Test: Sight Distance - Civil Engineering (CE) MCQ

The length of the road that is visible to the driver at any time is called sight distance, in night visibility it is headlight sight distance and in zone which it can be overtaken is called overtaking sight distance.

Stopping sight distance is given by:
= Lag distance + Breaking distance

Calculation:
SSD = 95 m
V = 50 kmph
t_R = 2.3 seconds

SSD = 57.87

SSD = 0.278vt+v²/254f
SSD = 0.278*45*2.5+452/(254*0.4)
SSD = 51m.

Stopping Sight Distance:-
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 travelling at design safely without collision with any other obstruction.
The stopping sight distance is the sum of lag distance and the braking distance.
Lag distance is the distance the vehicle travelled during the reaction time ‘t’ and is given by vt, where v is the velocity in m/sec².
Braking distance is the distance travelled by the vehicle during braking operation.
If F is the maximum frictional force developed and the braking 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,

SSD = Lag distance + Braking distance

Where, v is the design speed in m/sec²,
t is the reaction time in sec,
g is the acceleration due to gravity
f is the coefficient of friction.
Coefficient of longitudinal friction:-

Important Points

1. Intermediate Sight Distance (ISD) = SSD for 2-way 2- lane road
2. ISD = 2 × SSD -- for single lane 2 way
3. ISD = SSD --- for single lane one - way

SSD = 0.278vt + v²/ (254f)
In this equation t is the reaction time taken by driver for stopping the vehicle, v is the speed of the vehicle and f is the coefficient of friction, so the SSD is independent of the length of the vehicle.

Sight distance for geometric design of highway and traffic control are:

• Stopping Sight Distance (SSD)
• Overtaking Sight Distance (OSD)
• Sight distance at the intersection.
• Intermediate sight distance.
• Headlight sight distance.

The incorrect sight distance from the above option is the sight distance for merging two lanes.

The SSD is based on PIEV theory, it is the theory that estimates the time taken to perceive and react.

Sight distance available from a point is the actual distance along the road surface, over which a driver from a specified height above the carriageway has visibility of stationary or moving objects. Three sight distance situations are considered for design:

• Stopping sight distance (SSD) or the absolute minimum sight distance
• Intermediate sight distance (ISD) is defined as twice SSD
• Overtaking sight distance (OSD) for safe overtaking operation
• Headlight sight distance is the distance visible to a driver during night driving under the illumination of headlights
• Safe sight distance to enter into an intersection

The reaction time considered in SSD unless and until specified is 2.5 sec; it is based on PIEV theory.

Vb = V-16
V = Vb+16
V = 80+16 = 96Kmph.