its is a part of transportation engineering.

1. UNIT –III
Geometric Design of
highway
Dr. RAM VILAS MEENA
Assistant Professor (Civil Engineering
Dept.) JECRC university Jaipur
B.Tech , M.Tech, Ph.D MNIT Jaipur
Transportation Engineering

2. What is Geometric Design of
Highways?
• Geometric design deals with the visible elements of a
highway.
• It is concerned with the positioning of the visible elements
according to the standards and constraints.
• It includes the design elements of cross-section component,
sight distance, horizontal and vertical alignment etc.

3. Basic Principles Of
Geometric Design
• To develop a uniform practice to achieve optimum design
standards for rural roads.
• Geometric features do not allow stage construction.
• Improvements of features like grade , curvature , widening of
CD at later stage is expensive and sometimes impossible in
hilly and remote area.
Therefore, Ultimate geometric requirements to be kept in view
right from the beginning.

4. • Design standards are absolute minimum.
• Minimum values shall only be applied if technical or economical
considerations are there.
• General effort is to exceed minimum values.
• Geometric design also affects "livability," which is defined as
designing roads to foster broader community goals, including
providing access to employment, schools, businesses and
residences, accommodate a range of travel modes such as
walking, bicycling, transit, and automobiles, and minimizing fuel
use, emissions and environmental damage.

5. • Rural roads refer commonly to Other District Roads and
Village Roads.
• Other District Roads are roads serving rural areas of
production and providing them with outlet to market
centres, tehsil headquarters, block development
headquarters, or other main roads.
• Village Roads are roads connecting villages or group of
villages with each other and to the nearest road of a
higher category.
Rural Roads

6. • The geometric design of a highway is influenced significantly
by terrain conditions.
• Terrain is classified by the general slope of the country
across the highway alignment.
Terrain Classification Cross Slope of the country
Plain 0-10% More than 1 in 10
Rolling 10-25% 1 in 10 to 1 in 4
Mountainous 25-60% 1 in 4 to 1 in 1.67
Steep Greater than 60% Less than 1 in 1.67
Terrain Classification

7. Design Speed (in kmph)
Road
classification
Plain terrain Rolling
terrain
Mountainous
terrain
Steep terrain
Ruling Min. Ruling Min. Ruling Min. Ruling Min.
Other District
Roads 50 40 40 35 25 20 25 20
Village Roads 50 40 40 35 25 20 25 20
• It is the basic parameter which determines all other
geometric design features.
• Choice of design speed depends on the function of the
road as also terrain conditions.

8. • Ruling design speed should be the guiding criteria for
correlating the various geometric design features.
• Minimum design speed may be adopted in sections
where site conditions ,including costs, do not permit a
design based on the ruling design speed.
• The 98th percentile speed in the graph of cumulative
speed of vehicle is taken for the purpose of highway
geometric design.

9. Cross-Sectional Elements

10. Road Land Width/
Right of Way
• Road land width (also known as Right of way) is the land
acquired for the road purposes, along its alignment.
Road classification
Plain and Rolling Terrain
Mountainous and Steep
Terrain
Open Area Built-up Area Open Area
Built-up
Area
Normal
(m)
Range
(m)
Normal
(m)
Range
(m)
Normal
(m)
Normal
(m)
Other District Roads
15 15-25 15 15-20 15 12
Village Roads 12 12-18 10 10-15 9 9

11. • If a road is expected to be upgraded to a higher classification
in the foreseeable future, the land width should correspond to
the latter.
•In high banks or deep cuts, the land width should be suitably
increased.

12. Building Line & Control Line
• Building line is defined by a hypothetical line set back from the
road boundary to restrict the building activity within a prescribed
distance from the road.

13. • It will be desirable to exercise control on the nature of
building activity for a further distance beyond the building
line up to Control lines.

