeds, conflicts analysis,
and intersection sight distance studies
• The practices and previous experiences of the agency
Predictive Models
The Highway Safety Manual (HSM) Predictive Method was developed
due to the recognition of the need for standardized, quantitative
methods for crash estimation and crash evaluations that address the
limitations of use of data on past crashes. The HSM provides
quantitative methods to reliably estimate expected average crash
frequencies and severities at a site, facility, or roadway network. The
predictive method allows for crash estimation in situations where no
crash data are available. Where crash data are available, the Empirical
Bayes method is used to apply a weighting factor is to combine the
estimation from the HSM model with observed crash frequency.
The HSM predictive models vary by facility type, but all have the same
basic elements:
• Safety Performance Functions (SPFs) - statistical “base”
models that are used to estimate the average crash frequency
for a facility type (e.g., two-lane rural highway, urban and
suburban arterials) with specified base conditions; see more
description in the following section.
• Crash Modification Factors (CMFs) - the ratio of the
effectiveness of one condition in comparison to another
condition (e.g., 12-ft [3.6 m] lane width compared to 10-ft [3.0 m] lane width). CMFs are multiplied
with the crash frequency predicted by the SPF to account for the difference between site
conditions and specified base conditions. See more description of the CMFs later in this
document.

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• Calibration Factor C - multiplied with the crash frequency

predicted by the SPF to account for the differences between
the jurisdiction and time period for which the predictive
models were developed and the jurisdiction and time period
to which they are applied.
The prediction model to estimate the expected average crash
frequency generally takes the form:

𝑁𝑁𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝 = 𝑁𝑁𝑆𝑆𝑆𝑆𝑆𝑆 × (𝐶𝐶𝐶𝐶𝐶𝐶1 × 𝐶𝐶𝐶𝐶𝐶𝐶2 × … 𝐶𝐶𝐶𝐶𝐶𝐶𝑛𝑛) × 𝐶𝐶

where:
Npredicted = predictive model estimate of crash frequency for a specific year on the site type
(crashes/year)
NSPF = predicted average crash frequency determined for base conditions for the site type
(crashes/year)
CMFn = Crash Modification Factor for local conditions for site type.
C = Calibration Factor to adjust for local conditions for site type
Crash frequencies, even for nominally similar roadway segments or intersections, may vary widely from
one jurisdiction to another. Such differences may result from differences in weather or terrain, crash
reporting practices, and driver population. The Calibration factor adjusts the SPF to reflect the differing
crash frequencies from one state to another or one geographic region to another within a state.
Estimation of expected average crash frequency using only observed
crash frequency or only using the HSM statistical predictive models
may result in a reasonable estimate of crash frequency. However, the
statistical reliability (probability that the estimate is correct) is
improved by combining observed crash frequency with the estimate
from the predictive model. The HSM uses an Empirical Bayes method
to combine the two estimates together. A weighting factor is calculated that is dependent on the variance
of the SPF and is not dependent on the validity of the observed data. The Empirical Bayes method is only
applicable when both predicted and observed crash frequencies are available, as follows:

𝑁𝑁𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒𝑒 = 𝑤𝑤 × 𝑁𝑁𝑁𝑁𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟𝑟 + (1 − 𝑤𝑤) × 𝑁𝑁𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜𝑜

where:
Nexpected = expected average crash frequency for study period
w = weighting factor
Npredicted = predicted average crash frequency using an SPF
Nobserved = observed average crash frequency
Using this process to identify the anticipated effectiveness of the implementation of proposed
countermeasures, a proposed project can be developed.

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Safety Performance Functions

Safety Performance Functions are regression equations that estimate
the average crash frequency for a specific site type, with specified base
conditions, as a function of annual average daily traffic and segment
length. Base conditions typically include conditions such as lane and
shoulder width, presence of lighting, presence of turn lanes, etc. For
example, the SPF for rural two-lane highways is:

𝑁𝑁𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆𝑆 = (𝐴𝐴𝐴𝐴𝐴𝐴𝐴𝐴 × 𝐿𝐿) × (365) × 10−6 × 𝑒𝑒−0.4865

where:
NSPFrs = estimate of predicted average crash frequency for SPF
base conditions for a rural two-lane highway segment
(crashes/year)
AADT = average annual daily traffic volume on highway
segment (veh/day)
L = length of highway segment (mi or km)
Crash Modification Factors
Crash Modification Factors represent the relative change in crash
frequency due to a change in one specific condition (e.g., lane width,
shoulder width, curve radius). A CMF of 1.00 indicates that no change
in crash frequency is expected. A CMF less than 1.00 indicates that the
proposed change will result in a reduced crash frequency. A CMF
greater than 1.00 indicates that the change will result in increased
crash frequency. Sometimes Crash Modification Factors are confused
with Crash Reduction Factors.
When several types of improvements are included in a project, the
CMFs for the various improvements must be combined by multiplying
them together.
A searchable database of CMFs can be found in the Crash Modification
Factor Clearinghouse (https://www.cmfclearinghouse.org/), along
with guidance and resources on using CMFs in road safety practice.
Economic Appraisal of Proposed Improvements
Once improvements have been identified, a consideration of the cost-
effectiveness of alternative improvements is made, comparing
relevant costs of implementation with benefits in terms of a reduction
in crash frequency and/or severity.
Quantifying the impacts of each alternative countermeasure in
monetary terms can be quite difficult. This requires an estimate of the
effectiveness of various improvements in reducing crashes. Crash
reduction factors are contained in the Highway Safety Manual, the
FHWA Crash Modification Clearinghouse and other publications. These crash reduction factors estimate
the percentage of crashes that can be reduced or avoided by implementing various improvements. Such

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data must be used cautiously, because countermeasure applications

vary greatly, and published data usually apply to only “average”
conditions. In addition, when two or more countermeasures are
implemented together, the expected crash reduction cannot be taken
as the sum of the reduction for each countermeasure taken
individually.
Another significant difficulty in economic evaluation of proposed improvements is in valuing crashes that
can be avoided by the countermeasure(s). Various agencies have valued crashes avoided at significantly
different levels. Analysts should carefully consider the underlying assumptions behind crash values before
using them. It should be recognized that higher values placed on avoided crashes would tend to justify
more expensive countermeasures.
In evaluating the cost-effectiveness of a proposed improvement, it
may be necessary to compare the value of the initial costs of
construction and any right-of-way acquisition against the future value
of benefits of crash reduction that may accrue over a period of many
years into the future. There may also be future ongoing costs of
operation and maintenance of the improvement that need to be
considered in the evaluation. Future costs and benefits do not have
the same value as current costs and benefits and must be discounted based on the time value of money.
The present worth of some future cost or benefit is:

The present worth of a series of future amounts is:

where:
P = Present worth of future costs or benefits
F = A single future cost or benefit
A = Annual amount of a series of future costs or benefits
n = Number of years into the future
i = Annual interest rate representing time value of money
Alternative safety improvement projects that have a benefit/cost ratio (lifetime benefits divided by
lifetime costs) greater than 1.0 or a positive net present worth (lifetime benefits minus lifetime costs) may
be considered economically worth implementing.
The benefit / cost ratio of a project is:

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B/C Ratio = PBenefits / PCosts

In performing these economic analyses, it should always be remembered that some projects may involve
significant costs that cannot be readily measured in dollar value, such as environmental and social
impacts. Such impacts must be carried through the evaluation process and explicitly considered in
deciding which projects to go forward with.
Prioritization of Projects
The next question that must be addressed is which projects should be
implemented in order to use limited safety resources as efficiently as
possible, and in what order of priority?
A simple ranking of alternative projects according to any of the
following may sometimes be appropriate:
• Net present worth
• Project costs
• Monetary value of project benefits
• Total number of crashes avoided
An incremental benefit/cost analysis is more sophisticated, and generally will result in more efficient
allocation of resources.
Once safety improvement projects have been selected for implementation, the projects should move
forward as quickly as feasible. Any delay in implementation will potentially cost additional crashes.
Locations that are not chosen for implementation of safety improvement projects, or where the
implementation of the desired improvement projects must be delayed, should be examined to determine
if interim improvements are appropriate to improve safety pending the implementation of the desired
improvement.
Safety Effectiveness Evaluation of Implemented Improvements
A program of countermeasure evaluation is essential for intelligent
future countermeasure selection. Such an evaluation determines the
actual effectiveness of any improvements after they have been put
into place. The evaluation results, whether positive, negative, or
neutral, should be documented to improve the knowledge base to
improve future decisions.
Such evaluations are typically “before-and-after” evaluations. To conduct such a study, it is essential that
the study be designed prior to implementing the improvement so that adequate “before” data can be
collected.
As currently conducted by many practitioners, before and after studies often suffer from two serious flaws
and provide incorrect or misleading results. These flaws are:
• Failure to control for the effects of changing conditions during the lengthy time periods required
to amass before and after crash statistics. For example, changes in vehicle design, driver education
or enforcement campaigns, etc. may result in changes in crash frequency even if no improvements
were made in the roadway.

