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CE 6504 - HIGHWAY ENGINEERING
UNIT II
GEOMETRIC DESIGN OF HIGHWAYS
Typical cross sections of Urban and Rural roads –– Cross sectional elements - Sight distances –
Horizontal curves, Super elevation, transition curves, widening at curves – Vertical curves -
Gradients, Special consideration for hill roads - Hairpin bends – Lateral and vertical clearance
at underpasses.
Cross Sections:
Cross sections are created to provide a visual guide depicting the initial, interim, and ultimate
phase cross sections for these road classifications. The typical sections illustrated in the following
pages are recommendations tied to transportation planning aspects, such as right‐of‐way, sidewalk
width, etc. Threshold daily traffic can be used as a guide for a starting point when determining which
cross section is most applicable.
The width of a roadway is an important design consideration to ensure that it is appropriately
sized to serve its function. Because of the diversity within the County, two major roadway categories
have been established: 1. Rural Road Standards; 2. Urban Road Standards. Urban Road Standards
will serve those areas which tend to be more developed and need to provide for multiple users
(bicyclists, pedestrians, parallel parking, etc.) whereas, many rural roads will primarily serve only
vehicular traffic.
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1. What is cross section? Draw the cross section of rural and urban road.
2
Rural Roads
Rural roads are defined as low traffic roads located in forested and rangeland settings that
serve residential, recreational and resource management uses. Rural roads may be owned and/or
managed by governmental or private parties.
Rural Road Standards
The rural roadways will not typically require curb and gutter or sidewalk, although the
County may require either or both in unique circumstances. Widths of lanes and shoulders
will vary depending upon the specific classification and the potential traffic volume which the
roadway may carry. Roads carrying fewer than 200 vehicles per day need not be paved or
treated for dust control. The need for paved shoulders is also dependent upon the level of
traffic and safety.
3
2. Explain in detail about rural and urban roads with neat sketch.
TYPICAL CROSS SECTION OF RURAL ARTERIAL
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TYPICAL CROSS SECTION OF RURAL COLLECTOR
5
TYPICAL CROSS SECTION OF RURAL LOCAL
6
Urban RoadsAn arterial road, or arterial thoroughfare, is a high-capacity urban road. The primary function
of an arterial road is to deliver traffic from collector roads to freeways or expressways, and between
urban centres at the highest level of service possible.
Urban Road Standards
Three roadway classifications are identified for those areas that are associated with the
community’s urban growth areas. They include arterial, collector, and local street
classifications. Urban road standards will include 12‐foot lanes, sidewalk and curb & gutter;
arterials and collectors will also include a striped bike lane. Turn lanes may be necessary as
determined by the County. Since almost all of the municipalities have different right‐of‐way
cross sections adopted for their community, it makes it very difficult for the County to match
them all. Therefore, the philosophy was to encourage a baseline amount of right‐of‐way
reservation that could ensure the adjacent community enough area for coordination of future
roadway improvements, until such time the community would annex the road.
7
TYPICAL CROSS SECTION OF URBAN ARTERIAL
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TYPICAL CROSS SECTION OF URBAN COLLECTOR
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TYPICAL CROSS SECTION OF URBAN LOCAL
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CROSS SECTIONAL ELEMENTS
The features of the cross-section of the pavement influence the life of the pavement as well as
the riding comfort and safety. Of these, pavement surface characteristics affect both of these.
Camber, kerbs and geometry of various cross-sectional elements are important aspects to be
considered in this regard.
1. Pavement surface characteristics
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.
a) Friction
Friction between the wheel and the pavement surface is a crucial factor in the design of
horizontal curves and thus the safe operating speed. Further, it also affect the acceleration and
deceleration ability of vehicles. Lack of adequate friction can cause skidding or slipping of vehicles.
Skidding happens when the path traveled along the road surface is more than the
circumferential movement of the wheels due to friction
Slip occurs when the wheel revolves more than the corresponding longitudinal movement
along the road.
Various factors that affect friction are:
Type of the pavement (like bituminous, concrete, or gravel),
Condition of the pavement (dry or wet, hot or cold, etc),
Condition of the tyre (new or old), and
Speed and load of the vehicle.
The frictional force that develops between the wheel and the pavement is the load acting
multiplied by a factor called the coefficient of friction and denoted as . The choice of the value of
is a very complicated issue since it depends on many variables. IRC suggests the coefficient of
longitudinal friction as 0.35-0.4 depending on the speed and coefficient of lateral friction as 0.15. The
former is useful in sight distance calculation and the latter in horizontal curve design.
11
3. Write in detail about cross section elements of highway.
b) Unevenness
It is always desirable to have an even surface, but it is seldom possible to have such a one.
Even if a road is constructed with high quality pavers, it is possible to develop unevenness due to
pavement failures. Unevenness affect the vehicle operating cost, speed, riding comfort, safety, fuel
consumption and wear and tear of tyres.
Unevenness index is a measure of unevenness which is the cumulative measure of vertical
undulations of the pavement surface recorded per unit horizontal length of the road. An unevenness
index value less than 1500 mm/km is considered as good, a value less than 2500 mm.km is
satisfactory up to speed of 100 kmph and values greater than 3200 mm/km is considered as
uncomfortable even for 55 kmph.
c) Light reflection
White roads have good visibility at night, but caused glare during day time.
Black roads has no glare during day, but has poor visibility at night
Concrete roads has better visibility and less glare
It is necessary that the road surface should be visible at night and reflection of light is the
factor that answers it.
d) Drainage
The pavement surface should be absolutely impermeable to prevent seepage of water into the
pavement layers. Further, both the geometry and texture of pavement surface should help in draining
out the water from the surface in less time.
