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Difficulty: MediumAdjusting Margins in Wet Weather

A motorist travels at 40 mph on a dual carriageway during persistent rain. Under standard dry conditions, stopping safely from 40 mph requires a thinking distance of 12 metres and a braking distance of 24 metres. In accordance with the Highway Code's rules for driving in wet weather, what is the total stopping distance, in metres, required to bring the vehicle to a complete standstill?

Answer: 60 metres

Answer

60 metres
In wet weather, reduced road friction causes the braking distance to double from 24 m24\text{ m} to 48 m48\text{ m}. The thinking distance remains unchanged at 12 m12\text{ m}. Summing the thinking distance and the wet braking distance yields a total stopping distance of 12 m+48 m=60 m12\text{ m} + 48\text{ m} = 60\text{ m}.

Step-by-Step Solution

1
Calculate the braking distance on a wet road surface
48 metres (24 m×2=48 m24\text{ m} \times 2 = 48\text{ m})
The Highway Code establishes that wet conditions reduce tyre adhesion, causing braking distance to be twice as long as on a dry road.
2
Add the thinking distance to the adjusted wet braking distance
60 metres (12 m+48 m=60 m12\text{ m} + 48\text{ m} = 60\text{ m})
Thinking distance depends strictly on driver reaction time and vehicle speed, meaning it remains 12 metres regardless of the road surface.

Key Concept

In wet weather, braking distance is multiplied by two due to reduced tyre grip, whereas thinking distance remains unaffected.
Estimated Time:1m 0s
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Adjusting Margins in Wet Weather

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