# Geometric Design Of Highway

The alignment is the course of the road, described as a chain of horizontal tangents and curves. Geometric Design Of Highway, operating speed, drivers’ perception. The geometric design of roads is the department of highway engineering worried with the site of the bodily factors of the roadway in keeping with requirements and constraints.

## PAVEMENT UNEVENNESS

• 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.

## CROSS SLOPE/ CAMBER /CANT

• 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.

## TRAFFIC SEPARATORS / MEDIANS

These are accustomed forestall head-on collision between vehicles taking possession opposite directions.

KERBS : Kerb indicates the boundary between the pavement and shoulder, or typically islands or pathway or curbing parking space Low or mountable kinds kerb has a top of approximately 10 cm.

## SIGHT DISTANCE

• 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 dabble the (SSD) stopping sight distance ISD=2SSD. When overtaking (SD) sight distance can’t be supplied, (ISD) intermediate sight distance is supplied to provide limited overtaking possibilities to fast vehicles.

(ii) Headlight sight distance:Distance seen to the driving force at some point of the night time below headlights. This necessary for up gradients and declivitous stretch of the valley curves.

## STOPPING SIGHT DISTANCE (SSD)

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

(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
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 car at some point in general response time. L.D= vt, in which t is the whole response 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 through car after application of brakes Denoted as B.D= v2/2gf, in which v is in m/sec and f is that the longitudinal constant 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: