GEOMETRIC DESIGN
Geometric Design is the stage of road design process where the dimension and layout of road
elements are proportioned to meet the needs of road users.
TOPIC OUTLINE
• Basic Highway Design Data
• Sight Distance
• Horizontal Alignment
• Vertical Alignment
• Cross-Sectional Elements
• Intersection Design
• Interchanges
HIGHWAY CLASSIFICATION ACCORDING TO SYSTEM
CLASIFICATION MINIMUM DESIGN STATNDARDS
ROAD RIGHT OF WAY CARRIAGEWAY WIDTH ALLOWABLE GRADE
National Roads 20.00m 6.70m 6.0%
Provincial Roads 15.00m 6.10m 6.0%
City Roads 15.00m 6.10m 6.0%
Municipal Roads 15.00m 6.10m 6.0%
Barangay Roads 10.00m 4.00m 10%
Tourism Roads 15.00m 6.10m 6.0%
FMR 10.00m 4.00m 10%
MODE OF ACQUISITION
The government may acquire real property needed as right-of-way, site or location for any national
government infrastructure project through:
1. DONATION
2. NEGOTIATED SALE
3. EXPROPRIATION
Sufficient ROW should be acquired in order to avoid the expenses of purchasing developed properties.
FIELD INVESTIGATION
Proposed Sites for Stream Crossings
• Important for hydrologic and hydraulic considerations
Road Alignment
• Can produce a major impact on the environment, the fabric of community, and highway users.
Existing Utility Services
• Records obtained from utility service providers should be verified in the field.
SOIL INVESTIGATION
Obtaining design data and analyze in detail the soil problems in order to decide the most suitable
investigations, method and equipment to be used.
• Subsurface Investigation
• Subgrade Investigation (CBR value)
• Widening of Existing Pavements
• Sampling and Testing
EXISTING PAVEMENT EVALUATION
Whilst test pits and borings can give all the subgrade data, only all the pavement inspection combined
history of the pavement can guide the Pavement Engineer in his evaluation on the remaining life of the
pavement and the original quality of its construction.
• Spalling
• Scaling
• Polishing
• Transverse Crack
• Faulting
• Pothole
DESIGN CONTROLS
Topography, land use, traffic and vehicle data forms the major controls for highway design as these have
pronounced effect on highway location, geometrics and determination of the type highway.
• Anticipated Traffic Volume
• Character of Traffic
• Design Speed
• Design Traffic (Vehicles)
• Highway Capacity
• Classification of Highway
• Accident Information
ENVIRONMENT
- minimum effect to the environment
SAFETY
- provide with necessary roadside treatment, and road safety control devices.
CONSTRUCTION METHODOLOGY
- simple as possible from the standpoint of the builder
MAINTENANCE
- least/reasonable cost
MOTORISTS’ CONVENIENCE
- Suitable to traffic volume; safe for driving and ensure confidence for motorists.
MINIMUM HAZARD
- Consistent and mist avoid surprise changes in alignment, grade line, and sight distance.
AESTHETICS
- Pleasing to the user and to those who live along it.
HIGHWAY TYPES
2-lane Highways
4-Lane Undivided Highways
*Speed limit should be limited to 60kph or less, and they should
feature prominent road markings
Divided Highways
*Medians 1.20m to 1.80m
*If feasible: 4.50 to 18.50m (to obtain full advantage)
WHY TWO-WAY TWO-LANE ROAD ARE MORE DANGEROUS??
42.8% of the fatal collisions reporter on highway occurred on two-lane rural highways.
• Passing Maneuver
• Light from Opposing Direction
• Poorly Designed Shoulders
DESIGN VOLUME FOR ROAD WIDENING
Road widening is a network developed project which aims to increase the capacity of the existing
highway/road or to improve the safety aspects of the road. Widening shall be primarily considered along road
sections with Volume Capacity Ratio (VCR) of more than or equal to 0.60 as prescribed in the Department’s
Highway Planning Manual. The Improvement of the road shall only be considered “road widening” if the
increase in the carriageway width is equal or more than the minimum lane width of 3.05m, design as part of
carriageway width, with the same thickness as the existing carriageway width and should be provided with
shoulder or sidewalk.
