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Dr. Manoj M.
Asst. Professor
Department of Civil Engineering
IIT Delhi
Geometric designs for Safe Highways
WORKSHOP-CUM-TRAINING PROGRAMME ON ROAD SAFETY
17th – 21st September 2018
2
Outline
Introduction
Cross section elements
Horizontal Alignment
Vertical Alignment
3
Introduction
4
Introduction
Deals with the dimensions and layout - alignment, sight distance
and intersection
Objective is to provide optimum efficiency with maximum safety at
reasonable cost
Main Design elements:
Cross section elements
Sight distance considerations
Horizontal alignment
Vertical alignment
Intersection elements
5
Introduction
Maximize the comfort and economy of facilities
Efficiency in traffic operation
Safety at reasonable cost
Environmental impacts
6
Introduction
Design Speed
Topography
Traffic factors
Design hour volume and capacity
Environmental and other factors
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Design Controls and Criteria
Design Speedo Most important factor
o Affected by topography & road type
o Influences all geometric elements of roads
Topography o Plain; Rolling; Mountainous; Steep terrains
o Speed and cross slope governs the design of elements
Traffic Factors o Vehicular and Human Characteristics
o Design vehicle – speed, dimension, weight and acceleration,
o Physical, mental and psychological characteristics – driver
Design hourly volume and capacityo Knowledge of Peak and off-peak hour volume
Environmental and other factors o Aesthetics, landscaping, pollution, etc.
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Design Controls and Criteria
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Terrain and Speed
IRC 73 - 1980
Pavement Surface Characteristics
o Friction
Tyre and road surface – speed, acceleration, sight distance, curve design
o Pavement types – cement concrete, bituminous, WBM
o Roughness of pavement
o Condition of pavement – wet/dry, mud/oil spilled
o Tyre condition
o Speed of vehicles
o Load and tyre pressure
o Temperature, etc.
Longitudinal friction 0.35-0.40; Transverse – 0.15
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Highway Cross Sectional Elements
Pavement Surface Characteristics
o Unevenness
Influences operating speed – geometric standards
Wear tear; accidents; operating cost;
Low unevenness index – 150 – 250 cm/km (high speed highways)
o Light Reflecting Characteristics
Night visibility – wet conditions
Light coloured – night condition (rainy); strain and glare (day)
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Highway Cross Sectional Elements
Cross Slope / Camber
o Slope provided in the transverse direction to drain off the rain water
Provided by raising the carriage way with respect to the edges
Depends on – type of the pavement surface & amount of rainfall
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Highway Cross Sectional Elements
IRC 73 - 1980
Width of Pavement or Carriageway
o Depends on the width of traffic lane and number of lanes
o Carriageway intended for one line of traffic movement is traffic lane
o Lane width = vehicle width (2.44m )+ side clearance (0.625)
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Highway Cross Sectional Elements
IRC 73 - 1980
Traffic Separators / Medians
o To prevent head on collision between vehicles moving in opposite
directions on adjacent lanes
o Pavement markings, medians, dividing islands, etc.
o 5.0 m for rural highways (3.0 m land restriction)
o Long bridges – 1.2 to 1.5 m
o Transition 1 in 15 to 1 in 20
o Urban roads: (absolute min width 1.2 m; desirable 5.0 m)
o 1.2 m for pedestrian refuge
o 4.0 -7.5 m for protection of vehicles making right turn
o 9.0 to 12.0 m for protection of vehicles crossing at grade
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Highway Cross Sectional Elements
Kerb
o Indicates the boundary between the pavement and shoulder
Barrier, Semi-barrier, and Mountable
o Barrier – Built-up areas adjacent to footpaths with considerable
pedestrian traffic
o Semi-barrier – periphery of the roadway where pedestrian traffic is
light and a barrier could tend to reduce traffic capacity
o Mountable – Within the roadway at channelization schemes,
medians, outer separators and raised medians on bridges
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Highway Cross Sectional Elements
Kerb
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Cross Sectional Elements
Source: IRC 86-1983
o Road Margins
Shoulders – Emergency lane / service lanes (min 2.5 m width)
Parking lanes – for kerb parking (min 3.0 m width)
Lay-byes – to stop and clear off the carriageway
Busbays – 75m away from intersections
Frontage roads – access to properties
Driveways – connect commercial establishments
Cycle tracks – min 2 m width; 1 m for additional lane
Footpath – when vehicular and pedestrian volume is high (1.5 min )
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Highway Cross Sectional Elements
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Highway Cross Sectional Elements
o Width of roadway or formation – sum of width of carriageway;