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• Failure to correct for regression-to-the-mean. Regression to the mean occurs when locations with
high crash frequencies during one time period experience more normal frequencies during the
next time period even if no causative factor changes.
The best way to control for changing conditions is to use randomly selected control sites where crash data
are collected, but no changes are made. The Empirical Bayes method can be used to control for regression-
to-the-mean bias. Instead of a set of control sites, the SPF developed for the treatment site(s) is used to
predict crash frequency at the treatment site(s) in the after period had the treatment not been applied.
Road Safety Audits
The Road Safety Audit (RSA) is a proactive approach that can be applied before a crash history indicates a
problem exists. A road safety audit is defined as “a formal safety performance examination of an existing
or future road or intersection by an independent audit team.”
The key elements of this definition are:
• It is a formal examination with a structured process and not a
cursory review;
• It is conducted independently by professionals who are not
currently involved in the planning, design, construction, or
operation of the project;
• It is completed by a team of qualified professionals
representing appropriate disciplines; and
• It focuses solely on safety issues.
The RSA is not:
• a means to rate or rank a project.
• a check of compliance with standards.
• a redesign of a project.
• a part of the crash investigation process.
RSA’s can be performed at one or more stages of a roadway project:
• Planning or feasibility
• Preliminary design
• Final design
• Pre-opening or during construction
• On an existing roadway
RSA’s can have the greatest impact when they are early in the design process when the range of redesign
options is the greatest. The best advice is to conduct an audit as early as possible to be integrated with
the agencies existing safety program.
The following are the steps in the formal process of performing a road safety audit.
• Select the team. The road safety audit team should be composed of people who are independent
of the project and bring expertise from diverse backgrounds, experiences, and knowledge. There

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is no set number of people on the team, but teams of 3 to 5

people seem to be ideal. Core skills include geometric design,
traffic engineering, and safety. Other disciplines may include
enforcement, planning, human factors, and
pedestrian/bicycle specialists.
• Provide the background information. The project designer or
client (the person/organization who is commissioning the audit) provides information to the
team. This may include plans, design reports, traffic volume data, and crash histories if available.
• Hold a commencement meeting. At this meeting the audit team and the designer/client meet to
discuss the purpose of the audit and any other issues or concerns from both sides.
• Assessing the documents. The audit team meets to review all the documents and information
provided. At this meeting they review the checklist or prompt lists they will take to the field when
doing a site inspection.
• Inspecting the Site. The team makes a site visit to inspect the site and identify all potential safety
issues. The inspection should consider all potential users: not just motorists but also pedestrians,
bicyclists, and others. A daytime visit is essential but a subsequent nighttime visit should be part
of the field inspection process. The team should use checklists or prompt lists to make sure all
safety issues are addressed. However, these lists should only be used to help identify issues. The
primary value of the audit is the collected expert evaluations of the personal on the audit team.
• Write the audit report. The report should be a short, concise summary of the findings of the audit
team. It needn’t be any longer than 2 to 3 pages, except for large-scale projects. The report should
contain the following: purpose of the audit, names of the audit team, resources used by the audit
team, and identification of safety issues. Safety issues may be listed as findings or
recommendations, depending on the desires of the designer/client. It is becoming common
practice in the U.S. to hold a meeting with the designer/client prior to writing the report. The
purpose of this meeting is not to discuss what is in the report, but to establish an atmosphere of
cooperation and to assure that the audit is addressing pertinent issues.
• Hold a completion meeting. The audit team presents its report
at a formal meeting to the designer/client. This should be
viewed as an opportunity to disagree with the findings of the
audit.
• Write a response to the audit. It is the responsibility of the
designer/client to write a formal response to the audit report.
This response should address each and every audit
finding/recommendation and should identify which recommendations are fully accepted,
which are partially accepted, and which are rejected. Reasons for rejection should be given.
Solutions or changes that will be implemented should also be listed.
• Implement of solutions. The designer/client is responsible for
implementing the changes agreed upon in their report
responding to the audit findings.
• Close the loop. The designer/client should use the results of
the audit process to guide future projects. The experiences of

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the audit and the safety findings should be shared with others in the organization to improve
future designs.
The costs of an audit are reasonable, ranging from around $1,000 to $10,000 for large-scale projects. The
audit itself can be completed in one day, with an additional day’s time for each team member to review
materials and to prepare the final report.

