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

The trick in this question is realizing that Stopping Sight Distance is a one-driver quantity: it only asks whether a single driver, moving at the design speed, can stop in time for whatever appears ahead. It says nothing about how many directions of traffic share that same strip of road.

1. 360 m: This is the SSD already fixed for the carriageway's design speed under normal working. Diverting traffic through the median opening changes the traffic pattern (one-way becomes two-way on that stretch) but does not change the design speed, the driver's reaction time, or the braking friction, the three things SSD is actually built from. So the desired SSD stays exactly what it was.
2. 240 m: There is no rule in geometric design that shrinks SSD to two-thirds of its value because of a contraflow diversion; nothing in the SSD formula $SSD = 0.278\,V t + \frac{V^2}{254 f}$ depends on how many directions share the lane.
3. 180 m: Halving the SSD would mean assuming drivers now need only half the distance to stop, which is false, their speed and braking ability have not improved because of the diversion.
4. 720 m: Doubling the SSD would be correct only if the design requirement being asked for were a meeting or head-light sight distance for two vehicles closing on each other from opposite ends of the same lane, since then the combined stopping requirement of both drivers matters. But the question specifically asks for Stopping Sight Distance, which stays a single-driver, single-direction quantity even in a contraflow section.

Since SSD is defined from one vehicle's own speed and braking, and that design speed has not been stated to change, the SSD that must be kept available on the operating section is unchanged at 360 m.

Let's summarize:

• SSD = perception-reaction distance + braking distance for one driver at the design speed.
• It does not depend on whether the carriageway carries one-way or contraflow (two-way) traffic.
• Doubling SSD applies to meeting or head-light sight distance, a different criterion, not asked here.

So the correct answer is 360 m, option (A).