2. Elements:
i. Camber
Camber or cant is the cross slope provided to raise middle of the road surface in the transverse
direction to drain off rain water from road surface. The objectives of providing camber are:
Surface protection especially for gravel and bituminous roads
Sub-grade protection by proper drainage
Quick drying of pavement which in turn increases safety
Too steep slope is undesirable for it will erode the surface. Camber is measured in 1 in n or n
% (Eg. 1 in 50 or 2%) and the value depends on the type of pavement surface. The values suggested
by IRC for various categories of pavement are given in Table 1. The common types of camber are
parabolic, straight, or combination of them (Figure 1)
12
Figure 1: Different types of camber
Table 1: IRC Values for camber
Surface Heavy Light
Type rain rain
Concrete/Bituminous 2 % 1.7 %
Gravel/WBM 3 % 2.5 %
Earthen 4 % 3.0 %
ii. Width of carriage way
Width of the carriage way or the width of the pavement depends on the width of the traffic
lane and number of lanes. Width of a traffic lane depends on the width of the vehicle and the
clearance. Side clearance improves operating speed and safety. The maximum permissible width of a
vehicle is 2.44 and the desirable side clearance for single lane traffic is 0.68 m. This require
minimum of lane width of 3.75 m for a single lane road (Figure 2a). However, the side clearance
required is about 0.53 m on either side and 1.06 m in the center. Therefore, a two lane road require
minimum of 3.5 meter for each lane (Figure 2b). The desirable carriage way width recommended by
IRC is given in Table 2
13
Table 2: IRC Specification for carriage way width
Single lane 3.75
Two lane, no kerbs 7.0
Two lane, raised kerbs 7.5
Intermediate carriage 5.5
Multi-lane 3.5
Figure 2: Lane width for single and two lane roads
iii) Kerbs
Kerbs indicate the boundary between the carriage way and the shoulder or islands or
footpaths. Different types of kerbs are (Figure 3):
Low or mountable kerbs : This type of kerbs are provided such that they encourage the
traffic to remain in the through traffic lanes and also allow the driver to enter the shoulder
area with little difficulty. The height of this kerb is about 10 cm above the pavement edge
with a slope which allows the vehicle to climb easily. This is usually provided at medians and
14
channelization schemes and also helps in longitudinal drainage.
Semi-barrier type kerbs : When the pedestrian traffic is high, these kerbs are provided.
Their height is 15 cm above the pavement edge. This type of kerb prevents encroachment of
parking vehicles, but at acute emergency it is possible to drive over this kerb with some
difficulty.
Barrier type kerbs : They are designed to discourage vehicles from leaving the pavement.
They are provided when there is considerable amount of pedestrian traffic. They are placed at
a height of 20 cm above the pavement edge with a steep batter.
Submerged kerbs : They are used in rural roads. The kerbs are provided at pavement edges
between the pavement edge and shoulders. They provide lateral confinement and stability to
the pavement.
Figure 3: Different types of kerbs
iv. Road margins
The portion of the road beyond the carriageway and on the roadway can be generally called
road margin. Various elements that form the road margins are given below.
v. Shoulders
Shoulders are provided along the road edge and are intended for accommodation of stopped
vehicles, serve as an emergency lane for vehicles and provide lateral support for base and surface
courses. The shoulder should be strong enough to bear the weight of a fully loaded truck even in wet
conditions. The shoulder width should be adequate for giving working space around a stopped
vehicle. It is desirable to have a width of 4.6 m for the shoulders. A minimum width of 2.5 m is
recommended for 2-lane rural highways in India.
vi. Parking lanes
Parking lanes are provided in urban lanes for side parking. Parallel parking is preferred
because it is safe for the vehicles moving on the road. The parking lane should have a minimum of
15
3.0 m width in the case of parallel parking.
vii. Bus-bays
Bus bays are provided by recessing the kerbs for bus stops. They are provided so that they do
not obstruct the movement of vehicles in the carriage way. They should be at least 75 meters away
from the intersection so that the traffic near the intersections is not affected by the bus-bay.
viii. Service roads
Service roads or frontage roads give access to access controlled highways like freeways and
expressways. They run parallel to the highway and will be usually isolated by a separator and access
to the highway will be provided only at selected points. These roads are provided to avoid congestion
in the expressways and also the speed of the traffic in those lanes is not reduced.
ix. Cycle track
Cycle tracks are provided in urban areas when the volume of cycle traffic is high Minimum
width of 2 meter is required, which may be increased by 1 meter for every additional track.
x. Footpath
Footpaths are exclusive right of way to pedestrians, especially in urban areas. They are
provided for the safety of the pedestrians when both the pedestrian traffic and vehicular traffic is
high. Minimum width is 1.5 meter and may be increased based on the traffic. The footpath should be
either as smooth as the pavement or more smoother than that to induce the pedestrian to use the
footpath.
xi. Guard rails
They are provided at the edge of the shoulder usually when the road is on an embankment.
They serve to prevent the vehicles from running off the embankment, especially when the height of
the fill exceeds 3 m. Various designs of guard rails are there. Guard stones painted in alternate black
and white are usually used. They also give better visibility of curves at night under headlights of
vehicles.
xii. Width of formation
Width of formation or roadway width is the sum of the widths of pavements or carriage way
including separators and shoulders. This does not include the extra land in formation/cutting. The
values suggested by IRC are given in Table 3.
Table 3: Width of formation for various classed of roads
Road Roadway width in m
classification Plain and Mountainous and
rolling terrain steep terrain
16
NH/SH 12 6.25-8.8
MDR 9 4.75
ODR 7.5-9.0 4.75
VR 7.5 4.0
xiii. Right of way
Right of way (ROW) or land width is the width of land acquired for the road, along its
alignment. It should be adequate to accommodate all the cross-sectional elements of the highway and
may reasonably provide for future development. To prevent ribbon development along highways,
control lines and building lines may be provided. Control line is a line which represents the nearest
limits of future uncontrolled building activity in relation to a road. Building line represents a line on
either side of the road, between which and the road no building activity is permitted at all. The right
of way width is governed by:
Width of formation: It depends on the category of the highway and width of roadway and
road margins.
Height of embankment or depth of cutting: It is governed by the topography and the vertical
alignment.
Side slopes of embankment or cutting: It depends on the height of the slope, soil type etc.
Drainage system and their size which depends on rainfall, topography etc.
Sight distance considerations : On curves etc. there is restriction to the visibility on the inner
side of the curve due to the presence of some obstructions like building structures etc.
Reserve land for future widening: Some land has to be acquired in advance anticipating
future developments like widening of the road.
Table 4: Normal right of way for open areas
Road Roadway width in mclassification Plain and Mountainous and
rolling terrain steep terrainOpen areas
NH/SH 45 24MDR 25 18
17
ODR 15 15VR 12 9
Built-up areasNH/SH 30 20MDR 20 15ODR 15 12VR 10 9
The importance of reserved land is emphasized by the following. Extra width of land is
available for the construction of roadside facilities. Land acquisition is not possible later, because the
land may be occupied for various other purposes (buildings, business etc.) The normal ROW
requirements for built up and open areas as specified by IRC is given in Table 4. A typical cross
section of a ROW is given in Figure 4.
Figure 4: A typical Right of way (ROW)
(NOV/ DEC 2011), (MAY/JUNE 2012), (NOV/ DEC 2012), (MAY/JUNE 2013)
SIGHT DISTANCES
The safe and efficient operation of vehicles on the road depends very much on the visibility of
the road ahead of the driver. Thus the geometric design of the road should be done such that any
18
4. What is sight distance? Write in detail about types of sight distances.
obstruction on the road length could be visible to the driver from some distance ahead. This distance
is said to be the sight distance.
