What is Sight Distance? | What is the factor affecting Sight Distance? | SSD | OSD | Superelevation ~ Civil Engineer Knowledge

HIGHWAY ENGINEERING(Part 3)

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SIGHT DISTANCE

What is Sight Distance?

Sight distance is the distance along the road at which a driver from a specified height above the carriageway has a visibility of objects and can safely stop his vehicle or overtake another vehicle.

What is Sight Distance? | What is the factor affecting Sight Distance? | SSD | OSD | Superelevation

What is the factor affecting Sight Distance?

(i) Speed of Vehicle - Speed of vehicle directly related to sight distance. More will be the speed of vehicle , more will be the sight distance required.

(ii) Driver's Reaction Time- The reaction time is taken by the driver from the instant of seeing the object to the instant when the brakes are applied.

(iii) Brake's Efficiency- Efficiency of brakes depends upon the age and the characteristic of vehicle.

(iv) Gradient of Pavement- Gravitational force comes into action which causes the vehicle to take more time to stop the vehicle means more sight distance is required.

(v) Frictional Resistance- Less distance required by vehicle to stop when frictional resistance is more.

Stopping Sight Distance-

Stopping sight distance is the minimum distance over which the driver travelling at design speed can apply brakes and bring the vehicle to the stop position safely without collision with any other obstruction.

Stopping sight distance composed two components-

(i) Lag Distance(d)- It is the distance travel by the vehicle in total reaction time.

Lag Distance(d)= vt metres ( v= design speed in m/s t= total reaction time in sec)
Lag Distance(d)= 0.278vt metres ( v= design speed in kmph t= total reaction time in sec)

(ii) Braking Distance(l)- It is the distance travel by vehicle after the application of brakes.

Braking Distance(l) = (v^2/2gf) (f= coefficent of friction v= design speed in m/s)
Braking Distance(l) = (0.278v^2/2gf) (f= coefficent of friction v= design speed in kmph)

So, Stopping Sight Distance(SSD)= (vt + v^2/2gf). ( v= design speed in m/s )

Stopping Sight Distance(SSD)= (vt + v^2/2gf). (v= design speed in kmph)

Overtaking Sight Distance-

It is the minimum distance visible to driver of a vehicle who is intending to overtake the slow moving vehicle ahead safely against the traffic of opposite direction.

MEASUREMENT OF OVERTAKING SIGHT DISTANCE

SUPERELEVATION

Superelevation is the distance between the height of inner and outer edges of highway pavement.

In passing from a straight to a curve path, there should be two forces-

(i) the weight of the vehicle.

(ii) the centrifugal force.

SUPERELEVATION IN PAVEMENT SECTION

Superelevation(e)= tanθ= E/B

Relation between superelevation(e), coefficent of friction(f) and centrifugal ratio-

e + f = v^2/gR ( Here, v = m/s)

e + f = v^2/gR ( Here, v = kmph)

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