Importance, highway cross section elements, sight distance, design of horizontal alignment, design of vertical alignment, design of intersection

1. “HIGHWAY
GEOMETRIC DESIGN”
1
PREPARED BY : ASST. PROF. VATSAL D. PATEL
MAHATMA GANDHI INSTITUTE OF
TECHNICAL EDUCATION &
RESEARCH CENTRE, NAVSARI.

2. IMPORTANCE OF GEOMETRIC DESIGN
 Highway geometric design consists of determining the
physical dimensions of various elements needed for the
highway based on scientific and practical requirements.
 The geometrics of highway should be designed to provide
efficiency in traffic operation with maximum safety at
reasonable cost.

3. HIGHWAY GEOMETRIC DESIGN
 Highway geometric design includes the following elements:
 Cross-section elements
 Sight distance considerations
 Horizontal alignment details
 Vertical alignment details
 Intersection elements
 Vehicle dimensions and permissible axle loads
 Cross drainage works viz. bridges, culverts etc.
 Accommodation of utility services within the right of way

4. FACTORS AFFECTING HIGHWAY
GEOMETRIC DESIGN
 Design speed
 Topography
 Traffic factors
 Design hourly volume and capacity
 Environmental factors

5. FACTORS AFFECTING HIGHWAY
GEOMETRIC DESIGN
 Design speed :
 The maximum safe speed of vehicles used for the road
geometric design is known as design speed.
 Design speed is important for the following:
 Sight distance
 Super elevation
 Radius of horizontal curve
 Length of summit and valley curves
 Extra widening of pavement

6. HIGHWAY GEOMETRIC DESIGN
 Topography :
 The topography or the terrain conditions influence the
geometric design of highway significantly.
 Based on the cross slope of the country across the
alignment, the terrains are classified as under :
Terrain
classification
Cross slope of
the Country (%)
Plain 0-10 %
Rolling 10-25 %
Mountainous 25-60 %
Steep Greater than 60 %

7. FACTORS AFFECTING HIGHWAY
GEOMETRIC DESIGN
 Traffic factors :
 The following traffic factors affect the geometric design :
 Vehicular characteristics
 Road user characteristics

8. FACTORS AFFECTING HIGHWAY
GEOMETRIC DESIGN
 Environmental factors :
 Aesthetics
 Landscaping
 Air pollution
 Noise pollution

9. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 The cross-section of road is shown in figure.

10. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 The various components of highway cross-section are as :
 Carriage way :
 The width of pavement way on which vehicles travel is
called ‘Carriage way’ or ‘pavement width’.
 Width of carriage way is given in table.
Sr.
No.
Class of road Width of
carriage way
1 Single lane 3.75 m
2 Two lanes (without raised kerbs) 7.00 m
3 Two lanes (with raised kerbs) 7.50 m
4 Multi-lane Pavements 3.05 m per lane

11. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Formation width :
 Formation width (road way) is the top width of the highway
embankment or the bottom width of cutting excluding the
side drains.
 Formation width = Width of carriage way + Width of
shoulders

12. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Width of formation :
Sr.
No.
Road
Classification
Width of formation in m.
Plain and Rolling terrain Mountainous terrain
1. NH & SH
Single lane 12.0 6.25
Two lanes 12.0 8.80
2. Major District Roads
Single lane 9.0 4.75
Two lanes 9.0 -
3. Other District Roads
Single lane 7.5 4.75
Two lanes 9.0 -
4. Village roads- Single 7.5 4.0

13. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Right of way :
 Right of way is the area of land acquired for the road, along
its alignment. It is the distance between the boundary
stones on either side of the road.

14. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Road shoulders :
 Shoulders are provided along the road edge to serve as
emergency lane for vehicles.
 As per IRC, the minimum width of shoulders should be
2.5m.

15. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Side slope :
 The slope of earthwork in filling (embankment) or in
cutting is called side slope.
 Side slope imparts stability to the earthwork. It should be
as flat as possible for the purpose of safe traffic movement
and also for aesthetic reasons. But in case of flatter slopes
amount of earthwork increases on the other side in steeper
slopes erosion of soil will be more.

16. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Side slope (for embankment) :
 Normally,1:2 (1 vertical,2 horizontal) side slope is provided.
 Side slope (for cutting) :
 For cutting, side slope is provided as under :
Sr. No. Type of soil Slope
1. Ordinary soil 1:1 to 1:1/2
2. Broken rock 1:1/2 to 1:1/4
3. Soft rock 1:1/4 to 1:1/8
4. Hard rock Approximately
perpendicular

17. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Berm :
 The distance between the road toe and the inner edge of
borrow pit is called berm.
 It prevents the erosion of embankment soil.

18. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Boundary stone :
 To indicate the boundary of land acquired for road, stones
are driven into the ground at about 30 m distance on either
side from the centre line of the road. These stones are
known as boundary stones.

19. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Side drain :
 For the drainage of rain water, drains are provided on
either side of the road.
 Normally, side drains are required for road in cutting. For
road in embankment, side drain is not necessary.

20. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Building line :
 The distance from the centre line of road on either side
within which construction of buildings is not permitted is
called building line.

21. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Control line :
 At the locations like bank, hospital, factory, theatre, etc. on
the road, where more people gather disturbance to the
traffic will be more.
 The distance from the centre line of road to such building is
called control line.

22. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Spoil bank :
 The bank constructed from surplus excavated earth on the
side of road cutting parallel to its alignment, are known as
spoil banks.
 The soil from spoil bank can be used for the repair of
shoulders.

23. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Borrow pits :
 The pits dug along the road alignment for using excavated
earth in the construction of embankment, are known as
borrow pits.

24. CROSS-SECTION OF ROAD AND ITS
ELEMENTS
 Kerbs :
 The boundaries between pavement and shoulders of
footpaths are known as kerbs.

25. TYPES OF KERBS
 Low or mountable kerb :
 It allow the driver to enter the shoulder area with little
difficulty.
 The height of the this type of shoulder kerb is about 10 cm
above the pavement edge with slope to help the vehicle
climb the kerb easily.

26. TYPES OF KERBS
 Semi-barrier kerb :
 It is provided on the periphery of a roadway where the
pedestrian traffic is high.
 Height of about 15 cm above the pavement edge with a
batter of 1:1 on the top 7.5 cm.
 It prevents parking the vehicle but during emergency it is
possible to drive over this kerb with some difficulty.

27. TYPES OF KERBS
 Barrier type kerb :
 It is provided in built-up area adjacent to the foot paths
with considerable pedestrian traffic.
 The height of the kerb is about 20 cm above the pavement
edge with a steep batter of 1V:0.25H.

28. CROSS-SECTION OF DIFFERENT
TYPES OF ROADS AS PER IRC

29. CROSS-SECTION OF DIFFERENT
TYPES OF ROADS AS PER IRC

30. CAMBER
 Camber or cross slope is the slope provided to the road
surface in the transverse direction to drain off rain water
from the surface.

31. CAMBER
 Purpose of providing camber :
 To remove the rain water from the pavement surface as
quickly as possible.
 To prevent entry of water into bituminous pavement layers.
 To prevent entry of surface water into sub grade soil
through pavement.
 To make pavement surface attractive

32. CAMBER
 Camber for different road surfaces :

33. SIGHT DISTANCE
 Sight distance available from a point is the actual distance
along the road surface, which a driver from a specified
height above the carriageway has visibility of stationary or
moving objects.
 It is the length of road visible ahead to the driver at any
instance.

34. SIGHT DISTANCE
 There are three types of sight distance :
1. Stopping sight distance
2. Overtaking sight distance
3. Site distance at intersection

35. STOPPING SIGHT DISTANCE
 The minimum sight distance available on a highway to stop
a vehicle travelling at design speed , safely without collision
with any other obstruction is called stopping sight distance.
 For the purpose of measuring stopping sight distance, IRC
has suggested the height of eye level of driver as 1.2 m and
the height of the object as 0.15 m above the road surface.

36. STOPPING SIGHT DISTANCE
 Factors affecting SSD are :
1. Speed of vehicle
2. Efficiency of brakes
3. Total reaction time of driver
4. Frictional resistance between the road and the tyres
5. Gradient of the road.

37. STOPPING SIGHT DISTANCE
 Analysis of SSD :
 The stopping sight distance of a vehicle is the sum of:
1. Lag distance
2. Braking distance
 Lag distance:
 The distance travelled by the vehicle during the total
reaction time.
 Lag distance = v * t
v = speed of vehicle
t = total time reaction (as per IRC,total reaction time = 2.5)

38. STOPPING SIGHT DISTANCE
 Braking distance:
 The time taken from the instant the object is visible to the
driver to the instant the barkers are effectively applied.
 Braking distance= v²/2gf
 Where, v = velocity of the car (m/s)
f = coefficient of friction (unit less)
g = acceleration due to gravity (9.80 m/s2)
 SSD=lag distance + braking distance
= v * t + v²/2gf

39. OVERTAKING SIGHT DISTANCE
 The minimum distance open to the vision of the driver of a
vehicle intending to overtake slow vehicle ahead with safety
against the traffic of opposite direction is known as
overtaking sight distance.
 The overtaking sight distance is the distance measured
along the centre of the road 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.