Types of sight distance
Sight distance available from a point is the actual distance along the road surface, over which
a driver from a specified height above the carriage way has visibility of stationary or moving objects.
Three sight distance situations are considered for design:
Stopping sight distance (SSD) or the absolute minimum sight distance
Intermediate sight distance (ISD) is defined as twice SSD
Overtaking sight distance (OSD) for safe overtaking operation
Head light sight distance is the distance visible to a driver during night driving under
the illumination of head lights.
Safe sight distance to enter into an intersection.
The most important consideration in all these is that at all times the driver traveling at the
design speed of the highway must have sufficient carriageway distance within his line of vision to
allow him to stop his vehicle before colliding with a slowly moving or stationary object appearing
suddenly in his own traffic lane.
The computation of sight distance depends on:
a) Reaction time of the driver - Reaction time of a driver is the time taken from the instant the
object is visible to the driver to the instant when the brakes are applied. The total reaction time
may be split up into four components based on PIEV theory.
In practice, all these times are usually combined into a total perception-reaction time suitable
for design purposes as well as for easy measurement. Many of the studies show that drivers
require about 1.5 to 2 secs under normal conditions. However, taking into consideration the
variability of driver characteristics, a higher value is normally used in design. For example, IRC
suggests a reaction time of 2.5 secs.
b) Speed of the vehicle - The speed of the vehicle very much affects the sight distance. Higher the
speed, more time will be required to stop the vehicle. Hence it is evident that, as the speed
increases, sight distance also increases.
c) Efficiency of brakes - The efficiency of the brakes depends upon the age of the vehicle, vehicle
characteristics etc. If the brake efficiency is 100%, the vehicle will stop the moment the brakes
are applied. But practically, it is not possible to achieve 100% brake efficiency. Therefore the
sight distance required will be more when the efficiency of brakes is less. Also for safe geometric
design, we assume that the vehicles have only 50% brake efficiency.
d) Frictional resistance between the tyre and the road - The frictional resistance between the tyre
and road plays an important role to bring the vehicle to stop. When the frictional resistance is
19
more, the vehicles stop immediately. Thus sight required will be less. No separate provision for
brake efficiency is provided while computing the sight distance. This is taken into account along
with the factor of longitudinal friction. IRC has specified the value of longitudinal friction in
between 0.35 to 0.4.
e) Gradient of the road - Gradient of the road also affects the sight distance. While climbing up a
gradient, the vehicle can stop immediately. Therefore sight distance required is less. While
descending a gradient, gravity also comes into action and more time will be required to stop the
vehicle. Sight distance required will be more in this case.
Stopping sight distance
Stopping sight distance (SSD) is the minimum sight distance available on a highway at any
spot having sufficient length to enable the driver to stop a vehicle traveling at design speed, safely
without collision with any other obstruction.
There is a term called safe stopping distance and is one of the important measures in traffic
engineering. It is the distance a vehicle travels from the point at which a situation is first perceived to
the time the deceleration is complete. Drivers must have adequate time if they are to suddenly re-
spond to a situation. Thus in highway design, sight distance at least equal to the safe stopping dis-
tance should be provided. The stopping sight distance is the sum of lag distance and the braking dis-
tance.
Lag distance is the distance the vehicle traveled during the reaction time t and is given by vt,
where v is the velocity in m=sec2. Braking distance is the distance traveled by the vehicle during
braking operation. For a level road this is obtained by equating the work done in stopping the vehicle
and the kinetic energy of the vehicle. If F is the maximum frictional force developed and the braking
distance is l, then work done against friction in stopping the vehicle is Fl = fWl where W is the total
weight of the vehicle. The kinetic energy at the design speed is,
Therefore, the SSD = lag distance + braking distance and given by:
20
where v is the design speed in m=sec2, t is the reaction time in sec, g is the acceleration due to grav-
ity and f is the coefficient of friction. The coefficient of friction f is given below for various design
speed.
Co efficient of Longitudinal Friction
When there is an, ascending gradient of say +n%, the component of gravity adds to braking
action and hence braking distance is decreased. The component of gravity acting parallel to the sur-
face which adds to the braking force is equal to W sin α = W tan α = Wn / 100. Equating kinetic en-
ergy and work done:
Similarly the braking distance can be derived for a descending gradient. Therefore the general equa-
tion is given by,
Overtaking sight distance
The overtaking sight distance is the minimum distance open to the vision of the driver of a ve-
hicle intending to overtake the slow vehicle ahead safely against the traffic in the opposite direction.
The overtaking sight distance or passing sight distance is measured along the center line of the road
over which a driver with his eye level 1.2 m above the road surface can see the top of an object 1.2 m
above the road surface.
The factors that affect the OSD are:
Velocities of the overtaking vehicle, overtaken vehicle and of the vehicle coming in the oppo-
site direction.
Spacing between vehicles, which in-turn depends on the speed
Skill and reaction time of the driver
Rate of acceleration of overtaking vehicle
21
Gradient of the road
The dynamics of the overtaking operation is given in the _gure which is a time-space dia-
gram. The x-axis denotes the time and y-axis shows the distance traveled by the vehicles. The trajec-
tory of the slow moving vehicle (B) is shown as a straight line which indicates that it is traveling at a
constant speed. A fast moving vehicle (A) is traveling behind the vehicle B. The trajectory of the ve-
hicle is shown initially with a steeper slope.
The dotted line indicates the path of the vehicle A if B was absent. The vehicle A slows down
to follow the vehicle B as shown in the figure with same slope from t0 to t1. Then it overtakes the ve-
hicle B and occupies the left lane at time t3. The time duration T = t3 - t1 is the actual duration of the
overtaking operation. The snapshots of the road at time t0; t1, and t3 are shown on the left side of the
Figure.
From the Figure, the overtaking sight distance consists of three parts.
d1 the distance traveled by overtaking vehicle A during the reaction time t = t1 - t0
d2 the distance traveled by the vehicle during the actual overtaking operation T = t3 - t1
d3 is the distance traveled by on-coming vehicle C during the overtaking operation (T).
Therefore:
22
It is assumed that the vehicle A is forced to reduce its speed to vb, the speed of the slow mov-
ing vehicle B and travels behind it during the reaction time t of the driver. So d1 is given by:
Then the vehicle A starts to accelerate, shifts the lane, overtake and shift back to the original
lane. The vehicle A maintains the spacing s before and after overtaking. The spacing s in m is given
by:
Let T be the duration of actual overtaking. The distance traveled by B during the overtaking
operation is 2s+vbT. Also, during this time, vehicle A accelerated from initial velocity vb and over-
taking is completed while reaching final velocity v. Hence the distance traveled is given by:
The distance traveled by the vehicle C moving at design speed v m / sec during overtaking operation
is given by:
The overtaking sight distance is
23
where vb is the velocity of the slow moving vehicle in m=sec2, t the reaction time of the
driver in sec, s is the spacing between the two vehicle in m given by equation 13.5 and a is the over -
taking vehicles acceleration in m=sec2. In case the speed of the overtaken vehicle is not given, it can
be assumed that it moves 16 kmph slower the design speed.