Safety Countermeasures
Selecting one or more countermeasures to improve a safety concern
requires the determination of the crashes’ probably causes. Following
are some examples of possible countermeasures for different types of
crash patterns:
• Right-angle crashes at a two-way STOP controlled
intersection:
Potential Causal Factor Possible Countermeasure
Restricted sight distance Remove sight obstruction
Restrict parking near the intersection
Provide an all-way STOP installation
Install a traffic signal
Install an advance warning sign
Move stop line closer to intersection
Excessive speed Install an advance warning sign
Reduce speed limit with enforcement
Install rumble strips
Inadequate roadway lighting Improve lighting
Inadequate advance warning signs Install or improve warning sign
Large traffic volume / Insufficient gaps Provide traffic signal
Reroute traffic
Inadequate traffic control devices Upgrade traffic control devices

• Left-turn crashes at a two-way STOP controlled intersection:

Potential Causal Factor Possible Countermeasure

Large turn volume Create one-way street
Add left-turn lane
Prohibit left turn
Reroute left-turn traffic
Provide traffic signal with left-turn phase
Restricted sight distance Remove sight obstruction
Provide left-turn lane
Prohibit left-turn
Provide traffic signal with protected only left
turn phase
Excessive speed Reduce speed limit
Improve enforcement

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• Right-angle crashes at a signalized intersection:

Potential Causal Factor Possible Countermeasure

Restricted sight distance Remove sight obstruction
Install/improve warning sign
Reduce speed limit with enforcement
Excessive speed Reduce speed limit with enforcement
Adjust phase change interval
Install rumble strips
Inadequate roadway lighting Improve lighting
Poor visibility of traffic signal Install or improve warning sign
Install overhead signal heads
Install 12” signal lenses
Install visors
Install back plates
Add retroreflective strips to back plates
Relocate/add signal heads
Inadequate signal timing Retime signal
Adjust phase change interval
Provide red clearance interval
Provide progression
Provide signal actuation with dilemma zone
protection
Inadequate advance warning signs Install/improve warning sign
Large traffic volume Add lane(s)
Retime signal

More such examples are included in the Highway Safety Manual. Following are how specific
countermeasures can be effective in reducing crashes; consult the CMF Clearinghouse for specific
information about studies and effectiveness for various scenarios.
Traffic Control Devices
Traffic control devices provide can significantly reduce crashes and
crash severity if used appropriately:
• Various types of signing can be used to communicate and/or
emphasize a concern to a driver:
o Regulations and restrictions (including STOP, NO
TURN, SPEED LIMIT, etc.)
o Warnings (including CURVE, SIGNAL AHEAD, SLIPPERY WHEN WET, etc.)
o Guidance (proper advance guidance can help reduce last minute lane changes and turns)
It is in these conditions that signing conspicuity and visibility are critical. Sign condition,
retroreflectivity, enhanced colors (such as fluorescent yellow, orange, and yellow-green), and
placement are important considerations.
• Installed where warranted based on an engineering analysis, traffic signals can reduce certain
types of crashes (primarily right-angle and left turn crashes). However, a signal installation may
also increase rear-end and side-swipe crashes. The design of the signal, including placement of

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signal faces, phasing, and supporting structures, and operational functions such as timing, can all
have an impact on safety.
• Roundabouts have been shown to be very effective in reducing intersection type crashes and their
severity. The number of conflict points are reduced from a traditional intersection, and the
maneuvering through the roundabout causes a natural reduction in speed.
• Pavement markings provide longitudinal guidance along a roadway and through intersections, as
well as supplementing other traffic control devices. Retroreflectivity of the markings is important
for nighttime viewing. Pavement markings will gain increasing importance as autonomous vehicle
capabilities expand.
Pavement Treatments
Roadways must have an appropriate level of pavement friction to ensure that drivers are able to keep
their vehicles safely in the lane. Poor pavement conditions, especially when the pavement is wet, are a
major contributing factor in roadway departure crashes. When a pavement surface is wet, the level of
pavement friction is reduced, and this may lead to skidding or hydroplaning. Surface treatments to
increase the pavement friction can help to reduce crashes at locations where there are numerous wet
weather crashes or sharp curves with higher approach speeds.
Rumble strips and stripes are used to alert drivers to a potentially hazardous situation using both audible
and tactile feedback:
• Transverse strips can be used to alert drivers to the need to slow down or stop, such as on the
approach to an intersection, a toll plaza, or a work zone.
• Shoulder rumble strips are designed to reduce run-off-the-road crashes by alerting drivers they
have entered the shoulder. Edge line rumble stripes are a variation of the shoulder strips and are
combined with the edge line.
• Center line rumble strips are designed to reduce head-on collisions and opposite-direction
sideswipe crashes caused by motorists inadvertently crossing the centerline.
A consideration for rumble strips and stripes is the associated noise which may be undesirable in some
locations.
Roadside Barriers
Roadside barriers are designed to redirect an errant vehicle, preventing it from hitting a roadside hazard
or crossing into the path of oncoming vehicles. While striking a barrier is still a collision, the impact will
have a lower severity than striking the roadside hazard or oncoming traffic.
Flexible barriers are made from wire rope supported between frangible posts. Semi-rigid barriers are
usually made from steel beams or rails. Rigid barriers are usually made of concrete and do not deflect.
The use of roadside barriers requires end terminals. A collision with an untreated barrier end can result
in excessive deceleration and possible penetration of the barrier into the vehicle.
Improve Sight Distance
Maintaining adequate sight distance is important for roadway safety. Some sight distance issues are
relatively easy to resolve, while others may be prohibitively expensive to resolve and need a lower cost
countermeasure:

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• Vegetation overgrowth is often a cause of insufficient intersection sight distance. If located on

the right-of-way, this can be a relatively easy fix – remove or trim the vegetation. If on private
property, coordination with the property owner is generally required; acquisition of a corner
section of right-of-way or an easement may be needed.
• Obstacles in the right-of-way can also impact intersection sight distance – utility poles, traffic
signal cabinets, mailboxes, political signs, and more. These should be relocated if possible.
• Horizontal and vertical curves are more difficult to resolve cost-effectively. For sight distance
concerns with roadway alignments, traffic control devices including signs and pavement markings
may provide the only option.
Speed Control
As mentioned earlier in this module, speed is a factor in many severe crashes. Multiple countermeasures
exist for speeding issues, including:
• Highly visible enforcement
• Clearly posted speed limits on frequent signs; these can be supplemented by word message
pavement markings.
• Appropriate speed limits for the roadway facility and the adjacent land uses
• Speed feedback signs (radar signs advising motorists of their current speed)
• Roadway design features including narrower lane widths and reduced corner radii
• Coordinated traffic signal timing designed to stop motorists speeding faster than the progression
speed
• Traffic calming features.
Roadway Lighting
Adding roadway lighting can have a significant impact on nighttime crashes, either along segments of the
roadway or at intersections or other hazard locations. Lighting should be designed to provide illumination
without glare or uneven lighting levels within the lighted area.
The advent of LED lighting fixtures has greatly reduced the recurring energy costs of providing roadway
lighting.
Sidewalks And Crosswalks
Annually, around 4,500 pedestrians are killed in crashes; pedestrians killed while “walking along the
roadway” account for almost 8 percent of these deaths. Providing walkways separated from the
roadway’s travel lanes could help to prevent the vast majority of these crashes.
A sidewalk separated from the roadway by a buffer strip is the preferred accommodation for pedestrians.
Roadways without sidewalks are more than twice as likely to have pedestrian crashes as sites with
sidewalks on both sides of the street. By providing sidewalks on both sides of the street, the number of
midblock crossing crashes can be reduced.
Marked crosswalks help to alert motorists of the presence of a pedestrian crossing and help define the
intended crossing path for the pedestrians.

