CHAPTER 7

TRAFFIC ENGINEERING

Subject:
CE 353 – HIGHWAY AND RAILROAD ENGINEERING
 CHAPTER 7
TRAFFIC ENGINEERING:

o TRAFFIC CHARACTERISTICS-TRAFFIC STUDIES SPEED, VOLUME,

SPEED AND DELAY, ORIGIN-DESTINATION, PARKING AND ACCIDENT

o CAPACITY OF URBAN ROADS AND HIGHWAYS

o TRAFFIC OPERATIONS REGULATION AND CONTROL

o DESIGN OF INTERSECTIONS-AT GRADE AND GRADE SEPARATED

PREPARED BY:
(BSCE-3A)
ARANDA, NEILS JERMAINE
ANTOYA ANGELICA JONES
BALDERIAN, MTCHELLE REY
DISPO, JOHN MARLON

Prof. JOHN PAUL VILLANUEVA, RCE, Meng

INSTRUCTOR
 TRAFFIC ENGINEERING

Description:

This module serves as an Introduction to Traffic Engineering, providing a

solid base of understanding for students. The chapter covers the definition of Traffic
Engineering and the scope of it, expounding further on each topic.

Learning Objectives:

At the end of this Module, the student is expected to learn the basics of Traffic
Engineering, including the planning of roadways and traffic flow for existing roads.
The student is also expected to familiarize with the different designs of intersections
that minimizes delays in driving to ensure smooth flow of traffic.

In this module, the student shall become familiar with how Traffic Management is a
vital part of urban growth and develop appreciation for the amount of research is
done to provide comfort for the people who benefit from engineering marvels and
designs.

Pre-Activity:

1. Define Traffic Engineering in your own words.

______________________________________________________________
2. Present
What is Traffic Engineering?
Traffic Engineering is a branch of Civil Engineering that deals with “the improvement
of traffic performance of road terminals and networks” using systematic traffic
studies, scientific analysis and engineering applications. This generally includes
planning and designing of new roads as well as redesigning old roads. This also
includes the regulation and control of existing roads. Traffic engineers must always
consider the short-term impact they might do when implementing their proposed
changes (redirecting traffic, proposing alternative routes during construction) as well
as the long-term impact after the construction (impact of improved roads and traffic
routes on nearby commercial and residential areas).

The objective of traffic engineers is to achieve a free and rapid flow of traffic while
minimizing traffic accidents and congestions. Studies in traffic operations provide the
foundation for developing methods and solutions to deal with improvements and
problems. Typical traffic engineering ventures include structuring traffic regulating
devices (i.e. traffic lights, signs, asphalt markings).

Scope of Traffic Engineering

1. Traffic Characteristics
2. Traffic Studies and Analysis
3. Traffic Operations
4. Design of Intersections and Interchanges

TRAFFIC CHARACTERISTICS

Traffic Characteristics are factors that interact with each other in a Traffic
system. These characteristics help engineers fully visualize their plans and
designs for roads and highways by focusing on targeting these 5 factors:

1. Road Users
 Road Users are the human element of the traffic system and are given the
highest priority since they are the ones to dictate the flow of traffic the
most. Road users are not limited to drivers only, they also include
passengers of public transportation, bicyclists, and pedestrians.

2. Vehicles
Roads and highways are designed to cater to most commercially available
transportation (both public and private). Regular maintenance of vehicles
is a must to reduce potential accidents during travels. City Planners and
Traffic Engineers can also restrict certain vehicle types from passing
through designated roads as a way to control traffic volume.

3. Streets and Highways

These are the paths designed for vehicles to traverse on. Engineers and
City planners are responsible for the designing and structuring of these
roads that shape the surrounding area and help it grow.

4. Traffic Control Devices

These devices help manage traffic on roads and intersections. These
include road signs, traffic lights, speed bumps, etc. These devices help
communicate with the Road users to ensure safer travels and keep them
alert.