The acceleration values of the fast vehicle depends on its speed and given in Table 5.
Table 5. Maximum Overtaking Acceleration at different Speed
On divided highways, d3 need not be considered
On divided highways with four or more lanes, IRC suggests that it is not necessary to provide
the OSD, but only SSD is sufficient.
Overtaking zones
Overtaking zones are provided when OSD cannot be provided throughout the length of the
highway. These are zones dedicated for overtaking operation, marked with wide roads. The desirable
length of overtaking zones is 5 times OSD and the minimum is three times OSD (Figure 5).
Figure 5: Overtaking Zone
24
Intermediate Sight distance
Intermediate Sight distance (ISD) is defined as the twice the Stopping Sight Distance (SSD).
ISD = 2* SSD
The minimum distance at which, the driver travelling at design speed (v), has sufficient visi-
bility to take control o his vehicle and to avoid collision with another vehicle in an uncontrolled inter-
section is called ISD.
Head light Sight distance
Head light sight distance is the distance visible to a driver during night driving under the
illumination of head lights.
Sight distance at intersections
At intersections where two or more roads meet, visibility should be provided for the drivers
approaching the intersection from either sides. They should be able to perceive a hazard and stop the
vehicle if required. Stopping sight distance for each road can be computed from the design speed.
The sight distance should be provided such that the drivers on either side should be able to see each
other. This is illustrated in the figure 6.
The design of SD at intersections based on the following:
a) Enabling the approaching vehicle to change speed.
b) Enabling the approaching vehicle to stop.
c) Enabling the stopped vehicle to cross the road.
25
Figure 6: Sight Distance at Intersection
Horizontal curves
Curves are the place in highway used to change the direction o the traffic. As far as possible,
the curves are to be neglected in alignment and may be provided or the following reasons
1) To adjust the topography of the land.
2) To avoid religious and monumental structures in straight alignment.
3) To alert the drivers, from careless of very long straight roads.
4) To utilize the existing right of ways.
5) To preserve the existing amenities.
6) To provide access to certain locality.
7) To solve the land acquisition problem etc.,
Horizontal Curves Provides a transition between two tangent lengths of roadway. Horizontal
Curves are necessary for gradual change in direction when a direct point of intersection is not feasi-
ble.
Example: Highways, Interstates, high speed roads with constant flow of traffic
Types of Horizontal Curves
26
5. Write in detail about horizontal curves with its types.
a) Simple Curve
b) Compound Curve
c) Reverse Curve
d) Spiral Curve
e) Transition Curve
a) Simple Curve
Simple Curves are defined as a circular curve of single radius connecting two straight line
shown in figure.
The length o the curved portion in a highway is based on the degree o curvature. Simple
curves are laid where there is less traffic of fast moving vehicles and less traffic volume.
b) Compound Curve
Compound Curve is defined as the combination o two simple circular curves o different
radii, turning in the same direction and joining at one common junction.
This curve is provided where there is no possibility for providing simple curves.
c) Reverse Curve
Reverse Curve is defined as the compound curve of two different radii, or same radii, but
curving in opposite direction with a common tangent.
Drawbacks of providing Reverse curve in Highways:
27
Streets of vehicles in reverse curve are dangerous.
Very difficult to provide proper super elevation at the junction point.
The sudden changes of direction makes uncomfortable to the passengers.
(APRIL/ MAY 2011)
TRANSITION CURVE
Transition curve is a type of curve and it is introduced between a straight and circular
curve or between two branches of compound curve or between the two branches o reverse curve.
Transition curve is provided to change the horizontal alignment from straight to circular
curve gradually and has a radius which decreases from infinity at the straight end (tangent point) to
the desired radius of the circular curve at the other end (curve point.
A typical transition curve is shown in the following figure,
The primary purpose o transition curve is to enable vehicles moving at high-speed, to
make the change from the tangent section to the curved section and again to the tangent section in a
safe and comfortable made.
It is used to introduce the designed super elevation gradually and extra widening.
There are five objectives for providing transition curve and are given below:
1. To introduce gradually the centrifugal force between the tangent point and the be-
ginning of the circular curve, avoiding sudden jerk on the vehicle. This increases the
comfort of passengers.
28
6. What do you know about transition curve? Write its types with necessary diagram.
2. To enable the driver turn the steering gradually for his own comfort and security,
3. To provide gradual introduction of super elevation, and
4. To provide gradual introduction of extra widening.
5. To enhance the aesthetic appearance of the road.
Type of transition curve
Different types of transition curves are spiral or clothoid, cubic parabola, and Lemniscates.
IRC recommends spiral as the transition curve because it fulfills the requirement of an ideal transition
curve, that is;
(a) rate of change or centrifugal acceleration is consistent (smooth) and
(b) radius of the transition curve is 1 at the straight edge and changes to R at the curve point
(Ls ∞ 1 /R)
Length of transition curve
The length of the transition curve should be determined as the maximum of the following three crite-
ria:
i) Rate of change of centrifugal acceleration,
ii) Rate of change of super elevation, and
iii) An empirical formula given by IRC.
1. Rate of change of centrifugal acceleration
At the tangent point, radius is infinity and hence centrifugal acceleration is zero. At the end of
the transition, the radius R has minimum value R. The rate of change of centrifugal acceleration
29
should be adopted such that the design should not cause discomfort to the drivers. If c is the rate of
change of centrifugal acceleration, it can be written as:
Therefore, the length of the transition curve Ls1 in m is,
where c is the rate of change of centrifugal acceleration given by an empirical formula suggested by
by IRC as below:
2. Rate of introduction of super-elevation
The length of the transition curve Ls2 is:
Where, N = Rate o change o super elevation
W = Width o the pavement in m
We = Width o extra widening in m
3. By empirical formula
IRC suggest the length of the transition curve is minimum for a plain and rolling terrain:
and for steep and hilly terrain is:
30
and the shift s as:
The length of the transition curve Ls is the maximum of above equations,
(NOV/ DEC 2011), (MAY/JUNE 2012)
SUPER ELEVATION
When the vehicles moves in a horizontal curve, the vehicle may be overturned or skid by the
centrifugal force. Hence, to avoid the effect of centrifugal force on vehicle, it is necessary to provide
a special arrangement in road alignment.
A special arrangement in a road, in which the outer edge of the road is raised with respect to
inner edge of the road, for the purpose of avoiding the centrifugal force effect on vehicle is called su-
per elevation. It is also called as cant or banking.