separators (if provided) & shoulders
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Highway Cross Sectional Elements
IRC 73 - 1980
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Highway Cross Sectional Elements
Source: IRC 86-1983
Right of Way
Sight Distance
o Sight distance available from a point is the actual distance along the road surface
which a driver from a specified height above the carriage way has visibility of
stationary or moving objects
o The length of road visible to the driver at any instance
Should Satisfy:
o Length of road visible ahead to stop the vehicle
o Safely overtake at reasonable intervals
o Control vehicle and avoid collision at uncontrolled intersection
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Sight Distance
Stopping Sight Distance
The minimum sight distance available on a highway at any spot
Depends on:
o Features of the road ahead
o Height of the driver’s eye above the road surface (1.2 m)
o Height of the object above the road surface (0.15 m)
Stopping depends on:
Total reaction time of the driver
Speed of vehicle
Efficiency of brakes
Frictional resistance between the road and tyres
Gradient, if any
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Sight Distance
Total Reaction Time
o The time taken from the instant the object is visible to the driver to the instant the
brakes are effectively applied
o Total reaction time = perception time + brake reaction time
o Total reaction time = 2.5 sec
Speed of vehicle
o Higher the speed, longer the stopping sight distance
Efficiency of brakes
o 100% braking efficiency – skidding
o Braking force should not exceed friction
Frictional/skid Resistance
o Depends on road and tyre
o f = 0.35 to 0.40
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Sight Distance
Stopping Sight Distance
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Sight Distance
0.278𝑉𝑡 +𝑉2
254(𝑓 ± 0.01𝐺)
IRC 73 - 1980
Overtaking Sight Distance
The minimum distance open to the vision of the driver of a vehicle
intending to overtake the slow moving vehicle with safety against the
traffic in opposite direction
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Sight Distance
Overtaking Sight Distance – Optimum condition is one in which the
overtaking driver can follow the vehicle ahead for a short time while
he assess his chances of overtaking
o Assumptions:
o The vehicle being overtaking is travelling at a uniform speed which
is 16kmph less than the design speed of the road
o The overtaking vehicle follows the vehicle ahead for a short while to
perceive the clear road ahead
o Overtaking is done by accelerating rapidly to the design speed and
is considered completed when the vehicle returns to its own side of
the road
o Overtaking once began is finished in the face of an oncoming
vehicle travelling at design speed in such a way that the latter
arrives alongside the former just at the completion of maneuver
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Sight Distance
Overtaking Sight Distance
o Overtaking maneuver – 8 to 14 seconds
o One third of the total time is spent following the vehicle to be
overtaken
o The opposing vehicle’s travel distance in 2/3 of the total time is
added.
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Sight Distance
IRC 73 - 1980
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Intermediate Sight Distance
o Sections of roads where the customary overtaking sight distance
cannot be provided should be designed as far as possible for
intermediate sight distance.
o It is twice the normal safe stopping distance.
Sight Distance
IRC 73 - 1980
29
Headlight Sight Distance
o In valley curves – roadway ahead is illuminated by vehicle
headlights to a sufficient length enabling the vehicle to break stop
(equal to SSD)
Sight Distance
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o Directional transition of the roadway in a horizontal plane
o Relationship between design speed and curvature and on their joint
relationships with superelevation (roadway banking) and side friction
Horizontal Alignment
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Superelevation
Horizontal Alignment
𝑒 + 𝑓 =𝑉2
127𝑅
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Superelevation
o Plain and rolling terrain – 7%
o In snow bound areas – 7%
o In hilly areas not bound by snow – 10%
Horizontal Alignment
IRC 73 - 1980
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o Radius of Horizontal Curve
𝑅 =𝑉2
127(𝑒 + 𝑓)
Horizontal Alignment
IRC 73 - 1980
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Widening of Pavements on Horizontal Curve
o When curves are not of large radius
Horizontal Alignment
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Widening of Pavements on Horizontal Curve
o Extra Widening = Mechanical Widening + Psychological Widening
𝑊𝑒 =𝑛𝑙2
2𝑅+
𝑉
9.5 𝑅
Horizontal Alignment
IRC 73 - 1980
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Horizontal Transition Curve
A transition curve has a radius which decreases from infinity at the
tangent point to that of the circular curve.