14. Recommended Standards for Building Lines
and Control Lines
Road
Classificati
on
Plain and Rolling terrain Mountainous and Steep
Terrain
Open Areas Built-Up
Areas
Open Areas Built-Up
Areas
Overall
width
between
Building
Lines (m)
Overall
width
between
Control
Lines (m)
Distance
between
Building-line
and Road
boundary (m)
Distance between
Building Line and Road
Boundary (setback) (m)
Other
District
Roads
30 35 3-5 3-5 3-5
Village
Roads
25 30 3-5 3-5 3-5

15. Roadway Width
• Roadway width is the sum of widths of pavements or
carriageway including separators if any; and the shoulders.

16. Road Classification Roadway Width, m at :
Plain and Rolling
Terrain
Mountainous and
Steep Terrain
Other District Roads
-Single Lane Road
7.5 4.75
Other District Roads
-Two lane Road
9.0 -
Village Road
-Single Lane
7.5 4.00
Width Of Roadway for Rural-Roads

17. Roadway Width At Cross-
Drainage Structures
• Cross-drainage structures are difficult to widen at a later
stage. Therefore the roadwidth for them should be
decided very carefully at the planning stage itself
• The desirable roadway width at culvert locations in
mountainous and steep terrain is 4.25m.
• Roadway width at bridges between kerb is as:
Single-Lane Road : 4.25 m
Two-Lane Road : 7.5 m
Multi-Lane Road : 3.5 m per lane + 0.5 m per
carriageway

18. Carriageway Width
• The width of carriageway is taken as 3.75m.
• Carriageway width may be restricted to 3.0m, where traffic
intensity is less than 100 motorised vehicles per day and where
the traffic is not likely to increase due to situation, like dead
end, low habitation and difficult terrain condition.

19. Shoulder Width
• Shoulder width will be one-half the difference between
the roadway width and carriage way width.
• It is proposed to have {1.875 m to 1.5 m} wide shoulder
as the case may be on both sides of which at least
0.875m is hard shoulder where required.

20. Camber or Cross Slope
• It is the slope provided to the road surface in the
transverse direction to drain off the rain water from the
road surface.
• Usually the camber is provided on the straight roads by
raising the centre of the carriageway with respect to the
edges, forming a crown or highest point on the centre
line.

21. • The camber is given a parabolic elliptic, straight line, or
combination of both shape in the cross section.

22. Value of Camber
Surface type Camber (%)
Low Rainfall
Intensity
High Rainfall
Intensity
Earth road 3.0 4.0
WBM Gravel road 2.5 3.0
Thin bituminous road 2.0 2.5
Rigid Pavement or
High Type Bituminous Surfacing
1.7 2.0

23. Sight Distance

24. Sight Distance
• The safe and efficient operation of vehicles on the road
depends very much on the visibility of the road ahead of
the driver.
• Sight Distance available from a point is the actual
distance along the road surface, which is visible ahead
to the driver at any instance.
• Three types of sight distances are :
1) Stopping Sight Distance (SSD)
2) Overtaking Sight Distance (OSD)
3) Intermediate Sight Distance (ISD)

25. Stopping Sight Distance
• Stopping sight distance is the clear distance ahead
needed by a driver to bring his vehicle to a stop before
meeting a stationary object in his path on the road.
• Minimum stopping sight distance is given by the sum of :
i. Distance travelled during the perception and brake
reaction time and
ii. The braking distance.

26. Perception And Brake
Reaction Time
• It is the time interval between the instant the driver sights
a dangerous object for which a stop is necessary and the
instant the brakes are applied.
• A value of 2.5 sec is considered reasonable for most of
the situations.
• The distance travelled during this time is given as :

27. Braking Distance
• Braking distance is the distance required for a vehicle to
come to stop after the brakes are applied.
• On a level road, assuming friction remains constant
during deceleration, Braking Distance is given by :

28. • The braking distance formula amended to take the effect
of grades into account is :
• Minimum stopping sight distance is given by the sum of
the components d1 and d2.

29. Stopping Sight Distance for various speeds

31. Overtaking Sight Distance
• All vehicles do not move at the designated speed and in
such circumstances it is necessary for fast moving
vehicles to overtake or pass the slow moving vehicle.
• Overtaking Sight Distance is the minimum sight distance
that should be available to a driver to enable him to
overtake another vehicle safely.

32. • Design values for Overtaking Sight Distance :

33. • Dynamics of the overtaking operation is given in a
Time-Space diagram.

34. • The trajectory 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 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 vehicle B and occupies the left
lane at time t3.
• The time duration T = t3 - t1 is the actual duration of the
overtaking operation.

36. s
s
Vb X T d2

37. •Thus the overtaking sight distance is :

38. Overtaking Zones
• Overtaking Zones are provided when overtaking sight
distance cannot be provided throughout the length of the
highway.
• These are zones dedicated for overtaking operation.

39. • The desirable length of overtaking zones is five times
OSD and the minimum is three times OSD.

43. Intermediate Sight Distance
• The sections of roads where the customary overtaking
sight distance cannot be provided, should be designed
for Intermediate Sight Distance.
• Intermediate Sight Distance is defined as twice the
normal safe stopping distance i.e. (2 × S.S.D).
• It improves visibility appreciably.
• It give a chance to driver to overtake with caution.

44. • Recommended Values of Intermediate Sight Distance for
different speeds :

45. Headlight Sight Distance at
Valley Curve
• For night travel the design must ensure that the road-
way ahead is illuminated by vehicles headlight for a
sufficient length which enables the vehicle to brake to a
stop, if necessary.
This is known as the headlight sight distance.
• It is equal to the safe stopping sight distance.

46. Sight Distance At
Intersections

47. Sight Distance At
Intersections
• At intersections where two or more roads meet, the sight
distance should be provided such that the drivers on
either side should be able to see each other.
• Sight distance at intersections may be used on three
possible conditions:
i. Enabling approaching vehicle to change the speed
ii. Enabling approaching vehicle to stop
iii. Enabling stopped vehicle to cross a main road

48. Horizontal Alignment

49. Horizontal Alignment
• Often changes in the direction are necessitated in
highway alignment due to obligatory points.
• Obligatory are the control points governing the alignment
of highway for e.g. religious place, costly structure,
unsuitable land, bridge site, intermediate town, hill range,
high ridges etc.
• The alignment should enable consistent, safe and
smooth movement of vehicles operating at design
speeds. It is hence necessary to avoid those sharp
curves and reverse curves which could not be
conveniently negotiated by vehicles at design speed.

50. • Various design factors to be considered in the horizontal
alignment are :
 Horizontal Curves
 Super elevation
 Radii of Horizontal Curves
 Transition Curves
 Widening of Carriageway on curves.
• Improper design of horizontal alignment of roads would
necessitate speed changes resulting in increased vehicle
operation cost and higher accident rate.

51. Horizontal Curves

52. Horizontal Curves
• In general, Horizontal curves should consist of a circular
portion, flanked by spiral transitions at both ends.

53. • Short curves for particularly small deflection angle
should be avoided. Curve length should be atleast 150m
for a deflection angle of 5 degrees and should be
increase by 30m for each one degree decrease in
deflection angle.
• Reverse curve, needed in difficult terrain, should have
sufficient length between the two curves for introduction
of requisite transition curve.
• Compound curve may be used in difficult topography but
only when it is impossible to fit in a single curve.

54. Super-Elevation
• It is a transverse inclination to the pavement surface.
• To counter act the effect of centrifugal force.
• To reduce the tendency of the vehicle to overturn or
skid.

55. α
Fc
W 1
e
≈
Rv

56. • The super-elevation can be obtained from this
expression:

57. Radii of Horizontal Curve
• On horizontal curve, the centrifugal force is balanced by
the combined effects of super-elevation and side friction.

58. • The basic equation for this condition of equilibrium is :
Where :
v = vehicle speed in m/s
V = vehicle speed in km/h
g = acceleration due to gravity in m/ s2
e = superelevation ratio in m/m
f = coeff. of side friction between vehicle tyres and
pavement
R = radius in m

59. Classification
Of Roads
Plain Terrain Rolling
Terrain
Mountanious
Terrain
Steep terrain
Areas
not
affected
By
Snow
Snow
Bound
Areas
Areas
not
affected
By
Snow
Snow
Bound
Areas
Ruling
Min.
Absolut
e Min.
Ruling
Min.
Absolut
e Min.
Ru
lin
g
Mi
n.
Ab
sol
ut
e
Mi
n.
Ru
lin
g
Mi
n.
Ab
sol
ut
e
Mi
n.
Ru
lin
g
Mi
n.
Ab
sol
ut
e
Mi
n.
Ru
lin
g
Mi
n.
Ab
sol
ut
e
Mi
n.
ODR 155 90 90 60 30 20 33 23 20 14 23 15
Village Road 90 60 60 45 20 14 23 15 20 14 23 15
Radii of Horizontal Curve

60. Horizontal Transition Curve
• A Transition Curve is the curve which is introduced
between a straight and circular curve.
• The radius of the transition curve decreases from infinity
at the tangent point to a designed radius of the circular
curve.
• The ideal shape of a transition curve should be such that
the rate of introduction of centrifugal force should be
consistent.

61. Functions of Transition
Curve
• To introduce gradually the centrifugal force between the
tangent point and the beginning of the circular curve,
avoiding a sudden jerk on the vehicle.
• To enable the driver turn the steering gradually for his
own comfort and security.
• To enable gradual introduction of the desired
superelevation and extra widening of pavement at the
start of circular curve.

62. • Minimum length of the transition curve should be
determined from the following two considerations and the
larger of the two values are adopted for design :

63. Extra Widening of Pavement
• Extra widening refers to the additional width of
carriageway that is required on a curved section of a
road over and above that required on a straight
alignment.
• This widening is done due to two reasons:
i. Mechanical widening
ii. Psychological widening

64. Mechanical Widening
• It is due to the additional width required for a vehicle
taking a horizontal curve.
• When a vehicle negotiates a horizontal curve, the rear
wheels follow a path of shorter radius than the front
wheels. This phenomenon is called off-tracking, and has
the effect of increasing the effective width of a road
space required by the vehicle.
• The extra widening of a road with n lanes is given by:

65. • Let R1 is the radius of the outer track line of the rear wheel, R2
is the radius of the outer track line of the front wheel l is the
distance between the front and rear wheel.

66. Psychological Widening
• There is a tendency for the drivers to drive close to the
edges of the pavement on curves.
• Some extra space is to be provided for more clearance
for the crossing and overtaking operations on curves.
• IRC proposed an empirical relation for the psychological
widening at horizontal curves Wps :

67. • The Extra Widening of Pavement at Curve as per IRC
guideline is given below:
Radius of Curve
(m)
Upto 20 21 - 60 Above 60
Extra Widening
for 3.75 m wide
single lane
carriageway, (m)
0.9 0.6 Nil

70. • Length of the transition curve is the highest of the above
values, therefore it taken as 35.1 m or 35 m

71. Vertical Alignment

72. Vertical Alignment
• The Vertical Alignment should provide for a smooth
longitudinal profile, consistent with category of the road
and lay of the terrain.
• The Vertical Alignment of a road consists of
 Gradients and
 Vertical curves

73. Gradient
• Gradient is the rate of rise or fall along the length of the
road with respect to the horizontal.
• The positive(ascending) gradient is denoted as +n and
the negative gradient as −n.

74. Types of Gradient
• Gradients are divided into the following category:
 Ruling gradient
 Limiting gradient
 Exceptional gradient and
 Minimum gradient

75. Ruling Gradient
• The Ruling Gradient or the design gradient is the
maximum gradient with which the designer attempts to
design the vertical profile of the road.
• This depends on the terrain, length of the grade, speed,
pulling power of the vehicle and the presence of the
horizontal curve.