31
7. Describe briefly about super elevation with proper sketches.
Factors governing the Super Elevation:
The following factors, governing the super elevation of a road surface,
a) Design Speed
b) Type of the Highway
c) Radius of the Curve
d) Weight of the Vehicle
e) Type of the road surface
In general, the super elevation is defined as the ratio of the height of outer edge with respect
to horizontal width of the pavement.
e= Height of outer edgeHorizontal width of pavement
Analysis of Super Elevation
The forces acting on the vehicle, moving on a circular curve of radius R at the speed of v are,
i) Centrifugal Force F
ii) Weight of the Vehicle W
iii) Friction force between wheel and road surface
The centrifugal force F can be counteracted by two forces, as follows
a. Frictional force
b. Components of the force of gravity due to the super elevation provided.
In the figure, under the equilibrium condition,
32
F cosθ=W sin θ+FA+FB --------------------------- (1)
Where FA and FB are frictional forces at A and B respectively.
When the maximum values of frictional forces are developed, FA and FB reaches their max-
imum values of (f * RA) and (f * RB) respectively.
f – Coefficient of lateral friction
RA and RB - Normal reaction at wheels A and B
Equation (1), becomes,
F cosθ=W sin θ+ f (RA+RB)
From the figure,
RA=W cosθ∧RB=F sin θ and we get,
F cosθ=W sin θ+ f (W cosθ+F sin θ)
¿W sin θ+ fW cosθ+fF sin θ ¿¿
F cosθ−fFsinθ=W sin θ+ fW cosθ
F ¿
Dividing by W cosθ on both side,
For design purposes, the value of coefficient of lateral friction ‘f’ = 0.15 and the value of ‘tanθ’ ex-
ceeds 0.07 (or 1 / 15).
From the above assumptions,
f tanθ = 0.15*0.07 = 0.01
and 1-f tanθ = 1 – 0.01 = 0.99 (or) 1
The equation (3) becomes,
33
From the concept of centrifugal ratio, we have
Where, e = rate of super elevation (tan θ)
f = coefficient of lateral friction
Conditions
1. If f = 0, i.e., the coefficient of lateral friction is neglected, the equilibrium super elevation will be,
e+f = vl2
gR
e= vl2
gR
2.If the super elevation is not provided due to practical and topographical problems, then,
f = vl2
gR
In such cases, the frictional force only counteracts the centrifugal force.
Advantages of Super Elevation
It increases the stability of fast moving vehicle, passing the curves.
It reduces the intensity of stress on foundation.
It distributes the equal pressure on tyres.
It provides smooth and safe travelling to the vehicles.
It increases the traffic volume indirectly.
It reduces the maintenance cost on curves.
It removes the rain water and works like a camber on curves.
Drawbacks of Super Elevation
34
It is necessary to raise the center of the pavement.
The height of ‘e’ will be raised with respect to inner edge and hence, earthwork for filling will
be high.
Even though this method has some disadvantages, it does not affect the drainage system of
the road and hence, this method is suitable for all roads.
(APRIL/ MAY 2010), (NOV/ DEC 2011)
WIDENING AT CURVES:
Widening (or) mechanical widening on curve is defined as the concept of providing an extra
width of pavement on horizontal curves, having the radius of less than about 300m.
A vehicle has a rigid base and only the front wheels of the vehicle can be turned. On curve, if
the front wheel changes it direction, the rear wheels does not follow the same path, as that of the front
wheels. This concept is termed as Off-tracking.
The off – tracking depends on the length of the wheel based (l) and the turning angle or the ra-
dius of the horizontal curve. Hence, the extra widening of the pavement is required on horizontal
curves.
Analysis of Extra Widening on Curves
The extra widening of pavement on horizontal curves is divided into two parts,
i) Mechanical Widening
ii) Psychological Widening
Mechanical Widening
The widening required to account for the off-tracking due to the rigidity of wheel based is
called mechanical widening (Wm) and may be calculated as given below. Refer below figure,
35
8. What are the objectives of widening of road pavement at horizontal curves? Derive the expression for the extra widening.
R1 = Radius of the path traversed by the outer rear wheel (m)
R2 = Radius of the path traverse by the outer front wheel (m)
Wm = Off-tracking or the mechanical widening (m)
l = Length of wheel base (m)
Wm = OC – OA = OB –OA = R2 –R1
From Δ OAB, OA2 = OB2 - BA2
R12 = R2
2 - l2
R1 = R2 - Wm
(R2 - Wm)2 = R22 - l2
R22 - 2R2Wm + Wm
2 = R22 - l2
l2 = Wm (2 R2 - Wm)
Wm = l2 / (2 R2 - Wm)
= l2 / 2 R (approximately)
Here R is the mean radius of the curve. The mechanical widening calculated above is re-
quired for one vehicle negotiating a horizontal curve along one traffic lane. Hence in a road having
‘n’ vehicles can travel simultaneously, the total mechanical widening required is given by,
Wm = n l2 / 2 R
36
Psychological Widening
Extra width of pavement is also provided for psychological reasons such as, to provide for
greater maneuverability of steering at higher speeds, to allow for the extra space requirements for the
overhangs of vehicles and to provide greater clearance for crossing and overtaking vehicles on the
curve. Psychological widening is therefore important in pavements with more than one lane. An em-
pirical formula has been recommended by IRC for finding the additional psychological widening
‘Wps’ which dependent on the design speed V of the vehicle and the radius R of the curve. The psy-
chological widening is given by the formula:
Wps= V9.5√R
Hence, the total widening We, m required on a horizontal curve is given by:
We=Wm+Wps
i.e ., We= nl2
2 R+ V
9.5√R
Hence, n = Number of traffic lanes
l = Length of wheel base of longest vehicle, m. the value of l may normally be
taken as 6.1 m or 6 m for commercial vehicles, if not known.
V = Design Speed, kmph
R = Radius of horizontal curve, m
The extra widening recommended by the Indian Roads Congress for single and two lane pavements
are given below,
Radius of curve (m) Upto 20 20 to 40 41 to 60 61 to 100 101 to 300 Above 300
Extra width (m)
Two-lane 1.5 1.5 1.2 0.9 0.6 Nil
Single-lane 0.9 0.6 0.6 Nil Nil Nil
(APRIL/ MAY 2011), (NOV/ DEC 2012)
VERTICAL CURVES
37
9. What do you mean by vertical curves? Write its types.
Vertical curves are provided in elevation of change of gradients. These curves are convex
when two gradients meet as a ‘summit’ and concave when they meet at ‘sag’. Generally vertical
curves are not necessary when the total gradient change from one tangent to other tangent not exceed
0.5%.
The vertical curve may be in the form of either a circular arc or a parabola. The parabolic
curve produces the best riding qualities, because of the uniform rate of change of grade.