o Objectives:
o To introduce the centrifugal force gradually
o To steer the vehicle gradually and comfortably
o To allow for gradual introduction of superelevation and extra widening
o To improve aesthetic appearance
Horizontal Alignment
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Horizontal Transition Curve
o Spiral curve:
o Ideal transition
o Calculation and implementation are easy
Horizontal Alignment
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o Length of Transition Curve
i. Rate of change of centrifugal acceleration
𝐿𝑠 =0.0215𝑉2
𝐶𝑅; 𝐶 =
80
75 + 𝑉
ii. Rate of change of Superelevation
o For plain and rolling terrain
𝐿𝑠 =2.7𝑉2
𝑅o For mountainous and steep terrain
𝐿𝑠 =𝑉2
𝑅
Horizontal Alignment
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Horizontal Alignment
IRC 73 - 1980
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Setback distance
o Distance from the road center line within which the obstructions
should be cleared to ensure the needed visibility
Horizontal Alignment
𝑚 = 𝑅 − (𝑅 − 𝑛)𝑐𝑜𝑠𝜃
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o The vertical alignment is the elevation or profile of the center line of
the road – to accommodate changes in grades
o Consists of grades and vertical curves – vehicle speed, acceleration,
deceleration, SSD and comfort
Vertical Alignment
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Gradient – the rate of rise or fall along the length of the road with
respect to the horizontal (1 in x; n in 100)
o Gradient types
o Ruling gradient
o Limiting gradient
o Exceptional gradient
o Minimum gradient
Vertical Alignment
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o Ruling Gradient – The maximum gradient within which the designer
attempts to design the vertical profile of the road (design gradient)
o Limiting gradient – where topography compels adopting steeper
gradients than ruling gradients
o Exceptional gradient – steeper than limiting; not exceeding100 m at
a stretch
o Minimum gradient – from drainage point of view; 1 in 500 in concrete
drains
Vertical Alignment
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Vertical Alignment
IRC 73 - 1980
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Grade Compensation
o At horizontal curves, the gradients should be eased by an amount
known as the grade compensation (reduction in gradient)
o 𝐺𝑟𝑎𝑑𝑒 𝑐𝑜𝑚𝑝𝑒𝑛𝑠𝑎𝑡𝑖𝑜𝑛 % =30+𝑅
𝑅;𝑚𝑎𝑥.=
75
𝑅
o Not necessary for grades flatter than 4%
Vertical Alignment
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Vertical Curve
o At intersections of different grades to smoothen out the vertical
profile
Summit curves/Crest curves – Convexity upwards
Valley curves/Sag curves – Concavity upwards
Vertical Alignment
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o Summit Curve
o Governing design factor – Sight Distance
o Circular / Parabolic curves
Vertical Alignment
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o Length of Summit Curve for SSD
i. When L>SSD
ii. When L<SSD
Vertical Alignment
𝐿 =𝑁𝑆2
( 2𝐻 + 2ℎ)2=𝑁𝑆2
4.4
𝐿 = 2𝑆 −4.4
𝑁
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o Length of Summit Curve for OSD or ISD
i. When L>OSD/ISD
ii. When L<OSD/ISD
Vertical Alignment
𝐿 =𝑁𝑆2
8𝐻=𝑁𝑆2
9.6
𝐿 = 2𝑆 −8𝐻
𝑁= 2𝑆 −
9.6
𝑁
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Vertical Alignment
IRC 73 - 1980
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Valley Curve
o Governing design factor – comfort of passengers& availability of
stopping sight distance under headlights of vehicle
o Allowable rate of centrifugal acceleration influences the design
(transition curves)
Vertical Alignment
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o Length of Valley Curve
i. For comfort condition
𝐿 = 0.38(𝑁𝑉3)1/2
ii. L > Headlight Sight Distance
𝐿 =𝑁𝑆2
(1.5 + 0.035𝑆)
iii. L < Headlight Sight Distance
𝐿 = 2𝑆 −(1.5 + 0.035𝑆)
𝑁
Vertical Alignment
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1. IRC: 73-1980, geometric design standards for rural (non-urban)
highways
2. IRC: 86-1983, Geometric Design Standards for Urban Roads and
Plains
3. IRC: 66-1976, Recommended Practice for Sight Distance on Rural
Highways
References – IRC codes
Thank You
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