Capacity estimations have several purposes such as input to economic evaluation and forecast of VCR
levels. A VCR of around 0.60 is considered the trigger for alerting the planners to think heavily about remedial
measures (traffic management, road widening or bypass/diversion/flyover/ring road construction) to be
implemented over the next five years to relieve congestion.
HIGHWAY CAPACITY
NUMBER OF LANES (N)
Level of Service Volume – Capacity Ratio Drescription
A Less than 0.20 Free flow traffic
B 0.21 - 0.50 Free flow traffic
C 0.51 – 0.70 Moderate traffic
D 0.71 – 0.85 Moderate to heavy traffic,
approaches unstable flow
E 0.86 – 1.00 Heavy traffic, flow near capacity
F Greater than 1.0 Forced flow, stop and go
NOTE: Capacity gives a quantitative measure of traffic, level of service or LOS tries to give a qualitative
measure
LEVEL OF SERVICE
It is qualitative measurement that describes the traffic conditions in terms of speed, travel time,
freedom, maneuver, comfort, convenience, traffic interruptions and safety.
DESIGN HOURLY VOLUME
DDHV = AADT*K*D(for multilane highways)
DHV = AADT*K (for 2-lane or 3-lane, 2-way highways)
• DDHV – directional design hour volume
• DHV – design hourly volume
• AADT – Average annual daily traffic
• K – proportion of daily traffic occurring during peak hour, expressed as a decimal
• D – proportion of peak-hour traffic travelling in the peak direction, expressed as a decimal
INTERSECTION DESIGN
Intersections play an important role in any network system. They are the points where traffic flow
converges and where direction of the travel changes. Intersections may be categorized according to shape,
type of structure, and type of operation.
TOPIC OUTLINE
• Hierarchy of Intersection Control
• Design Elements of At-Grade Intersection
• Turning Roadways
• Auxiliary Lanes
• Islands and Channels
• Loading and Unloading Facility
• Indirect Left-Turn and U-Turn
• Design Alternative
HIERARCHY OF INTERSECTION CONTROL
Level 1 – NO CONTROL (Basic Rule)
If two cars get an intersection at the same time, the one to the right has right of way?
Republic Act No. 4136, Article III, Section 42 to 44 AKA the Land Transportation and Traffic Code –
Right of Way and Signals
NOTE: The driver of the vehicle on the left shall yield the right of way to the vehicle on the right
The corner sight distance available for a vehicle approaching an intersection to see oncoming vehicles
approaching from crossing legs (the left and right)
Level 2 – Priority Intersection
Republic Act No. 4136 - The Land Transportation and Traffic Code – Right of Way and Signals
Level 3 – Signal Controlled Intersections
RELATIVE SPEED
The vectorial speel of convergence of the vehicles in a conflict maeuver.
CONTROL OF CONFLICTS
Conflict Point and Area for Different Intersections
• 4-Legged Intersection
• Roundabout Intersection
• 3-Legged Intersection
CONTROL OF SPEED
The speed of vehicle through an intersection depends on: Alignment; Road environment; Traffic
volume and composition; and Traffic control devices.
AT-GRADE INTERSECTION
Summary of Turning Design
Design Procedure for Right-Turn Movements at
Intersections:
1. Select the design vehicle.
2. Determine the acceptable inside clearance.
3. Determine the acceptable encroachment.
4. Consider impacts on pedestrians.
5. Select the type of right-turning treatment.
6. Check the location of the stop bar.
The boundaries of the turning paths of each design vehicle for its sharpest turns are established by the outer
trace of the front overhang and the path of the inner rear wheel.
2. INSIDE CLEARANCE
NOTE: Desirably, the selected design
vehicle will make the right turn while
maintaining approximately a 600mm
clearance from the pavement edge or
face of curb.
3. ENCROACHMENT
Method of Corner Design
4. PEDESTRAIN IMPACT
The larger the right-turning radius, the farther pedestrians must walk across the street. This is especially
important to persons with disabilities.
5. SELECT TYPE OF RIGHT-TURN DESIGN
6. CHECK STOP BAR LOCATION
Width of Stop Line = 300mm to 450mm wide on urban roads and up to 600mm on rural roads.
STORAGE LENGTH
Storage length should be based on the number of turning vehicles likely to arrive within the peak hour.
For unsignalized intersection:
• Based on an average two minute period within the
peak hour
• At least two (2) passenger cars should fit in the
storage bay.
• With over 10% turning truck traffic, provision should
be made for at least one car and one truck.