The general equation of a parabola with a vertical is,
y = ax2 + bx
The slope at any point dydx is,
dydx = 2 ax + b
The rate of change of slope or the rate of change of grade is d2 ydx 2 is,
d2 ydx 2 = 2 a = constant
Hence a parabola, with vertical axis, the rate of change of slope (or) rate of change of grade is
uniform (or a constant), which ensures the comfortable movement of the vehicle.
Types of Vertical Curve:
1. Summit Curve (crest or converse curves)
2. Valley curves (sag curves)
1. Summit Curves:
Summit Curves are defined as the convex type of vertical curves; when two grades are meet a
‘summit’.
When a fast moving vehicle is travelling along a summit curve, the centrifugal force acts upwards
against gravity and hence a part of the pressure on the tyres and spring of the vertical is relieved. The
deviation angle on roads is quite small and hence, the problem of causing discomfort to the passen-
gers does not arise seriously on summit curves. However, adequate sight distance should invariably
be provided on the summit curves to grant safety and to avoid contacts. The summit curves are
shown in figure.
38
Summit curves are introduced at,
A positive grade meets a negative grade
A positive grade meets an another smaller positive grade
A positive grade meets a level stretch
A negative grade meets a steeper negative grade.
2. Valley Curves:
Valley curves are defined as the type of vertical curves, when two grades meet ‘sag’.
The centrifugal force generated by the vehicle moving on a valley curve acts in the same direc-
tions as the weight of the vehicle. The length of the vertical curve depends upon the algebraic differ-
ence by the rate of change of grade. The following figure shows the valley curves.
39
Valley curves are introduced at,
A negative grade meets a positive grade
A negative grade meets an another smaller negative grade
A negative grade meets a level stretch
A negative grade meets a steeper positive grade.
Design of Vertical curves:
Assumptions in the Design of Vertical curves:
a. The length of the curve is equal to the length of the road.
b. The portions of the curve along the two tangents, on either side of the point of intersection are
equal.
c. The angels subtended by the tangents with horizontal are very small.
Design of Summit Curve
The centrifugal force along a summit curve acts in a direction opposite to the direction in
which its weight acts. The design of summit curve (the length of the summit curve) is decided by
the choice of sight distance, whether SSD or PSD (Passing Sight Distance).
40
From T1 and T2 are tangent points and n1 and n2 are the grades of tangents. The equation of the
parabolic summit curve is given by,
At P, Point x = L
y= L2
N
Substituting we get,
Where, L = Length of the curve
N = Deviation angle (n1+n2)
1. Length of Summit for SSD
Case I: (L > SSD)
The length of the summit curve L is given by,
Where, H = 1.2 m (Height of the driver’s eye)
h = 0.1 m (Height of the object)
S = Stopping Sight Distance
41
Case II: (L < SSD)
The length of the summit curve L is given by,
and H = 1.2 m and h = 0.1 m as same as above, we get,
L=2 S− 4N
2. Length of Summit Curve for PSD
Case I: (L > S)
From the equation,
and H = 1.2 m and h = 1.2 m as same as above, we get,
L= NS 29.6
Where, S = Required passing sight distance
Case II: (L < S)
From the equation
and H = 1.2 m and h = 0.1 m as same as above, we get,
Design of Valley Curves:
The centrifugal force at these curves acts downwards along the weight of the vehicle. The cu-
bic parabola serves for this purpose as a best valley curve. The design of a valley curve is depending
on the following four important factors.
The vehicle headlight sight distance
Motorists comfort
Drainage control
General aesthetic factors
42
As per the IRC Recommendations, the length of the valley curve selected should be in such a
way that, the distance of the headlight beam for night time is equal to the stopping sight distance. The
valley curves should be designed as a square parabola and according with the IRC conditions, the
length of the Valley curve can be calculated as follows.
Assumptions in the Design of Valley Curves:
The height of the headlight is 0.75 m
Upward divergence of the light beam from the longitudinal axis of the vehicle is 10
The minimum length of the curve should not be less than 0.5 v, where ‘v’ is the design speed
in kmph.
Length of the Valley Curve
Case I: (L > S)
Case II: (L < S)
Where, L = Length of the valley curve (in m)
S = SSD (in m)
N = Deviation angle (n1-n2)
GRADIENTS
Gradient is the rate of rise or fall along the length of the road with respect to the horizontal. It
is expressed as a ration of 1 in x (1 vertical unit to x horizontal unit). Sometime it is expressed as per -
centage. While aligning a highway, the gradient is decided for designing the vertical curve. Before fi-
nalizing the gradients, the construction cost, vehicular operation cost and the practical problems in
the site also has to be considered. Usually steep gradients are avoided as far as possible because of
the difficulty to climb and increase in the construction cost.
Effect of gradient
The effect of long steep gradient on the vehicular speed is considerable. This is particularly
important in roads where the proportion of heavy vehicles is significant. Due to restrictive sight dis-
43
10. EXPLAIN IN DETAIL ABOUT GRADIENTS.
tance at uphill gradients the speed of traffic is often controlled by these heavy vehicles. As a result,
not only the operating costs of the vehicles are increased, but also capacity of the roads will have to
be reduced. Further, due to high differential speed between heavy and light vehicles, and between up-
hill and downhill gradients, accidents abound in gradients.
Representation of gradient
The positive gradient or the ascending gradient is denoted as and the negative gradient as
. The deviation angle is: when two grades meet, the angle which measures the change of direc-
tion and is given by the algebraic difference between the two grades
. Example: 1 in 30 = 3.33% is a steep gradient, while 1
in 50 = 2% is a flatter gradient. The gradient representation is illustrated in the figure 1.
Figure 1: Representation of gradient
Gradients are categories into the following categories. They are,
Rulling Gradient
Limiting Gradient
Exceptional Gradient
Minimum Gradient
Rulling Gradient
It is the maximum gradient within which the designer attempts to design the vertical profile of
a road. Gradients up to the rulling gradients are adopted as a normal course in design of vertical
alignment and accordingly the quantities of cut and fill are balanced.
Hence rulling gradient is also known as design gradient. However flatter gradients may be
preferred where ever practicable. The selection of rulling gradient for the purpose of design is a com-
plex job as several factors such as type of terrain, the length of the grade, the speed, pulling power of
vehicles and presence of horizontal curves are considered.
IRC has recommended rulling gradient values of 1 in 30 on plain and rolling terrain, 1 in 20
on mountainous terrain and 1 in 16.7 on steep terrain.
44
Limiting gradient
This gradient is adopted when the ruling gradient results in enormous increase in cost of con-
struction. On rolling terrain and hilly terrain it may be frequently necessary to adopt limiting gradi-
ent. But the length of the limiting gradient stretches should be limited and must be sandwiched by ei -
ther straight roads or easier grades.
