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Difficulty: MediumStopping Distances

You are travelling at 40 mph40\text{ mph} (64 km/h64\text{ km/h}) along an open road while it is raining. Given that the road surface is wet, what is the expected overall stopping distance for a standard car?

  1. A
    36 metres36\text{ metres} (120 feet120\text{ feet})
  2. B
    24 metres24\text{ metres} (80 feet80\text{ feet})
  3. 72 metres72\text{ metres} (240 feet240\text{ feet})Answer
  4. D
    48 metres48\text{ metres} (160 feet160\text{ feet})

Answer

72 metres72\text{ metres} (240 feet240\text{ feet})
According to The Highway Code, travelling at 40 mph40\text{ mph} (64 km/h64\text{ km/h}) requires a total stopping distance of 36 metres36\text{ metres} (120 feet120\text{ feet}) under dry conditions (12 metres12\text{ metres} thinking distance + 24 metres24\text{ metres} braking distance). Because wet road surfaces double the overall stopping distance, the correct total distance required is 72 metres72\text{ metres} (240 feet240\text{ feet}).

Step-by-Step Solution

1
Determine the standard total stopping distance on a dry road at 40 mph40\text{ mph} (64 km/h64\text{ km/h}).
Thinking distance is 12 metres12\text{ metres} (40 feet40\text{ feet}) and braking distance is 24 metres24\text{ metres} (80 feet80\text{ feet}), giving a total dry stopping distance of 36 metres36\text{ metres} (120 feet120\text{ feet}).
The Highway Code establishes baseline stopping distances for dry, level asphalt.
2
Apply the wet weather environmental multiplier.
36 metres×2=72 metres36\text{ metres} \times 2 = 72\text{ metres} (120 feet×2=240 feet120\text{ feet} \times 2 = 240\text{ feet}).
Water on the road reduces tyre adhesion and significantly increases the distance required to bring the vehicle to a halt, doubling normal stopping distances.

Key Concept

Total stopping distance equals thinking distance plus braking distance, and doubles in wet weather conditions.
Estimated Time:1m 0s
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Topics

Stopping Distances

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