By going through this slides we can understand the geometric design of roads.

1. UNIT- 3 GEOMETRIC DESIGN
HIGHWAY ENGINEERING
FACULTY NAME : - Mr. R N RANGANATH

2. INTRODUCTION
1. The geometric design of highway deals with the dimensions and layout of visible features of the highway
such as alignment, sight distances and intersections.
2. Geometric design delas with following elements: -
1-Cross section elements
2-Sight distance considerations
3-Horizontal alignment details
4-Vertical alignment details
5-Intersection element.

3. INTRODUCTION
Design Controls and criteria
1-Design speed
2-Topography
3-Traffic factors
4-Design hourly volume and capacity
5-Environmental and other factors
Design speed:
Different speed standards have been assigned for different class of road. Design speed may be modified depending upon the
terrain conditions.
Topography:
Classified based on the general slope of the country.
1. Plane terrain <10%
2. Rolling terrain 10-25%
3. Mountainous terrain 25-60%
4. Steep terrain >60%

4. INTRODUCTION
Traffic factor
1-Vehicular characteristics and human characteristics of road users.
2-Different vehicle classes have different speed and acceleration characteristics, different dimensions and weight.
3-Human factor includes the physical, mental and psychological characteristics of driver and pedestrian.
Design hourly volume and capacity
1-Traffic flow fluctuating with time
2-Low value during off-peak hours to the highest value during the peak hour.It is uneconomical to design the roadway for
peak traffic flow.
Environmental factors and others
1-Aesthetics
2-Landscaping
3-Air pollution
4-Noise pollution

5. HIGHWAY CROSS-SECTION ELEMENTS
1. Pavement surface characteristics
For safe and comfortable driving four aspects of the pavement surface are important;
1. The friction between the wheels and the pavement surface
2. Smoothness of the road surface
3. The light reflection characteristics of the top of pavement surface,
4. Drainage
1. Friction: -
1. Friction between the wheel and the pavement surface is a crucial factor in the design of horizontal curves and thus the safe
operating speed.
2. Lack of adequate friction can cause skidding or slipping of vehicles.
Various factors that affect friction are:
1. Type of the pavement (like bituminous, concrete, or gravel),
2. Condition of the pavement (dry or wet, hot or cold, etc),
3. Condition of the tyre (new or old), and Speed and load of the vehicle.
IRC suggests the coefficient of longitudinal friction as 0.35-0.4 depending on the speed and coefficient of lateral friction as 0.15.

6. HIGHWAY CROSS-SECTION ELEMENTS
2. Unevenness: -
1. Even if a road is constructed with high quality pavers, it is possible to develop unevenness due to pavement failures.
2. Unevenness affects the vehicle operating cost, speed, riding comfort, safety, fuel consumption and wear and tear of
tyres.
3. Unevenness index( Bump indicator)
4. IRC recommend 150cm/1Km.
3. Light reflection characteristics
1. White roads have good visibility at night, but caused glare during day time.
2. Black roads has no glare during day, but has poor visibility at night
3. Concrete roads has better visibility and less glare

7. HIGHWAY CROSS-SECTION ELEMENTS
4. Cross-slope & Camber: -
Camber or cant is the cross slope provided to raise middle of the road surface in the transverse direction to drain of rain
water from road surface.
The objectives of providing camber are:
1. Surface protection especially for gravel and bituminous roads
2. Sub-grade protection by proper drainage
3. Quick drying of pavement which in turn increases safety
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 and
weather condition.
The common types of camber are parabolic, straight, or combination of them .

8. HIGHWAY CROSS-SECTION ELEMENTS
The common types of camber are parabolic, straight, or combination of them .

9. HIGHWAY CROSS-SECTION ELEMENTS
The values suggested by IRC for various categories of pavement is given in Table below

10. HIGHWAY CROSS-SECTION ELEMENTS
2. Width of the Pavement/ carriage way
1. Width of the carriage way or the width of the pavement depends on the width of the traffic lane and number of lanes.
2. Width of a each traffic lane depends on the width of the vehicle and the clearance.
3. IRC recommendations
The maximum permissible width of a vehicle is “2.44”.

11. HIGHWAY CROSS-SECTION ELEMENTS
3. Kerbs
Kerbs indicate the boundary between the carriage way and the shoulder or islands or footpaths.
1. Low or mountable kerbs: - The height of this kerb is about 10 cm above the pavement edge with a slope which allows
the vehicle to climb easily.
2. Semi-barrier type kerbs:
Their height is 15 cm above the pavement edge.
3. Barrier type kerbs:
They are placed at a height of 20 cm above the
pavement edge with a steep batter.

12. HIGHWAY CROSS-SECTION ELEMENTS
4.Traffic Separator/ medians: -
The main function of traffic separator to prevent the head of collision between vehicles moving opposite.
The separator may also help as
1. Channelized traffic into streams
2. Shadow the crossing
3. Segregate slow traffic
The separator may be in the form of pavement marking or physical divider.
IRC recommends minimum of 5m medians, in case of bridges 1.2 to 1.5 m.

13. HIGHWAY CROSS-SECTION ELEMENTS
5. Road margin: -
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:
1. Shoulders: -
1. Shoulders are provided along the road edge and is intended for accommodation of stopped vehicles, serve as an
emergency lane for vehicles and provide lateral support for base and surface courses.
2. A minimum width of 2.5 m is recommended
2. Parking lanes
3. Bus-bays
4. Service roads
5. Cycle track
6. Footpath
7. Guard rails

14. HIGHWAY CROSS-SECTION ELEMENTS
6. Width of formation/ roadway
1. Width of formation or roadway width is the sum of the widths of pavements or carriage way including separators and
shoulders.
2. The values suggested by IRC are given in Table

15. HIGHWAY CROSS-SECTION ELEMENTS
7. Right of way
Right of way (ROW) or land width is the width of land acquired for the road, along its alignment.
The right of way width is governed by:
1.Width of formation
2. Height of embankment or depth of cutting
3.Side slopes of embankment or cutting
4.Drainage system and their size which depends on rainfall, topography etc.
5. Sight distance considerations
6. Reserve land for future widening
To prevent ribbon development along highways, control lines and building lines may be provided.

16. SIGHT DISTANCE
Sight distance 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:
1. Safe Stopping sight distance (SSD) or the absolute minimum sight distance
2. Intermediate sight distance (ISD) is defined as twice SSD
3. Overtaking sight distance (OSD) for safe overtaking operation

17. SIGHT DISTANCE
Safe Stopping sight distance( SSD): -
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.
The computation of sight distance depends on:
1. Total Reaction time
The reaction time of the driver from instant of object visible to applied brakes.
PIEV Theory
1. Perception
2. Intellection
3. Emotion
4. Volition
2. Speed of Vehicle: -Higher speed, more time to stop and sight distance.
3. Efficiency of Brakes: -The sight distance required will be more when the efficiency of brakes are less
5. Frictional Resistance: -IRC has specified the value of longitudinal friction in between 0.35 to 0.4
6. Gradient of the road: -Gradient of the road also affects the sight distance.

18. SIGHT DISTANCE
Safe Stopping sight distance( SSD): -
1. The stopping sight distance is the sum of lag distance and the braking distance.
2. Lag distance is the distance the vehicle traveled during the reaction time “t”. As per IRC t = 2.5 sec.
𝐿𝑎𝑔 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 = 𝑉 ∗ 𝑡
Where, V= design speed of vehicle or actual speed of vehicle.
3. 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

19. SIGHT DISTANCE

20. SIGHT DISTANCE
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 surface which adds to the the braking force
is equal to W sin(ᶿ) = W tan(ᶿ) = Wn=100. Equating kinetic energy and work done:-
Similarly the braking distance can be derived for a descending gradient. Therefore the general equation is given