Exceptional gradient
Exceptional gradient are very steeper gradients given at unavoidable situations. They should
be limited for short stretches not exceeding about 100 meters at a stretch. In mountainous and steep
terrain, successive exceptional gradients must be separated by a minimum 100 meter length gentler
gradient. At hairpin bends, the gradient is restricted to 2.5%.
Minimum gradient
This is important only at locations where surface drainage is important. Camber will take care
of the lateral drainage. But the longitudinal drainage along the side drains requires some slope for
smooth flow of water. Therefore minimum gradient is provided for drainage purpose and it depends
on the rain fall, type of soil and other site conditions. A minimum of 1 in 500 may be sufficient for
concrete drain and 1 in 200 for open soil drains are found to give satisfactory performance.
Critical length of the grade
The maximum length of the ascending gradient which a loaded truck can operate without un-
due reduction in speed is called critical length of the grade. A speed of 25 kmph is a reasonable
value. This value depends on the size, power, load, grad-ability of the truck, initial speed, final desir -
able minimum speed etc.
Creeper lane
When the uphill climb is extremely long, it may be desirable to introduce an additional lane so
as to allow slow ascending vehicles to be removed from the main stream so that the fast moving vehi-
cles are not affected. Such a newly introduced lane is called creeper lane. There are no hard and fast
rules as when to introduce a creeper lane.
But generally, it can be said that it is desirable to provide a creeper lane when the speed of the
vehicle gets reduced to half the design speed. When there is no restrictive sight distance to reduce the
speed of the approaching vehicle, the additional lane may be initiated at some distance uphill from
the beginning of the slope.
But when the restrictions are responsible for the lowering of speeds, obviously the lane
should be initiated at a point closer to the bottom of the hill. Also the creeper lane should end at a
point well beyond the hill crest, so that the slow moving vehicles can return back to the normal lane
45
without any danger. In addition, the creeper lane should not end suddenly, but only in a tapered man-
ner for efficient as well as safer transition of vehicles to the normal lane.
Grade compensation
While a vehicle is negotiating a horizontal curve, if there is a gradient also, then there will be
increased resistance to traction due to both curve and the gradient. In such cases, the total resistance
should not exceed the resistance due to gradient specified.
So if a curve need to be introduced in a portion which has got the maximum permissible gra-
dient, then some compensation should be provided so as to decrease the gradient for overcoming the
tractive loss due to curve.
Thus grade compensation can be defined as the reduction in gradient at the horizontal curve
because of the additional tractive force required due to curve resistance ( ), which is in-
tended to offset the extra tractive force involved at the curve.
IRC gave the following specification for the grade compensation.
1. Grade compensation is not required for grades flatter than 4% because the loss of tractive
force is negligible.
2. Grade compensation is %, where is the radius of the horizontal curve in meters.
3. The maximum grade compensation is limited to %.
(NOV/ DEC 2011), (MAY/JUNE 2013)
SPECIAL CONSIDERATION FOR HILL ROADS
A hill road is the one, which passes through the mountainous or steep terrain. Border Road
Development Board (BRDB) classified the hill roads, as follows
Class 9 roads
Class 5 roads
Class 3 roads
Construction of Hill Roads
Construction of Hill Road is a difficult, time consuming and a hazardous work. It is also very
difficult to maintain the stability of hill roads. The reason is in hills, the road stability may be affected
by various factors, as follows:
Type of Rock
46
11. Explain the factors influencing the geometric design of hill roads.
Inclination or dip of the strata
Presence of ground water, etc
When the embankment is constructed on hill side slopes, it is necessary to bench the surface
of the hill slope with a height of 0.5 m and width of 1.5 m to 3 m to add the stability.
The important stages of construction on the hill roads are as follows:
a) Reconnaissance
b) Trace out
c) Detailed surveys
d) Formation cuttings
e) Drainage and protective works
f) Pavement design and construction
Geometric Design of Hill Roads
The geometric design of hill roads is generally different from the highways in plain terrain.
The following factors affect the geometric design of highway
Type of the surface to be provided
Topography of the area
Types of the vehicle using the road
Total daily tonnage
Difficulty in construction
The following aspects are to be considered for the geometric design of highway in hilly roads.
1. Widths of carriageway, shoulder, roadway and land
The following table shows the widths of carriage way, shoulder, roadway and land for differ-
ent types of highways. The following factors are to be considered while selecting the widths in the
geometric design of hill roads.
a) The minimum setback for building line beyond the right of way should be 5m in normal
cases and 3m in exceptional situations.
b) For roads subjected to heavy rainfall and snowfall, an additional roadway width of 1.5 m
may be increased in MDR, ODR, and VR.
c) The separate side drains and parapets are to be provided in the roadway widths
S.No Types of the high-
way
Carriageway
Width (m)
Shoulder
Width (m)
Roadway
Width (m)
Roadway Width (m)
Normal Exceptional
1 NH and SH
a) Single lane 3.75 2 x 1.25 6.25 24 18
b) Double lane 7 2 x 0.90 8.80 24 18
47
2 MDR 3.75 2 x 0.50 4.75 18 15
3 ODR 3.75 2 x 0.50 4.75 15 12
4 VR 3 2 x 0.50 4 9 09
2. Stopping Sight Distance (SSD)
By using the following expression, the SSD for hill road can be calculated.
Where, V = Design speed of the vehicle (in kmph)
t = Total reaction time of the driver (in sec)
= 2 to 3 secs
f = Coefficient of friction
= Normally 0.4
The following table shows the value of SSD for various design speeds on hill roads recom-
mended by,
S.No SSD (m) Design Speed (kmph)
1 20 20
2 30 25
3 35 30
4 50 40
5 70 50
3. Overtaking Sight Distances
By using the following equation, the OSD for hill roads can be calculated.
Length of OSD = 0.278 V2 t +0.278V2 T +2S+0.278VT
Where,
V = Speed of the overtaking vehicle (in kmph)
V2 = Speed of the overtaken vehicle (in kmph)
= V – 16
t = Total reaction time of the driver
= normally 2 to 3 secs
48
S = Spacing between vehicles (in m)
= (0.2 V2 +6)
T = Overtaking time (in sec)
= √ 14.4 Sa
a = Acceleration of overtaking vehicle (in kmph/sec)
4. Super-elevation
The super-elevation (e) provided in the hill roads can be calculated as follows:
e= V 2225 R
Where, V = Design speed (in kmph)
R = Radius of curve (in m)
IRC has recommended that the super- elevation for hill roads should not exceed 7% and max-
imum of 10%.
