Geometric Design Of Highway Notes


The alignment is the route of the road, defined as a series of horizontal tangents and curves. highway geometric design, operating speed, drivers’ perception. The geometric design of roads is the branch of highway engineering concerned with the positioning of the physical elements of the roadway according to standards and constraints.


  • The pavement surface conditions are measured using equipment called “Bump integrator” in terms of unevenness index, which is the cumulative measure of vertical undulations of the pavement surface recorded per unit horizontal length of the road.
  • Uneven ness index should be kept below 150cm/km for good pavement surface of high-speed high ways. A value of 250 cm/km is satisfactory up to the speed of about 100kmph. Value more than 350 cm/km is considered very uncomfortable even at speed of 50kmph.


  • It is the slope provided to the road surface in the transverse direction to drain off rainwater from the road surface.
  • The requirement of camber of a pavement depends upon-

(i) The type of pavement surface.
(ii) The amount of rainfall.

  • The camber may be given a parabolic, elliptic or straight-line shape.
  • Parabolic is preferred by fast-moving vehicles because they required frequent crossing the crown line during overtaking operations.
  • When a very flat cross slope is provided as in cement concrete pavement, the straight-line shape of the chamber may be provided.
Design Of Highway


These are used to prevent head-on collision between vehicles moving in opposite directions.

KERBS : Kerb indicates the boundary between the pavement and shoulder, or sometimes islands or footpath or kerb parking space. Low or mountable types kerb has a height of about 10 cm.

Design Of Highway


  • It is the length of the road visible ahead to the driver at any instance.
  • Three sight distance situations are considered in the design
  1. Stopping or absolute minimum sight distance
  2. Safe overtaking or passing sight distance.
  3. Safe sight distance for entering into uncontrolled intersections
  • Apart from the above three situations, the following sight distance is considered by the I.R.C. in highway design.

(i) Intermediate sight distance: This is defined as twice the stopping sight distance. When overtaking sight distance cannot be provided, intermediate sight distance is provided to give limited overtaking opportunities to fast vehicles. ISD=2SSD

(ii) Headlight sight distance: Distance visible to driver during the night under headlights. This important for up gradients and descending stretch of the valley curves.


(Non-passing sight distance) sight – It is the length of road visible ahead of the driver at any instance which depends upon

(i) Feature of the road
(ii) Height of the driver eye’s above the road (1.2 m as per IRC)
(iii) Height of the object above the road (0.15 m as per IRC)

— Stopping distance depends upon the following factors.
(a) Total reaction time of driver.
(b) Speed of the vehicle
(c) Efficiency of brakes
(d) Friction
(e) Gradient of the road if any.
Total reaction time – It based upon PIEV Theory which consist of
(i) Perception time
(ii) Brake reaction time

  • Perception time is the time required for driver to realize that brakes must be applied.
  • Brake reaction time depends upon skill and type of situation.

PIEV Theory

The total reaction time is split into 4 parts.

  1. Perception: Time required to perceive an object
  2. Intellection: Time required to understand the situation
  3. Emotion: time elapsed during emotional sensation
  4. Volition: Time is taken for final action.

The total reaction time varies 0.5 to 4 sec, IRC recommends a value of 2.5 sec for total reaction time.

  • Stopping distance of vehicle is the sum of

A. Lag distance: Distance travelled by vehicle during total reaction time. L.D= vt, where t is the total reaction time in seconds. And v is in m/sec. Also denoted as L.D=0.278Vt where V is in kmph

B. Braking distance: distance is travelled by vehicle after application of brakes Denoted as B.D= v2/2gf, where v is in m/sec and f is the longitudinal coefficient of friction

The longitudinal coefficient of friction decreases with increased speed.
I.R.C.recommends that
f=0.40 for speed of 20 to 30 kmph
f=0.35 for speed of 100 kmph.
Equation of SSD:

SSD = vt + v22gf  or 0.278vt + v22gf

SSD (Inclined surface-Ascending Gradient)

Geometric Design Of Highway Notes

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