40. OVERTAKING SIGHT DISTANCE
 OSD depends upon :
1. Speed of overtaking vehicle ,speed of overtaken vehicle and
the speed of vehicle coming from opposite direction
2. Distance between overtaking vehicle and overtaken vehicle
3. Skill and reaction time of the driver
4. Rate of acceleration of overtaking vehicle
5. Gradient of the road.

41. SIGHT DISTANCE AT INTERSECTIONS
 It is very important that on all approaches of intersecting
roads, there is a clear view across the corners from a
sufficient distance so as to avoid collision of vehicles.
 It is very important at uncontrolled intersections.
 The site line obstructed by structures or other formed by
the lines of vision is called the sight triangle.

42. SIGHT DISTANCE AT INTERSECTIONS
 The design of sight distance at intersections may be based
on three possible conditions :
1. Enabling the approaching vehicle to change speed.
2. Enabling approaching vehicle to stop.
3. Enabling stopped vehicle to cross a main road.

43. DESIGN SPEED
 The maximum safe speed of vehicles used for highway
geometric design is known as design speed.
 In cumulative speed distribution curve , 98% speed is taken
as the design speed.
 Factors affecting design speed :
 Class of road
 Class of terrain
 Curves on the road
 Type of road surface
 Intensity & nature of traffic
 Condition of road surface

44. DESIGN SPEED
 Design speeds :
Sr
No.
Type
of
Road
Design speed in km.ph for various terrains
Plain Rolling Mountainous Steep
Rulling Min. Rulling Min. Rulling Min. Rulling Min.
1.
2.
3.
4.
NH or
SH
MDR
ODR
VR
100
80
65
50
80
65
50
40
80
65
50
40
65
50
40
35
50
40
30
25
40
30
25
20
40
30
25
20
30
20
20
20

45. SUPER ELEVATION
 It is the slope across pavement surface and is fully
developed in the circular curve.
 Super-elevation (banking) is the transverse slope provided
at horizontal curve to counteract the centrifugal force, by
raising the outer edge of the pavement with respect to the
inner edge, throughout the length of the horizontal curve.

46. SUPER ELEVATION
 So super elevation helps the vehicle to over come the
centrifugal force on the curves on pavements.
 The need for super-elevation on road curves, to ensure
safety against skidding and over turning with the advent of
fast moving traffic.

47. TYPES OF CURVES
 There are two types of curves in highway geometric design :
 Transition curves OR Horizontal curves
 Vertical curves
 Transition OR Horizontal curves :
 A transition curve is a curve which is provided between
straight & circular or between two compound curves or
between two reverse curves.
 Types of transition curve :
 Spiral or clothoid
 Lemniscate
 Cubic parabola

48. TYPES OF CURVES
 Objectives for providing transition curve :
 To introduce gradually the centrifugal force between the
tangent point and the beginning of the circular curve,
avoiding sudden jerk on the vehicle. This increases the
comfort of passengers.
 To enable the driver turn the steering gradually for his own
comfort and security.
 To provide gradual introduction of super elevation.
 To provide gradual introduction of extra widening.
 To enhance the aesthetic appearance of the road.

49. TYPES OF CURVES
 Vertical curves :
 Due to change in grade in vertical alignment of highway, it
is necessary to introduced vertical curves at the intersection
of different grades to smoothen out the vertical profile.
 Object of vertical curves :
 To introduced change of grade gradually.
 To provide safety and visibility.
 To provide comfort to the passengers.

50. TYPES OF CURVES
 Types of vertical curves :
 Vertical curve are classified into two categories :
 Summit curves
 Valley curves
summit
valley

51. WIDENING OF CURVES
 On horizontal curve especially when they are not of very
large radial, it is common practice to widen the pavement
slightly more then the normal width. This is called
Widening on curves.
 The extra widening on the horizontal curves divided into
two parts :
a. Mechanical widening
b. Psychological widening

52. WIDENING OF CURVES
 Mechanical widening :
 The widening required to account for the off-tracking due
to the rigidity of wheel base is called mechanical widening
(wm).
 Wm =nl2 /2R
 Where ,wm = Mechanical widening
n = Number of lanes
l = Length of wheel base (m)
R = Mean radius of curve (m)

53. WIDENING OF CURVES
 Psychological Widening :
 Extra width of pavement is also provided for psychological
reasons such as to provide for greater manoeuvrability 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
curves.
 Wps = V/9.5√R
 Where , V = design speed in km.ph

54. WIDENING OF CURVES
 Total widening = Wm + Wps
We = nl2 / 2R + V / 9.5 √R
We = 18n / R + 0.1V / √R
Where , l = wheel base = 6m

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