21. SIGHT DISTANCE

22. SIGHT DISTANCE

23. SIGHT DISTANCE
2. Overtaking Sight distance: -
The overtaking sight distance is the minimum distance open to the vision of the driver of a vehicle intending to overtake
the slow vehicle ahead with safely against the traffic in the opposite direction.
The factors that affect the OSD are:
1. Velocities of the overtaking vehicle, overtaken vehicle and of the vehicle coming in the opposite direction.
2. Spacing between vehicles, which in-turn depends on the speed
3. Skill and reaction time of the driver
4. Rate of acceleration of overtaking vehicle
5. Gradient of the road
Analysis of overtaking sight distance: -

24. SIGHT DISTANCE
Overtaking sight distance consists of three parts.
1. d1 is the distance traveled by overtaking vehicle A during the reaction time “t”.A1 to A2
2. d2 is the distance traveled by the vehicle during the actual overtaking operation.A2 to A3
3. d3 is the distance traveled by on-coming vehicle C during the overtaking operation. C1 to C2

25. Therefore: -
OSD = d1 + d2 + d3
It is assumed that the vehicle A is forced to reduce its speed to (𝑣𝑏), the speed of the slow moving vehicle B and travels
behind it during the reaction time t of the driver.
So, d1 is given by:
d1 = 𝑣𝑏 t
From the position A2, the vehicle A starts to accelerate, shifts the lane, overtake vehicle B 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:
s = 0.7vb + 6
Let “T”, sec be the duration of actual overtaking. The distance traveled by vehicle B during the overtaking operation is
b = 𝑣𝑏 T
Also, during this time, vehicle A accelerated from initial velocity (𝑣𝑏) and overtaking is completed while reaching final
velocity v. Hence the distance traveled is given by:
d2 = b + 2s
Where, T depends on Speed of vehicle B and Acceleration of Overtaking Vehicle A. therefore, The time T can be
calculated by equating d2 to 𝑣𝑏 T + ½ a𝑇2
.

26. The distance traveled by the vehicle C (d3) moving at design speed V (m/sec) during overtaking operation is given by:
d3 = V x T
The then overtaking sight distance is
In case the speed of the overtaken vehicle is not given, it can be assumed that it moves 16 kmph slower then the design speed.
Reaction time t = 2 sec as per IRC

27. 1. For two lane and two way traffic, OSD = d1 + d2 + d3
2. On divided highways or one way traffic , d3 need not be considered OSD = d1 +d2
3. 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 time OSD and
the minimum is three times OSD

29. Design of Horizontal Alignment: -
Here, we are going to studying on geometrical horizontal curve
1. Due to Obligatory points
2. Terrain
Most important parameter in Horizontal Alignment
1. Design speed
Depend on 1. Class of Road
2. Terrain
There are two values of Design speed : - 1.Ruling Design Speed
2. Min. Design Speed
As per IRC

30. Horizontal Curve: -
Exactly, what happen in horizontal curve ?
1. Centrifugal force (P)
2. Friction force (F)
The effect of Centrifugal force:-
1. Overturing Effect
2. Skidding Effect

31. Overturning Effect: -

32. Skidding Effect: -

33. Super Elevation (e) : -
To counteract Centrifugal force (or) reduce tendency of overturning and skidding by the rise in outer edge with inner
edge.
The inclination of pavement surface is know as Super- elevation or cant or banking.
e = ratio of outer edge height with respect to road width =
𝐸
𝐵
Analysis of Super elevation: -
Forces acting on the vehicle while moving on horizonal curve of a radius R and speed v (m/s)
1. Centrifugal force p =
2. Weight of vehicle acting vertically downwards
3. Frictional Force develop between pavement and wheel

35. Max. super elevation: -
The maximum limiting of super elevation is 7% or 0.007 for plain and rolling terrain.
For mountainous and steep terrain is up to 10%.
Min. Super elevation: -
From drainage consideration it is necessary to have minimum cross-slope to drain off the surface water. So, the minimum
super elevation provide on horizontal curve may be limited to the camber of the surface.
Super elevation Design: -
Consider the mixed traffic, what is the required super elevation for fast moving and slow moving. From practical
consideration, it is suggested the max. limiting super elevation has to counteract the Centrifugal force fully at 75% of
design speed.
Steps for super elevation design: -
Step 1: - the super elevation for 75% of design speed (v m/s) is calculated neglecting the friction.