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School Zones
A variety of countermeasures may be used to enhance the safety of children in school zones:
• Well-trained adult crossing guards are one of the most effective measures for helping children to
cross streets safely
• Reduced speed limits using flashing beacons during arrival and dismissal times help to reduce the
potential of a serious injury in a school zone
• Sidewalks or separated walkways and paths are essential for a safe trip from home to school on
foot or by bike
• Police enforcement in school zones may be needed in situations where drivers are speeding or
not yielding to children in crosswalks.
Students and parents need to be educated about school safety and the proper access to and from the
school.
Removal, Relocation, Or Protection Of Fixed Objects
Earlier in this module we discussed the forgiving roadside concept, which allows for errant vehicles leaving
the roadway, for whatever reason, to do so without serious consequences. Ideally, any substantial fixed
objects, including trees, utility poles, and other non-yielding structures, should be removed or relocated
from the clear zone. For any of these objects that cannot be removed or relocated, impact protection is
appropriate.
Geometric Design Improvements
Improvements to the geometric design can serve as crash countermeasures. Modifications may improve
sight distance, help to reduce speeds, provide protection for turning vehicles, and provide separation
between conflicting movements. Geometric improvements may have a significant cost factor, so the
economic evaluation is critical for demonstrating the effectiveness of such an improvement.
Enforcement
Enforcement is a key element of traffic safety. High Visibility Enforcement incorporates strategies
designed to make enforcement efforts obvious to the public. It is supported by a coordinated
communication strategy and publicity. Checkpoints, saturation patrols, and other HVE strategies include
increased publicity and warnings to the public. Although forewarning the public might seem
counterproductive to apprehending violators, the advance notice increases the deterrent effects.
Unfortunately, many law enforcement agencies are understaffed and unable to perform traffic
enforcement at high levels on a continuous basis. Some agencies have adopted automated enforcement
to support these efforts, but that has been a controversial subject in many locations.
Traffic Calming
Traffic Calming is the combination of mainly physical measures that reduce the negative effects of motor
vehicle use, alter driver behavior and improve conditions for non-motorized street users. Roadway
elements such as chicanes, chokers, diagonal diverters, roundabouts, speed cushions, and on-street
parking have proven to be successful in speed reductions and reducing cut-through traffic in
neighborhoods.

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Regulations
Regulatory countermeasures, including turn restrictions and parking restrictions (either by time
of day / day of week or full time), may help to resolve certain types of crashes. These do require
enforcement to be effective.
Visibility
Traffic control devices were addressed earlier as countermeasures. However, enhancing their visibility
can also have an impact on safety. Pavement markings are often enhanced by the use of wider lines or
the placement of raised reflective pavement markers. Signs can be enhanced using flags, special border
area patterns, reflective posts, flashing beacons, and fluorescent colors. Traffic signals can be enhanced
using backplates with yellow reflective borders. These all help to improve the conspicuity of the
device…to help them stand out in a crowded environment.

Safe System Approach

Over twenty-five years ago, Sweden adopted the concept of Vision
Zero – the standpoint that no one should die or be seriously injured in
a traffic crash. For years, while the elimination of serious injuries and
deaths was a goal of the nation’s rail and air transportation systems,
there was a general acceptance of the same on the nation’s streets
and highways. Vision Zero was intended to bring the goals of highway
safety to be on par with railroads and airways.
The concept of Vision Zero has been adopted throughout the world, and a major element of implementing
Vision Zero is known as the Safe System approach.
Principles
There are six principles of the Safe System approach, as described by
the Federal Highway Administration:
• “Deaths and serious injuries are unacceptable – No one
should be killed or injured when using the road system. The
Safe System approach attempts to bring a public health-type
focus to road safety, with emphasis on minimizing the harm of
crashes. Successfully adopting the Safe System approach requires a safety culture that
unequivocally places safety first and foremost in our road system investment decisions.
• Humans make mistakes – People will inevitably make mistakes, and those mistakes can lead to
crashes. The Safe System approach expects the road system is planned, designed, and operated
to be forgiving of inevitable human errors, so that injury outcomes are unlikely to occur.
• Humans are vulnerable – People have a limited ability to tolerate crash impacts. Although the
exchange of kinetic energy in collisions among vehicles, objects, and people has multiple
determinants, applying the Safe System approach largely depends on managing the kinetic energy
of crashes to avoid injury outcomes.
• Responsibility is shared – Road users, vehicle manufacturers, road designers and operators, law
enforcement, and post-crash care providers all share responsibility to ensure that crashes do not
lead to fatal or serious injuries. As part of a shared responsibility for safety, road users are
expected to comply with traffic laws. Education, enforcement, and vehicle feedback components

