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Difficulty: Very hardStopping Distances

You are initially travelling at 70 mph70\text{ mph} (112 km/h112\text{ km/h}) on a dry motorway. Later in your journey, you drive at 30 mph30\text{ mph} (48 km/h48\text{ km/h}) on an icy residential road where braking distances are ten times longer than normal. According to the official Highway Code, what is the exact difference in your overall stopping distance between these two specific scenarios?

  1. It is 53 metres53\text{ metres} greater in the 30 mph30\text{ mph} icy condition.Answer
  2. B
    It is 134 metres134\text{ metres} greater in the 30 mph30\text{ mph} icy condition.
  3. C
    It is 73 metres73\text{ metres} greater in the 70 mph70\text{ mph} dry condition.
  4. D
    It is 65 metres65\text{ metres} greater in the 30 mph30\text{ mph} icy condition.

Answer

The overall stopping distance is 53 metres greater in the 30 mph icy condition.
At 70 mph, the total stopping distance is 96 metres. At 30 mph on ice, the standard 14m braking distance increases tenfold to 140m, while the 9m thinking distance stays the same, creating a total stopping distance of 149m. Subtracting the dry distance (96m) from the icy distance (149m) results in a difference of 53 metres.

Step-by-Step Solution

1
Calculate the total stopping distance at 70 mph on a dry road.
Thinking distance = 21 metres, Braking distance = 75 metres. Total = 96 metres.
To establish the baseline stopping distance for the first scenario using standard Highway Code figures.
2
Calculate the total stopping distance at 30 mph on an icy road.
Thinking distance = 9 metres. Braking distance (14 metres) x 10 = 140 metres. Total = 149 metres.
Ice increases braking distance by a factor of ten, but human reaction time (thinking distance) remains unaffected by the road surface.
3
Determine the difference between the two total stopping distances.
149 metres (icy) - 96 metres (dry) = 53 metres.
To answer the specific question asked in the stem regarding the exact difference.

Key Concept

Calculating and comparing total stopping distances across different speeds and severe weather conditions by separating thinking and braking components.
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Topics

Stopping Distances

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