36. Step 2 : - If the calculated value of “e” is less then 0.07, the value so obtain is provide. If the calculated value of “e” is
greater then 0.07, then provide the max. super elevation equal to 0.07.
Step 3 : - Check the coefficient of friction developed for the max. value of e= 0.07 at the full value of design speed(v m/s).
If the value of the f calculated is less then 0.15, the super elevation of 0.07 for design speed is safe. If not, calculated the
restricted speed.
Step 4: -limiting the speed, allowable speed at the curve is calculated by considering the design coefficient of lateral
friction and the max. super –elevation
If allowable speed is greater than design speed, then the design is adequate and provides the super elevation of 0.07.
If not, the speed is restricted to allowable speed, Va.

39. Radius of Horizontal curve: -
For certain speed of vehicle the centrifugal force is dependent on the radius of horizontal curve. To limit the centrifugal
force, the radius of horizontal will kept higher.
If the deign speed is decided for the highway, then the minimum radius to adopt can be found,
Thus, the ruling minimum radius of the curve for ruling design speed, v, m/s.

40. When minimum design speed adopted, then absolute minimum radius of horizontal curve is,

41. Widening of pavement on Horizontal curves: -
General practice, when the radius of curve is not large to attend greater speed, then widening of pavement is introduced.
Objective of widening of pavement on Horizontal curves is for the reason: -
1. Off tracking
2. Psychology spacing
3. Clear vision in curves
We =Wm +Wps

42. 1. Off tracking: -
1. The rear wheel will not follow the path of front wheel.

43. Mechanical Widening: -

44. Psychological Widening: -
Hence, Widening of pavement on Horizontal curves is,

47. Transition curves: -

48. Transition curves: -
A transition curve may be defined as a curve of varying radius of infinity
at tangent point to a design circular curve radius provided in between the
straight and circular path in order that the centrifugal force was Gradually
increases.
Objective of providing Transition curves: -
1. To gradually introduce the centrifugal force between the tangent
point and the beginning of the circular curve thereby avoiding sudden
jerk on the vehicle.
2. To increase the comfort of passengers.
3. To introduce designed superelevation at a desirable rate
4. To enable the driver to turn the steering gradually for his
own comfort and security
5. To introduce designed extra widening at a desirable rate

49. The ideal transition curve should such that the rate of introduction of
Centrifugal acceleration should be consistent. This means the Radius
Of transition curve should be decrease from infinity to radius of circular
Curve.
TYPES OF TRANSISTION CURVES: -
1. Spiral
2. Cubic parabola
3. Lemniscate
Calculation of design Length of Transition curve : -
The transition curve is designed to fulfill the three conditions
1. Rate of change of centrifugal acceleration to be developed gradually
2. Rate of introduction of designed superelevation
3. Minimum length by IRC empirical formula

50. 1. Rate of change of centrifugal acceleration to be developed gradually
1. At the tangent point the centrifugal acceleration (
𝑣2
𝑅
) is zero,as the radius is infinity.
2. At the end of the transition the radius, R has the minimum value Rm.
3. Hence the centrifugal acceleration is distributed over length Ls of the transition curve.
4. If the “t” is the time taken in seconds to traverse this transition length at uniform design speed of v m/sec.
t =
𝐿𝑆
𝑉

51. 1. Rate of change of centrifugal acceleration to be developed gradually

52. 2. Rate of introduction of designed superelevation

53. 3. Minimum length by IRC empirical formula
Adopt the highest value of Ls given by all this three condition.

79. Example 1: - While Aligning a hill road with a ruling gradient of 6% , a horizontal curve of radius 50 m is encountered. the
grade compensation to provided for this case would be ………?

82. Types of summit curve
1. Circular summit curve
Constant radius of curvature throughout its length and gives constant sight distance all along
2. Simple parabola curve
Easy in arithmetical calculation, coordinates , filed layout
There is no requirement to provide a transition curve.
The main criteria to design summit curve are sight distance
1. Stopping Sight distance
2. Over taking Sight distance
3. Intermediate Sight distance