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(e.g., speedometer and automated driving systems) are all critical in enabling and encouraging
safe road use.
• Safety is proactive – Tools can be used to identify and mitigate risks in the road system to
proactively prevent crashes, rather than react after crashes occur.
• Redundancy is crucial – With shared responsibility comes inter-relationships and opportunities
for synergy. Weaknesses in one area of the system may be compensated with enhancements in
other areas. For example, intersection geometry design could correlate to occupant-protection
features offered in current vehicle design. Timely and effective emergency response when crashes
do occur is also a critical element of a Safe System. Redundancy helps ensure that if one part of
the system fails, other parts still protect road users from death or serious harm.”
Elements
A commitment to reach zero traffic deaths also addresses all aspects
of safety via five key elements that, combined, create a holistic
approach with multiple layers of protection for road users:
• Safe Road Users – The Safe System approach addresses the
safety of all road users, including those who walk, bike, drive,
ride transit, and travel by other modes.
• Safe Vehicles – Vehicles are designed and regulated to minimize the frequency and severity of
collisions using safety measures that incorporate the latest technology.
• Safe Speeds – Humans are unlikely to survive high-speed crashes. Reducing speeds can
accommodate human-injury tolerances in three ways: reducing impact forces, providing
additional time for drivers to stop, and improving visibility.
• Safe Roads – Designing to accommodate human mistakes and injury tolerances can greatly reduce
the severity of crashes that do occur. Examples include physically separating people traveling at
different speeds, providing dedicated times for different users to move through a space, and
alerting users to hazards and other road users.
• Post-Crash Care – People who are injured in collisions rely on emergency first responders to
quickly locate and stabilize their injuries and transport them to medical facilities. Post-crash care
also includes forensic analysis at the crash site, traffic incident management, and other activities.
The Haddon Matrix has been developed to apply the principles of public health toward improving highway
safety. The matrix provides for the identification of factor contributing to injury or death before, during,
and after a crash. An example of the Haddon Matrix is shown below; for any pattern of crashes, the
identified factors may differ:
Human Vehicle / Equipment Physical Environment Socioeconomic
Poor vision or Failed brakes, missing Narrow shoulders, Cultural norms
Pre-Crash reaction time, alcohol, lights, lack of warning improperly timed permitting speeding,
speeding, risk taking systems signals red light running, DUI
Failure to use Malfunctioning safety Poorly designed Lack of vehicle design
During Crash occupant restraints belts, poorly guardrails regulations
engineered air bags
High susceptibility to Poorly designed fuel Poor emergency Lack of support for
Post-Crash injury, alcohol tanks communication EMS and trauma
systems systems

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Kinetic Energy Management

A key element of the Safe System approach is the management of
Kinetic Energy. Going back to Physics classes, Kinetic Energy is the
energy an object possesses due to its motion. It is defined as the work
needed to accelerate a body of a given mass from rest to a stated
velocity, or conversely, the work needed to decelerate a body of a
given mass from a velocity to rest. It is this latter definition that is of
concern in the Safe System approach.
The Kinetic Energy of an object of mass m traveling at a speed v is ½mv2. In our case, the object is a vehicle.
As the speed of a vehicle increases, the Kinetic Energy of the vehicle increases in proportion to the square
of the speed; a vehicle traveling at 40 mph (65 km/hr) has four times the Kinetic Energy compared to the
same vehicle traveling at 20 mph (35 km/hr).
In a collision, some or all the vehicle’s Kinetic Energy is expended and is a primary factor in the severity of
the crash. If all the Kinetic Energy is released immediately upon impact, the damage to the vehicle and
its occupants, or to the other road users, is at its greatest. However, if the release of Kinetic Energy can
be designed to occur over an extended time, the damaging effects are reduced. An example of this is a
vehicle striking a concrete bridge abutment, incurring the greatest damage, as opposed to striking an
impact attenuator, which is designed to extend the release of the Kinetic Energy, or striking a glancing
blow to a guardrail and being redirected without a substantial change in velocity.
When one vehicle strikes another vehicle, the angle of the collision is also a factor. Two vehicles with the
same Kinetic Energy, traveling in opposing directions and colliding head on, will expend the combined
Kinetic Energy of the two vehicles – resulting in the greatest level of damage.
To minimize the effects of a crash, the speeds should be kept at an
appropriate level for the potential type of collision. Johansson1
proposed the following boundary values for safe speeds:
• Vulnerable road users (pedestrians and bicyclists) should not
be exposed to motor vehicles with speeds exceeding 20 mph
(30 km/hr).
• For 90-degree crossings, car occupants should not be exposed to other motorized vehicles at
speeds exceeding 30 mph (50 km/hr).
• For oncoming traffic, car occupants should not be exposed to other motorized vehicles at speeds
exceeding 45 mph (70 km/hr) when the opposing motorized vehicles are of the same size, or 30
mph (50 km/hr) when the opposing motorized vehicles are of considerably different weight.
• Car occupants should not be exposed to the roadside at speeds exceeding 45 mph (70 km/hr).
This maximum speed is 30 mph (50 km/hr) if the roadside contains trees or other fixed objects.
There are numerous approaches to achieving these boundary values, but they center around three key
factors:

1
Johansson, R. (2009). Vision Zero – Implementing a Policy for Traffic Safety. Safety Science, 47(6), 826-831.

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• Speed Reduction – the Kinetic Energy of vehicular traffic can be reduced by reducing their speeds.
Techniques to reduce speeds include traffic calming features, roundabouts, appropriate speed
limits, and visible enforcement.
• Reducing Conflict Angles – as the angle of the collision influences the release of Kinetic Energy, a
roadway design that minimizes the possibility of the maximum release of Kinetic Energy is
desirable. For example, a merging maneuver is better than a crossing maneuver; the possibility
of a head-on crash should be avoided.
• Reducing Conflict Points – if conflicts between vehicles, or between vehicles and other road users,
cannot be eliminated, efforts should be made to reduce them. A good example occurs in
intersection design and the use of displaced left turns, roundabouts, and similar concepts that
reduce the number of conflicts.

Road Safety Management

The principles of road safety management under the Safe Systems concept are not unlike those previously
described in terms of identifying high crash locations, developing countermeasures, prioritizing projects,
and implementing improvements. However, the Safe Systems concept also focuses on the initial design
of a roadway facility with a goal of enhanced substantive safety instead of just the nominal safety achieved
by meeting design standards.

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REFERENCES
Questions for the certification examination are derived and/or documented from a number of
professional sources. Some of the most frequently cited references are:
Title: Highway Capacity Manual, 7th Edition: A Guide for Multimodal Mobility Analysis
Author(s): Transportation Research Board Inc.
Publisher: TRB, ISBN: 978-0-309-08766-7
ITE Publication Number: LP-674C
Publication Date: 2022

Title: Manual of Transportation Engineering Studies, 2nd Edition

Author(s): Edited by H. Douglas Robertson
Publisher: ITE, ISBN: 978-1-933452-53-1
ITE Publication Number: TB-012A
Publication Date: 2010

Title: Manual on Uniform Traffic Control Devices, 2009 Edition

Author(s): FHWA/ITE/ATSSA/AASHTO
Publisher: FHWA/ITE/ATSSA/AASHTO, ISBN: 978-1-56051-473-2
ITE Publication Number: MUTCD-10
Publication Date: 2009

Title: A Policy on Geometric Design of Highways and Streets, 7th Edition

Author(s): AASHTO
Publisher: AASHTO, ISBN: 978-1-56051-676-7
Publication Date: 2018

Title: Traffic Engineering Handbook, 7th Edition

Author(s): ITE, Brian Wolshon and Anurag Pande
Publisher: Wiley, ISBN: 978-1-118-76230-1
ITE Publication Number: LP-691
Publication Date: 2016

Title: Traffic Safety Toolbox: A Primer on Traffic Safety

Author(s): ITE
Publisher: ITE, ISBN: 0-935403-43-4
ITE Publication Number: LP-279A
Publication Date: 1999

Title: Transportation Planning Handbook, 4th Edition

Author(s): Edited by Michael D. Meyer
Publisher: ITE, ISBN: 978-1-118-76235-6
ITE Publication Number: LP-695
Publication Date: 2016

Title: Highway Safety Manual

Author(s): AASHTO
Publisher: AASHTO, ISBN: 978-1-56051-477-0
ITE Publication Number: LP-672
Publication Date: 2010

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Title: Signal Timing Manual - NCHRP Report 812, Second Edition

Author(s): Tom Urbanik, Alison Tanaka, et al.
Publisher: TRB, National Cooperative Highway Research Program
Publication Date: 2015

Website References
Connected Vehicles, https://www.pcb.its.dot.gov/eprimer/module13.aspx

Freight and Commercial Vehicle ITS, https://www.pcb.its.dot.gov/eprimer/module6.aspx#is

USDOT, ATDM Program Brief: An Introduction to Active Transportation and Demand Management.
http://www.ops.fhwa.dot.gov/publications/fhwahop12032/fhwahop12032.pdf

In addition to these professional references, a candidate may find it advantageous to review a general
traffic or transportation engineering text. Among the excellent texts currently available, the following
was frequently cited in question documentation:

Title: Fundamentals of Traffic Engineering, 16th Edition (Currently not Available)

Author(s): Homburger, W., et al.
Publisher: University of California
Publication Date: 2007
Additional references related to this module include:

Roadside Design Guide, American Association of State Highway and Transportation Officials, Washington,
DC, 2011.
Crash Modification Factor Clearinghouse, Federal Highway Administration, www.cmfclearing-house.org
Zero Deaths – Safe System Approach, Federal Highway Administration
https://safety.fhwa.dot.gov/zerodeaths/zero_deaths_vision.cfm

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