Question

An incident occurred on one side of a median on a four-lane dual carriageway road section. This incident disrupted the traffic movement in one direction. Traffic police diverted the traffic from the disrupted side to the other side through an opening in a median. The desired sight distance on the four-lane dual carriageway was kept as 360 m under normal conditions. The Stopping Sight Distance (SSD, in m), which should be available on the operating road section after the incident, is

• A. 360
• B. 240
• C. 180
• D. 720

Hint:

SSD depends only on one driver's own speed and braking ability, not on how many directions share the carriageway.

Answer 1

The Correct Option is A

Step 1: Recall the definition of Stopping Sight Distance.
Stopping Sight Distance (SSD) is the minimum distance a driver, travelling at the design (operating) speed of the road, needs to see an obstruction ahead and bring the vehicle to a safe halt before reaching it. It is the sum of the lag distance covered during the driver's perception and reaction time plus the braking distance needed to stop.
SSD is worked out for a single driver in a single direction of travel; it depends only on that driver's own speed, reaction time and the coefficient of friction between tyre and road, not on what vehicles coming from any other direction are doing.

Step 2: Understand what actually changed on the road after the incident.
Before the incident, the four-lane dual carriageway carried one-way traffic on each carriageway, and each carriageway was designed for a desired SSD of 360 m at its design speed.
After the incident, the police diverted traffic from the blocked side onto the opposite (undamaged) carriageway through a median opening. This changes the carriageway from one-way flow to a shared, two-way (contraflow) operation, but it does not change the speed at which a single vehicle is expected to travel, nor the braking and reaction requirement placed on a single driver.

Step 3: Apply the definition to the operating condition.
Because SSD is defined purely from one driver's own ability to stop for whatever is ahead, including a vehicle coming from the opposite direction now sharing the same carriageway, the distance a driver needs to see and stop within does not double or halve just because the carriageway is being shared. The operating road section must still provide the same 360 m of unobstructed sight distance for each driver to be able to stop safely. Doubling to 720 m would only be relevant if the requirement were a meeting or head-light sight distance based on the combined closing speed of two vehicles travelling towards each other on the same lane, which is a different, separate criterion from SSD and is not what the question asks for. Reducing it to 240 m or 180 m has no basis at all in the SSD formula, since neither the design speed, the reaction time nor the friction value have changed.

Final Answer:
The desired SSD required on the operating road section after the incident remains 360 m, same as under normal conditions. \[ \boxed{360 \text{ m}} \]