5. The General Environment

The environment the road is built on can affect the quality of the commute
of the road users. Sometimes, the environment becomes unpredictable
and cause damages to the road. Hence, the only thing engineers can do is
to mitigate potential damages through planning and renovation.

Traffic Studies and Analysis

1. Traffic Speed studies
Speed is considered as a quality measure of travel as drivers and passengers
are more concerned about the speed of their travel rather than the design of
the road, hence traffic engineers use speed as a metric to determine the
quality of the road. Of course, the speed of different vehicles will vary so traffic
engineers classify these types into different variations.

a. Spot Speed
Spot speed is the instantaneous speed of a vehicle at a specified location.
Commonly, it is measured using a speedometer, time-lapse photographic
methods or hi-speed cameras.
The spot speed data is used to design the geometry of the road, location
and size of road signs, determining speed zones, accidental analysis, and
road maintenance.

b. Running Speed
Running Speed is the average speed maintained over a particular course
or route. The speed does not consider stops made or delays during travel.
It is calculated by dividing the length of the route with the time of vehicle in
motion.

c. Journey Speed
Journey Speed is the effective speed of the vehicle during travel. Unlike
running speed, journey speed data takes stops and delays into
consideration. The journey speed data is used to compare to the running
speed data of the same route to determine if the vehicle’s journey is
comfortable and smooth. If the journey speed is less than the running
speed, the vehicle may have run into enforced or unenforced traffic.

d. Time mean speed and space mean speed

Time mean speed is the average speed of all vehicles passing a point on a
highway over a period of time. Space mean speed is the average speed of
all vehicles passing through a specified section on the highway over a
period of time.

i. Calculating Time mean speed

Time mean speed (vt) is given by,
 n
1
v t= ∑ v i
n t =1

where vi is the spot speed of the ith vehicle and n is the number of
observations.

In most speed studies, speed is represented in the form of a frequency

table, so we use the following,

∑ qi vi
v t= t=1n
∑ vi
i=1

where qi is the number of vehicles having speed v i, and n is the number

of such speed categories.

ii. Calculating Space mean speed

Space mean speed (vs) also uses the average spot speed but
considers length of the road section rather than time. Consider the unit
length of a road, let vi be the spot speed of the ith vehicle and ti be the
time it takes for the ith vehicle to complete the unit distance and is
1
expressed by .
vi
If there are n such vehicles, then the average travel time ts is given by,

∑ ti= 1
t s=
n n
∑ v1
i

Since ts is average travel time, the average speed vs = 1/ts.

Hence,
 n
v s= n

∑ v1
i=1 i

If expressed in a frequency table, the equation becomes,

n

∑ qi
v s= i=1
n
q
∑ vi
i=1 i

2. Traffic Volume Studies

Systematic traffic surveys, traffic studies, and their scientific analysis are
essential in traffic engineering and find wide application in planning for future
needs of roads, improvement of existing facilities, geometric design aspects,
pavement design, and traffic regulation and control.
Traffic surveys include types of traffic, size and weight, traffic flow, traffic
volume per hour and per day, including seasonal changes and distribution in
different parts of the road network.

The purpose of conducting traffic volume studies is to determine which roads

are in need of development, plan routes and/or traffic management devices
along the routes, make analysis of traffic patterns for future plans and
designs, and design the most optimal type of road based on said analysis.
When conducting traffic volume studies, there are several methods that can
be used.

a. Manual Method
In this method, a team of observers can record a traffic volume study in
the assigned area. They can collect not only traffic volume data, but also
the types of vehicles on the road, and direction of turns in an intersection.
Having a team can also increase redundancy in the data gathered which
helps reduce possible errors.
 However, because this method requires human action, it would not be
possible to conduct a 24-hour study on an assigned area every day,
unless you increase the manpower which can
be quite expensive.

b. Video Photographic Method

Quite simply, traffic engineers can utilize cameras that can record on a
specific area for 24 hours at a more reasonable cost compared to the
manual method. As technology progresses, the amount of data that can be
collected using state-of-the-art traffic cameras would be substantial,
especially with the use of AI technology and HD cameras.

c. Automatic counters
Automatic counters allow less use of observers and can continuously
record data in a set time or period. However, there is limited data that can
be gathered by these counters.

i. Pneumatic Counters
This counter is activated by an air switch attached to a flexible
hose set across the road over which vehicles pass.
ii. Electrical Counters
The Counter is activated by the closing of an electrical circuit
every time a vehicle passes over it.
iii. Photo-Electric counter
The counter is activated by the interruption of light on the photo-
electric cell placed on the side of the road as a vehicle drives
past it.

3. Speed and Delay studies

Speed and Delay studies give the running speeds, overall speeds,
fluctuations in speeds and the delay between two stations on the road. The
study provides information on the number of vehicles, location, duration, and
 cause of delay in the traffic stream which is useful in spotting congested roads
along a specified route.

Delays in travel along a route can be classified into two types:

a. Fixed delay – primarily occurs at intersections due to traffic signals
and at level crossings.
b. Operational delay – are caused by interference on the road such as
turning vehicles, parking/parked vehicles, pedestrians, lane
switching.

Floating car/ Riding check method

The floating car method is where a test vehicle is driven over a given course
of a travel at approximately the average speed of the “stream”, thus trying to
“float” with the traffic “stream”. 4 observers ride in the floating car while the
other observers are placed along the stretch of the route record other various
details. The first observer carries two stopwatches, one to record the time of
the individual delays and the other to record the time it took to reach various
control points such as intersections, bridges, and any other fixed points on the
journey. A second observer records the time, location, and cause of each
encountered delay. A third observer notes the number of vehicles overtaking
the floating vehicle. The fourth observer notes the number of vehicles
travelling in the opposite direction of the floating vehicle.

4. Origin-destination
An origin and destination (OD) study is a survey that determines where
people start and end their trips in a specific area over a set period of time. The
data collected from these studies helps transportation planners and agencies
design new roads or improve existing ones.
The data gathered can also be a good indicator in the satisfaction of the
drivers and commuters who use the roads

5. Parking and Accident Studies

 Parking is a reoccurring issue that worsens as the city or town grows. Without
available parking spaces especially in high- density areas in the urban
environment, it can become a major cause of traffic or worse, accidents.

As such, it is crucial for traffic engineers and analysts to determine or design

safe parking areas can keep up with the growth of the urban environment.

There are 2 types of parking facilities

 On-street parking - vehicles are parked on the side of the road itself,
usually controlled by traffic enforcers as to what vehicles may park in
that area.
 Off-street parking - areas or buildings some distance away from the
road exclusively used for parking. They may be operated by a private
entity and some may charge for parking.

CAPACITY OF URBAN ROADS AND HIGHWAYS

Highway capacity is associated with traffic volume and traffic density

wherein Capacity is the maximum traffic flow that can be accommodated in a
highway facility during a given period under prevailing roadway, traffic and
control conditions. It is also the ability of a road to accommodate traffic
volume and is the maximum hourly rate at which vehicles can reasonably be
expected to cross a point on a roadway during a given time period under said
conditions.

Highway capacity is important for city planners and engineers to

determine the current traffic conditions and is useful for planning
improvements to road design, traffic management control, and road safety
measures. The capacity of a highway should be enough to serve the needs of
the drivers, which is why studies in highway capacity are important in
designing roads and highways These studies allow engineers to improve and
 redesign roads deemed outdated or are considered overloaded by vehicles
which may cause heavy traffic in the area.

When conducting capacity studies, there are 3 types of capacity that we can
measure:

a) Basic Capacity – is the maximum number of vehicles that can pass a

given point on a lane or roadway during one (1) hour, under the ideal
roadway and traffic conditions that can possibly be attained.
b) Possible Capacity – is the maximum number of vehicles that can pass a
given point on a lane or roadway during one (1) hour, under the prevailing
roadway and traffic conditions.
c) Practical / Design Capacity – Capacity without the traffic density being so
great as to cause unreasonable delays, hazards or restrictions to the
driver’s freedom under the prevailing condition of road way, traffic and
control.