For signalized Intersection:
• Determined by an analysis which considers the signal
cycle length, signal phasing arrangement, and the rate of
arrivals and departures of left-turning vehicles.
STORAGE LENGTH (Unsignalized)
Given: Design = 60km/h
DHV (in one direction) = 250 vph
Right Turns = 100 vph
Problem: Determine if a right-turn lane is warranted.
Solution: To read the vertical axis, use 100 -20
=80vph. The figure indicates that right-
turn lane is not necessary, unless other
factors (e.g high crash rate indicate a lane
is needed.
Right-turn lanes at unsignalized intersections on two-way single carriageway
TRAFFIC ISLAND
Separation/regulation of conflicting traffic movements into definite paths of travel by traffic islands or
pavement marking to facilitate the orderly movements of both vehicles and pedestrians.
Design of Channelized Islands
• Minimum area of corner island: 5 sq. m. (urban intersection)
7 sq. m. (rural intersection))
Preferable area = 9 sq.m.
• Curb height = 150mm
• Corner triangular islands should not be less than 3.5 m, and preferably should be 4.5 m on a side after
the rounding of corners.
ENTRY APPROACH
Entry Lanes (Lane Width)
Width may affect the motorists0 behavior when entering the intersection
TOO WIDE – may encourage drivers to speed up
TOO NARROW – May cause safety issues
“LANES at intersection may be equal or narrower that the midblock lane width”
W intersection < W midblock
U-TURN
• Normally, U-turns should not be permitted from through lanes. However, where medians have
adequate width to shield a vehicle stored in the median opening, through volumes are low and left-
turns/ U-turns are infrequent, this type of design may be permissible.
• Medians of 5.0 m and 15.0m or wider are needed to permit U-turn maneuvers by passenger and
single -unit truck traffic, respectively.
• The optimum location for the median U-turn roadway is 200m from the main intersection with
preferred spacing of 0.40 to 0.80 km.
BASIC PRINCIPLE IN PROVIDING INTERSECTION SAFETY
• Give Priority to major traffic.
• Separate conflicts in space and time
• Define and minimize conflicts areas.
• Reduce the relative speed between vehicles.
• Define the vehicle paths.
• Provide for all vehicular and non-vehicular traffic.
• Keep simple.
SHAPE
This refers to the configuration of the intersection and would depend largely on the number of legs.
Carefully planned and properly designed road networks often lead to intersections with simpler shape having
lesser number of legs.
TYPE OF OPERATION
Rules and regulations applied to a given intersection depend largely on the type of control which is in
operation at the intersection. The main objective is to simplify flow.
TYPE OF STRUCTURE
Most of the intersections are either designed as at-grade intersection or grade separation such as
flyovers or interchanges. Almost all intersections are initially designed at-grade and are planned to be grade
separated in the future to cope the high traffic volume.
ISLAND AND CHANNELIZATION
• Merge traffic streams at small angles to ensure low relative speed between conflicting stations.
• Reduce areas of conflict by causing opposing traffic streams to intersect generally at right angles
(desirable range is in the order of 70 to 90 degrees).
• Improve and define the alignment of major movements.
• Control the speed of traffic entering an intersection by changing alignment ot bending their approach
path.
• Control the speed of traffic by restricting width or funneling.
• Provide a refuge or median to shelter a turning of crossing vehicle.
• Provide protection for pedestrians.
• Improve conspicuity of an intersection e.g. a splitter island on an approach.
• Prohibit certain turns/movement; and
• Provide locations for traffic signal poles or traffic signs.
Channelization often leads to simplified movements of vehicles as it leads drivers to one conflict at a time
ROUNDABOUT
Circular shape, yield control on entry, and geometric features that create a low-speed environment
ROUNDABOUT CONTROLS
• Channelized approaches.
• Yield control on all entries.
• Counterclockwise circulation of all vehicles around the central island; and
• Appropriate geometric curvature to encourage slow traffic movement through the intersection.
• Unsatisfactory functioning occurs when two or more legs approach have unequal design capacity.
• The large area involved to satisfy proper geometric design of a rotary is extensive.
• Most rotaries are designed to function at low speeds. If high speeds are anticipated the large lengths
of required weaving sections may cause prohibitive land costs.