5. Radius of horizontal curve
The minimum radius of the horizontal curve for a hill road can be calculated by,
Radiusof curve , R=0.008 V 2e+ f in m
Where, V = Design speed (in kmph)
e = Super- elevation rate
= ‘0’ for snow bound area
= 0.10 for snow free area
f = coefficient of lateral friction = 0.15
The following table shows the minimum radii of curves in hill roads for various types of hill
roads.
S.No Type of the
highway
Minimum radius (m)
Mountainous terrain Steep terrain
Snow free area Snow bound area Snow free area Snow bound area
1 NH &SH 50 60 30 33
2 NDR 30 33 14 15
3 ODR 20 20 14 15
4 VR 14 14 14 15
6. Extra widening of curve
49
The extra width of carriageway required at the curve is calculated from the following equa-
tion.
Extra width, We=( 18 nR )+(0.1 V
√R )Where, n = number of lanes
R = Radius of curve (in m)
V = Design speed (kmph)
7. Camber
Flatter camber may be provided, if the longitudinal gradient greater than 1 in 20. Otherwise,
the steeper camber may be adopted given in the following table, recommended by IRC.
S.N
o
Types of Surface Camber
1 Sub-grades, earth roads and shoulders 3 to 4
2 Gravel and WBM surface 2.5 to 3
3 Thin bituminous surface 2.5
4 High type bituminous surface and C.C surface 2
8. Gradients
Ruling gradient of 6% and limiting gradient of 7% may be taken and in critical circumstances
the maximum gradient of 7% is provided.
HAIRPIN BENDS
A hair pin bend is defined as the three centred compound curve, which is designed as the cir-
cular curve with transition curve at each end, located on a hill side having the minimum slope with
maximum stability. For reducing the construction problems and expensive protection works, the hair-
pin bends should be provided with long arms and spacing.
The design standards of a hairpin bend
1) Minimum design speed - 20 kmph
2) Minimum radius of the inner curve - 14m
3) Minimum length of the transition - 15m
4) Minimum gradient - 1 in 200
5) Maximum gradient - 1 in 40
6) Minimum width of carriageway at apex of the curve
a) NH and SH
50
12. Define hairpin bends. Write its design standards
i) Single lane - 11.5m
ii) Double lane - 9m
b) MDR and ODR - 9.5m
c) VR - 6.5m
7) Minimum standard length between two successive hairpin bends – 60 m
8) Approach gradient - should not be steeper than 5% for 40m
9) Super-elevation - 1 in 10
LATERAL AND VERTICAL CLEARANCE AT UNDERPASSES
Underpass
Underpass implies a short passage beneath a grade-separated structure to carry one or more
streams of traffic.
Lateral clearance
Lateral clearance is the distance between the extreme edges of the carriageway to the face of
the nearest support whether it is a solid abutment, pier or column.
Vertical clearance
Vertical clearance stands for the height above the highest point of the travelled way, i.e., the
carriageway and part of the shoulders meant for vehicular use, to the lowest point of the overhead
structure.
Vertical clearance at underpasses shall be at least 5 metres. However, in urban areas, this
should be increased to 5.50 metres so that double-decker buses could be accommodated.
51
13. What do you mean by vertical Clearance? Explain it briefly.
Overall Considerations
Conscious effort must be made to create a sense of freedom for the drivers travelling through
the underpass. As far as possible, the underpass roadway should conform to the natural lines
of the highway at the approaches as regards alignment, profile and cross-section. Road profile
should not dip too sharply under the structure as that will produce a considerably enhanced
sense of restriction when compared with a profile that proceeds smoothly through.
To promote a feeling of openness and unrestrained lateral clearance, preferably structures
with open-end spans should be employed. Fig. 1. Where it becomes inescapable to have struc-
tures with solid abutments, these should be set back from the roadway edge as much as possi-
ble, Fig. 2. From considerations of cost, these treatments are meant for higher categories of
roads, especially with divided carriageways.
Since width at an existing underpass cannot be easily increased later on, initial construction
should be sufficient for the standards to which the underpass roadway would need to be im-
proved within the near future. This is essential especially for important routes like National
and State Highways to be widened soon from single-lane to two-lane standards, as also busy
two-lane roads which are in the planning stage for being upgraded to a four-lane divided
cross-section.
Protect vehicles from accidents with abutments or piers. Guard-rails must be provided at a
suitable height. These should be of a robust design to effectively resist disturbance of the sup-
port in the event of a collision. In addition, ends of the guard-rails should be turned away
from the line of approaching traffic, as shown in Fig. 3, so as to deflect runaway vehicles
which may otherwise hit the underpass structure. As a general rule, guard-rails must be pro-
vided on both sides of the central piers or columns, though these could be dispensed with on
the abutment side when a raised footpath forms part of the cross-section.
52
Lateral Clearance on Rural Roads
Single Carriageway
Desirably the full roadway width at the approaches should be carried through the underpass.
This implies that the minimum lateral clearance on either side must equal the shoulder width. This
rule should be relaxed only in exceptional circumstances.
Normal and exceptional values of lateral clearance for different classes of highways are given
below (see Fig. 4a) :
(i) National and State Highways - Normal 2.5 metres; (exceptional 2.0 metres)
(ii) Major District and Other - District Roads Normal 2.0 metres (exceptional 1.5 metres)
(iii) Village Roads - Normal 1.5 metres (exceptional 1.0 metre)
If a footpath is needed on a rural road, lateral clearance in the underpass portion should be the
width of the footpath plus one metre, Fig.4 (b). Footpath width depends upon the expected pedestrian
traffic and might be fixed with the help of following capacity guidelines, subject to not being less
than 1.5
metres :
53
Divided Carriageways
Lateral clearance on the right to a pier or column in the central median shall be 2 metres
desirably, and 1.5 metres at the minimum. Where the central median is ker.bed, the carriageway
width should be increased by the side safety margin of 0.5 metre as shown in Fig. 4 (c).
Lateral clearance in that event could be reduced to 1.5 metres (desirable value) or 1 metre
(exceptional). If the median is not wide enough to permit these clearances, either it should be
widened gradually at the approaches or a single span structure provided across the full cross-section
thereby avoiding a central pier.
Lateral Clearance on Urban Roads
Single Carriageways
Usually roads in urban areas are bordered by kerbs on both sides. If so, these should be
extended across the underpass. However, to offset the effect of kerb shyness, the carriageway in the
54
underpass area should be widened on both sides by the side safety margin of 0.25 metre in the case of
lower category urban roads and 0.5 metre in the case of higher category urban roads Fig. 5(a).
Divided Carriageways
Right lateral clearances to the face of any structure in the central median over and above the
side safety margin shall be at least 1 metre in the case of higher category urban roads and 0.5 metre in
the case of lower category urban roads, Fig. 5 (c).
55