Level of Service (LOS)

Level of Service is defined as a qualitative measure describing the operational

condition within a traffic stream, and their perception by motorist and
passengers. There are 7 levels that describe the current condition of the
highway.

Level A: Free flowing traffic with low traffic density. Drivers are not restricted
on maneuverability due to presence of other vehicles.

Level B: Stable flow with operating speeds beginning to be restricted by traffic

conditions and allows reasonable freedom of movement for speed and
lane.

Level C: Still allows stable flow of traffic but speeds are more controlled due to
increased volume, with speeds still relatively fast.

Level D: Flow of traffic start to become unstable with speeds affected by

fluctuations in volume and restrictions.
 Level E: Unstable traffic flow where it is only measured in volume and
highway capacity with stoppages along the route.

Level F: Drivers are forced to operate on low speeds due to a full highway
capacity, sometimes even resulting in full stops for a long duration resulting in
a continuous congestion along the highway.

Factors that can affect Capacity and Level of Service (LOS)

 Roadway conditions – Restrictive physical and geometric elements

such as lane-width, lateral clearance, and horizontal alignment of road
shoulders, gradients and pavement surface condition are some factors
which affect capacity and level of service. Even road intersections
affect Capacity.
 Traffic conditions – The capacity and level of service are affected by
the composition of different types of vehicles in the stream, variation of
traffic flow, traffic interruption, and lane distribution. The number of
traffic lanes, vehicular and driver characteristics, and one or two-way
traffic movements have a direct bearing on the lane or highway
capacity.
 Control conditions – Refer primarily to interrupted flow facilities such as
traffic signs and signals which have significant impact on capacity.

TRAFFIC OPERATIONS REGULATIONS AND CONTROL

Traffic Operations involve planning for and controlling the movements of

vehicles and groups of vehicles on streets and highways for the purpose of
attaining maximum efficiency and safety.

Several agencies in the Philippines are responsible for maintaining traffic

operations and regulation as well as enforce the laws that cover traffic
operations:
 Traffic Operation Management Enforcement and Control Office
(TOMECO)

TOMECO personnel primarily manage the regulation of traffic especially

during rush hours and special events. TOMECO is also in charge of enforcing
traffic laws and are granted the authority to withhold driver’s licenses should
any violation of any traffic law be found.

Land Transportation Office (LTO)

The LTO is responsible for issuing driver’s licenses and permits to those who
have been deemed responsible enough to carry said documents. They are
also responsible for maintaining the overall road worthiness of vehicles. LTO
is tasked with maintaining a standard for both vehicle and driver, educating
road users about current traffic laws and traffic etiquette.

Philippine National Police – Highway Patrol Group (PNP – HPG)

The HPG, while serving directly under the PNP, are also tasked with enforcing
traffic laws. Unlike TOMECO though, they have the authority to detain and
arrest violators if caught. In some cases, they can serve as traffic regulators
but are generally enforcers.

Department of Public Works and Highways (DPWH)

When it comes to Traffic Management, DPWH can be considered to hold the

highest authority as they are responsible with the planning, implementation,
regulation and maintenance of infrastructure projects such as roads, railroads,
and bridges.

Department of Transportation and Communications (DOTC)

A government agency primarily responsible for the development of

transportation and communications system.

Land Transportation Franchising and Regulatory Board (LTFRB)

 LTFRB’s jurisdiction covers vehicles used for public conveyance, regulating &
adjusting fares for public transport services, prescribing the routes/areas of
operation for these services.

Traffic Regulation

Traffic regulations are driving laws put in place by the government to regulate or
control the attitude of all road users to achieve an orderly and free flow of traffic.
It is a body of rules and regulations made by the government to guide and keep
safe the road users. It is very important that government put in place these laws
with punitive measures for road users.