76. Limiting Gradient
• This gradient is adopted when the ruling gradient results
in enormous increase in cost of construction.
• On rolling terrain and hilly terrain it may be frequently
necessary to adopt limiting gradient.

77. Exceptional Gradient
• Exceptional gradient are very steep 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 meters length gentler gradient.

78. 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 require 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

79. IRC Specifications for Gradients
Plain Terrain Rolling Terrain
Ruling Gradient
3.3 % 3.3 %
Limiting Gradient
5 % 5 %
Exceptional Gradient
6 % 6 %

80. 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 horizontal curve and the gradient.
• 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.

81. Vertical Curves

82. Vertical Curves
G1
G2
Valley Vertical Curve
• Vertical curves are introduced for smooth transition at
grade changes.
• Convex vertical curves are known as Summit Curves.
• Concave vertical curves are known as Valley Curves.
G1 G2
Summit Vertical Curve

83. Summit Curve
• Summit curves are vertical curves with gradient upwards.
• They are formed when two gradients meet in any of the
following four ways:
a) when a positive gradient meets another positive gradient

84. c) when an ascending gradient meets a descending
gradient
b) when positive gradient meets a flat gradient

85. d) when a descending gradient meets another
descending gradient.
• Generally parabolic curves are used in summit curves due
to following reason :
 The ease with it can be laid out and
 It allows a comfortable transition from one gradient to
another.

86. Design Consideration
• Sight distance requirements for the safety is most
important on summit curves.
• The stopping sight distance or absolute minimum sight
distance should be provided on these curves.
• Where overtaking is not prohibited, overtaking sight
distance or intermediate sight distance should be
provided as far as possible.

87. • The length of the vertical curves is controlled by sight
distance requirements.
G1
G2
h2
h1
Line of Sight
L
SSD

88. Length of the Summit Curve

89. •N is the deviation angle
•h1 driver's eye height (1.2 m) and
•h2 the height of the obstruction

91. Valley Curve
• Types of valley curve:
a) when a descending gradient meets another descending
gradient
b) when a descending gradient meets a flat gradient

92. • when a descending gradient meets an ascending gradient
• when an ascending gradient meets another ascending gradient

93. Design Consideration
• The most important design factors considered in valley
curves are:
• Impact-free movement of vehicles at design speed and
• Availability of stopping sight distance under headlight of
vehicles for night driving
• Cubic parabola is generally preferred in vertical valley
curves

94. G1
G2
h2=0
h1
L
Head Light Beam Distance (SSD)
•During night, under headlight driving condition, sight distance
reduces and availability of stopping sight distance under head
light is very important.
•The head light sight distance should be at least equal to the
stopping sight distance.

95. Length of the Valley Curve

96. Side slope
• Side slope for rural road (where embankment height is
less than 3.0m) is given in the table below:
Condition Slope (H:V)
Embankment in silty/sandy/gravel soil 2:1
Embankment in clay or clayey silt or inundated
condition
2.5:1 to 3:1
Cutting in silty/sandy/gravelly soil 1:1 to 0.5:1
Cutting in disintegrated rock or conglomerate 0.5:1 to 0.25:1
Cutting in soft rock like shale 0.25:1 to 0.125:1
Cutting in medium rock like sandstone, phyllite 0.083:1 to 0.0625:1
Cutting in hard rock like quartzite, granite Near vertical

97. References
• IRC : 73-1980 Geometric Design Standards For Rural
Highways
• IRC : 66-1976 Recommended Practice For Sight
Distance On Rural Highways
• PMGSY website
(www.pmgsy.nic.in/circulars/Annex%20III_210108.doc
24/1/2012)
• PMGSY DPR template
(www.pmgsy.nic.in/circulars/DPR_Template.doc
24/1/2012)
• PMGSY operations manual
(http://pmgsy.nic.in/downloads/opman_feb.pdf
28/1/2012)

98. THANK YOU.