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Difficulty: MediumFactors Affecting Braking Distance

A driver is travelling on a road where surface water has pooled after heavy rainfall. If the driver is forced to brake firmly from a speed of 40 mph40\text{ mph} (64 km/h64\text{ km/h}), how will these wet road conditions affect the vehicle's braking distance compared to normal, dry tarmac?

  1. A
    It will remain unchanged, but the driver's thinking distance will double due to poor visibility
  2. It will be at least doubled because water reduces friction between the tyres and the roadAnswer
  3. C
    It will increase by ten times because surface water completely eliminates tyre contact
  4. D
    It will decrease slightly because surface water cools the brake discs and prevents fading

Answer

The vehicle's braking distance will be at least doubled because water reduces friction between the tyres and the road surface.
According to the UK Highway Code, wet road surfaces significantly diminish tyre grip and braking efficiency, causing the vehicle's braking distance to be at least doubled compared to dry conditions.

Step-by-Step Solution

1
Identify the specific component of stopping distance affected by road surface moisture
Braking distance is the physical distance travelled while the brakes are applied, which depends directly on tyre-to-road friction.
Thinking distance depends on driver perception and reaction, whereas braking distance depends on vehicle and road physics.
2
Apply standard UK Highway Code multiplier rules for wet road conditions
Wet roads reduce tyre traction, resulting in at least double (2×2\times) the standard dry braking distance.
Water forms a lubricating layer between the tread blocks and tarmac, reducing maximum deceleration.

Key Concept

Braking distance on wet asphalt is at least double that of dry asphalt due to reduced friction between tyres and the road.
Estimated Time:1m 0s
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Topics

Factors Affecting Braking Distance

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