There are 6 traffic laws in the Philippines that is being enforced with the help of
the relevant agencies:

1. Land Transportation and Traffic Code (RA 4136)

The most important driving law ever enacted. It lays the groundwork for all
Philippine traffic laws and regulations. It also created the Land Transportation
Commission in the Philippines, currently known as the Land Transportation
Office (LTO).

RA 4136 mandates all motor vehicles to be registered in a national registry. It

also sets the registration requirements and fees for every car owner. Aside
from that, this law also requires drivers to apply for a professional or non-
professional license and sets the penalty fees for failing to comply with the
LTO's registration and licensing requirements.

In terms of traffic regulations, RA 4136 sets the legal speed limits for every
driver to follow.
RA 4136 Violations and Penalties - The penalties range from ₱100 to ₱500 for
RA 4136 violations, which include the following:
● Overtaking and passing a vehicle
● Right of way and signaling
● Turning at intersections
● Reckless driving
 ● Right of way for police and other emergency vehicles
● Tampering with vehicles
● Hitching to a vehicle
● Driving or parking on the sidewalk
● Obstruction of traffic

2. Anti-Distracted Driving Act of 2016 (RA 10913)

Republic Act 10913 or the Anti-Distracted Driving Act of 2016 prohibits drivers
from using electronic and communication devices while their car is in motion
or at a standstill at a red light.
This law is one of the most expensive traffic laws in the Philippines because
of its hefty penalty fees. Here are the fines for private vehicle violators:
a. First offense: ₱5,000
b. Second offense: ₱10,000
c. Third offense: ₱15,000 and a three-month suspension of driver's
license
d. Fourth offense: ₱20,000 and revocation of license

3. Anti-Drunk and Drugged Driving Act of 2013 (RA 10586)

One of the most serious driving laws in the Philippines. Aside from the penalty
fees, driving under the influence could lead to injury and even death of all
parties involved—the driver, passengers, and pedestrians.

Suspected drivers will undergo a series of tests to determine if they're driving

under the influence of alcohol, drugs, and other similar substances. These
tests include breath analyzers, field sobriety tests, and chemical tests.

In case you’re found guilty, you'll end up paying the following penalty fees:
A. If the violation didn't result in physical injuries or homicide: Between
₱50,000 to ₱100,000 and a six - month prison sentence
B. If the violation resulted in physical injuries: Between ₱150,000 to
₱250,000 and imprisonment
 C. If the violation resulted in homicide: Between ₱350,000 to ₱500,000
and imprisonment

4. Seat Belts Use Act of 1999 (RA 8750)

RA 8750 makes the use of seat belts mandatory for drivers, front seat, and
back seat passengers of public and private vehicles. This law prohibits
infants, toddlers, and children under six years old from sitting in front.
Seat Belts Use Act of 1999 Violations and Penalties
Follow this traffic law to avoid the following penalty fees:
A. First offense: ₱100 to ₱1,000
B. Second offense: ₱200 to ₱2,000
C. Third and succeeding offenses: ₱500 to ₱5,000 and one-week
suspension of license

5. Children’s Safety on Motorcycles Act of 2015 (RA 10666)

The law prohibits children from boarding two-wheeled vehicles running faster
than 60 kph on public roads.

RA 10666 Violations and Penalties Endangering your children could end up

costing you the following:
A. First offense: ₱3,000
B. Second offense: ₱5,000
C. Third offense: ₱10,000
6. Motorcycle Helmet Act of 2009 (RA 10586)
The Motorcycle Helmet Act of 2009[7] or RA 10054 requires motorbike drivers
and riders to wear DTI-prescribed standard protective motorcycle helmets.
According to the No Helmet Policy, a helmet should bear the Philippine
Standard (PS) mark or Import Commodity Clearance (ICC) of the Bureau of
Product Standards (BPS).