INTERCHANGE
An interchange is a grade separation in which vehicles moving in one direction of flow transfer to another
direction by using of connecting roadways or ramps.
• Grade-separated junctions are however large, and costly.
• Their height can be obtrusive, and combined with the large traffic volumes that grade-separated roads
attract, tend to make them unpopular to nearby landowners and residents.
• Grade-separation is expensive, time-consuming and requires significant engineering effort compared
to provision of an at-grade intersection.
FOUR MAIN TYPE OF INTERCHANGE
T and Y Interchange
Partial and Full Cloverleaf
Directional Interchange
CROSS- SECTIONAL ELEMENTS
The characteristics of cross-sectional elements are important in highway geometric design because
they influence the safety and comfort. The principal elements and comfort. The principal elements of a highway
cross section consist of the travel lanes shoulders, and medians (for some multilane highways). Marginal
elements include median and roadside barriers, curbs, gutters, guard rails, sidewalks, and side slopes.
Effect of Poor Cross-Sectional Elements
TRAFFIC LANES
For a two-lane national road, the minimum width is 2 x 3.35 m lanes (total 6.7 m). As the traffic volume
increases, so the need for extra width is justified. The width can increase up to a maximum of 3.65m. Where
warranted and where road space is available, an additional lane can be provided to improve the safety and
capacity of the road.
• Lane width V.S. Accident Rate
• Typical Lane Width Design Values
• Passing Stalled Vehicle
• Traffic Flow along EDSA
• Traffic Shockwave
Pavement Surface Type
The selection of a pavement type depends on several factors such as the traffic volume and
composition, soil characteristics, weather, availability of materials, initial cost, and maintenance cost.
• High type (Rigid Concrete) – 20 years
• Intermediate Type (Flexible Asphalt) – 10-15 years
• Low type
• According to DPWH RBIA Data (2018), small portion of the total length is unpaved and almost half of
the paved length has road conditions ranging from poor to fair.
• For safe and comfortable driving four aspects of the pavement surface are important: the friction
between the wheels and the pavement surface, smoothness of the road surface, the light reflection
characteristics of the top of pavement surface, and drainage to water.
CROSS SLOPE
HORIZONTAL ALIGNMENT
Horizontal curves are, in effect, transitions between two tangents. These deflection changes are
necessary in virtually all roadway alignments to avoid impacts on a variety of field conditions (e.g. right of
way, natural features, man-made features).
Simple Curves
Simple curves are continuous arcs of constant radius that achieve the necessary roadway deflection
without an entering or existing taper. It is the most commonly used. The radius of the circle determines the
‘sharpness’ or ‘flatness’ of the curve.
Compound Curves
Simple curves are continuous arcs of constant radius that achieve the necessary roadway deflection
without an entering or exiting taper. It is the most commonly used. The radius of the circle determines the
“Sharpness’ or ‘flatness’ of the curve.
Reverse Curve
A reverse curve consists of two simple curves joined together but curving in opposite directions. For
safety reasons, the designer should not use this curve unless absolutely necessary.
Spiral Curves
Spiral curves provide an entering transition into a simple curve with a variable rate of curvature along
its layout.
DESIGN ELEMENTS OF HORIZONTAL CURVES
• Design Speed
• Radius of Curve
• Widening on curves
• Superelevation – The tilting of roadway to help offset centripetal forces developed as the vehicle goes
around a curve. Along with friction they are what keeps a vehicle from going off the road.
• Topography
• Construction Cost
WIDENING ON CURVES
The objective of widening along horizontal curve is to make operating conditions on curves
comparable to those on tangents.”
“Widening on curves facilitates OFFTRACKING.”
Mechanical Widening:
Vehicles occupy a greater width on curves because their rear wheels generally track inside front
wheels in rounding a curve.
Psychological Widening:
Drivers generally experience difficulties in steering their vehicle in the center of a lane
PRINCIPAL POINTS OF CONCERN:
• On simple curves, widening should be applied on the inside edge only
• On curve design with spiral, widening may be placed on the inside or divided equally between the
inside and outside curve.
• Curve widening should be attained gradually over a length sufficient to make the whole of the
traveled way fully usable.
• Recommended minimum width of widening is 0.60m
TYPE OF SIGHT DISTANCE
1. Passing Distance
2. Stopping (Non-Passing) Sight Distance
3. Decision Sight Distance
4. Intersection Sight Distance