Motorcycle Helmet Act of 2009 Violations and Penalties

Breaking one of the most violated traffic laws in the Philippines will
have you paying the following penalty fees:
A. First offense: ₱1,500
 B. Second offense: ₱3,000
C. Third offense: ₱5,000
D. Fourth and succeeding offenses: ₱10,000 and confiscation of
license

Traffic Control

Traffic control is the supervision of the movement of people, goods, or

vehicles to ensure efficiency and safety. One of the principal challenges in
traffic control is to accommodate the traffic in a safe and efficient way.
Efficiency can be thought of as a measure of movement levels relative to the
objectives for a particular transportation system and the finances required for
its operation.

Traffic control is a critical element in the safe and efficient operation of any
transportation system. Elaborate operational procedures, rules and laws, and
physical devices (e.g., signs, markings, and lights) are but a few of the
components of any traffic control system.

Traffic Control Devices

 Signs - Regulatory sign, warning sign, guiding or informational sign,

traffic signals
 Road Markings and road construction
 Barriers and channelizers
 Traffic control devices for private facilities
 Humps, Bumps, and stops

DESIGN OF INTERSECTION

An intersection is defined as the general area where two or more highways

join or cross. This area is designated for the vehicles to turn to different
directions to reach their desired destinations. This is because vehicles moving
in different direction want to occupy same space at the same time.
There are different forms of commonly designed intersections for large towns,
small cities or rural areas:

These intersections can cater to relatively small urban centers serve different
functions depending on the city planners and engineers.

Elements of an Intersection
Each roadway extending from the intersection is referred to as a leg. The
intersection of two roadways has usually four legs (or three if there one of the
roadways is ended). The leg used by traffic approaching the intersection is the
approach leg, and that used by traffic leaving is the departure leg. The major
street is typically the intersecting street with greater traffic volume, larger
cross section, and higher functional class.

The minor street is the intersecting street likely to have less traffic volume,
smaller cross-section and lower functional classification than the major street.
Channelization is the separation or regulation of conflicting traffic movements
into definite paths of travel by traffic islands or pavement markings (regulation
of traffic).

When designing intersections there are a few principles that should be

followed to ensure a good and effective design:

• The number of intersections should be kept minimum.

• The geometric layout should be so selected that hazardous movement

by drivers are eliminated.

• The design should permit the driver to find distinguish quickly either
from the lay out or from traffic signs the path he should follow and the
actions of merging and diverging.

• The layout should follow the natural vehicle path. It must be smooth
avoid abrupt and sharp corners.

• The number of conflicts point should be minimized by separating some

of the many cuttings, merging or diverging movement.

• Vehicles that are forced to wait to cross a traffic should be provided

with adequate space at the junction.

Types of Intersections

1. Grade – Separated intersections or interchanges

 Grade – Separated intersections are where roads cross at different levels.
It is a bridge that eliminates crossing conflicts at intersections by vertical
separation of roadways in space. Route transfer at grade separations is
accommodated by interchange facilities consisting of ramps. The
interchange configurations are designed in such a way to accommodate
economically the traffic requirements of flow, operation on the crossing
facilities, physical requirements of the topography, adjoining land use, type
of controls, right-of-way and direction of movements.

Grade-Separated intersections can be classified as:

a. Underpass
b. Overpass
There are also 3 types of Grade-Separated intersections:

a. Direct
b. Semi-Direct
c. Indirect

When it comes to interchanges, which are essentially intersections that

occupy a considerable space designed to minimize delays on the road due
to turning of vehicles, they can be classified into 4:

a. Trumpet Interchange
b. Diamond Interchange
c. Full cloverleaf Interchange
d. Partial cloverleaf Interchange

Advantages of Grade-Separated Intersection

Grade-Separated intersections offer maximum facility given to the crossing

traffic. There is increased safety for turning traffic and by indirect
interchange ramp right which makes turn movement safe. Overall, the
increase in comfort and convenience to motorists and save travel time and
 vehicle operation cost. Capacity of grade operated intersection can
practically approach that of two crossroads.

Grade separation is an essential part of a controlled access to the highway

like expressway and freeways. It is possible to adopt grade separation for
all likely angles and layout of intersecting roads. Stage construction of
additional ramps are possible after the structure of the grade separation
main roads are constructed.

Disadvantages of Grade-Separated Intersection

It is costly to provide complete grade separation and interchange facilities.

Where there is limited right of way like build up or urban areas or where
topography is not favorable, construction of grade separation is costly,
difficult and often undesirable. In a flat or plain terrain, grade separation
may introduce undesirable crests and sags in the vertical alignment.

2. At Grade Intersection

At Grade intersections are much simpler intersections as compared to

Grade-Separated in terms of complexity of design and cost of both
construction and maintenance since the exchanges on this intersection lie
on the same plane. This intersection also requires less space to operate
and can be integrated in the road systems of highly urbanized cities.

When it comes to designing At Grade intersections, there are a few basic

requirements that must be met:
1. Area of conflict should be as small as possible at the intersection.
2. Relative speed and angle of approach of vehicle should be small.
3. Adequate visibility should be available for vehicles approaching the
intersection.
4. Sudden change of path should be avoided.
5. Proper sign should be installed.
 6. Good lighting at night is desired.

It is also important to note where the intersection will be placed and the
area that will use the intersection. If the intersection is to be placed in a
rural area, it must have a design speed of about 40 kph whereas in urban
areas it must be 30 kph.

There are two types of At Grade Intersections:

1. Standard at-grade intersection

The most basic type of intersection. Most common example of this type
of intersection are cross intersections and occupy the least amount of
space to construct. However, without traffic signs and signals, it would
be hard to prevent speeding towards the crossing or irresponsible
turning.

This intersection can also be divided into two types:

a. Channelized Intersections – where the roads are divided by
islands to prevent aggressive lane switching. There is also an
island in the middle of the intersection that helps facilitate the
flow of exchanges.
b. Unchannelized Intersections – are similar to channelized
intersections but without the island to control the flow of traffic
and exchanges.

2. Round-a-bout at-grade intersection

Roundabouts are circular intersections where traffic flows around a
central island in a counterclockwise direction. They are designed to
control traffic without the use of traffic signals, allowing a smooth flow
while slowing down drivers that yield to traffic already in the
roundabout.

They can also be called Rotary Intersections, where a large central

island facilitates the flow of traffic and exchange, with islets before the
 intersection that can control the speed of vehicles entering the
intersection.

Choice of Intersection

The choice between an at grade and grade separated junctions at a particular

site depends upon various factors such as traffic, economy, safety, aesthetic,
delay etc.

Grade separated junctions are generally more expensive initially and are
justified in certain situations on high type facilities such as expressways,
freeways and motor ways. Certain at grade intersection which have reached
the maximum capacity. At certain locations which have a proven record of bad
accidents history when functioning as at grade junction. At junctions where the
traffic volume is heavy, and delays and loss caused justify economically. At
certain specific topographical situations where it is logical to provide grade
separated rather than at grade.

References

1. Mannering Fred, Washburn Scott, Kilaresky Walter. 200. Principles of

Highway Engineering & Traffic Analysis. Muze Inc.
2. Wright, Paul H. 2003. Highway Engineering. Wiley & Sons.
3. Garber, Nicholas; & Hoel Lester. 2001. Highway and Traffic Engineering.
Brookes/Cole Publshing.
4. Fajardo, Max Jr. B., Elements of Roads and Highways, Second Edition, 5138
Merchandising Publisher, Manila, 1998
5. Department of Public Works and Highways. 1995. Standard Specification for
Public Works and Highways (Volume II – Standard Specification for
Highways, Bridges and Airports), DPWH, Office of the Secretary, Bonifacio
Drive, Port Area, Manila
6. Hay, W. H. 1982. Railroad Engineering. 2nd Edition. Wiley
7. Armstrong J. H. 2008. The Railroad: What It Is, What It Does, 5 th Edition.