Explain the factors which affect the distance required for road transport…

The idea

This statement asks students to explain why road vehicles need different distances to stop in emergencies and what this means for safety. The total stopping distance is thinking distance plus braking distance. Thinking distance depends on the driver's reaction time and the vehicle's speed. Braking distance depends on speed, the friction between tyres and road, and the condition of tyres and brakes. For example, at higher speed the vehicle has more kinetic energy to transfer, and because kinetic energy depends on speed², braking distance increases sharply. On ice, friction is low, so the braking force is small and the vehicle travels much further. Longer stopping distances increase the chance of hitting a hazard, so speed limits, safe gaps and vehicle maintenance all improve safety.

What earns the marks

• Stopping distance is the sum of thinking distance and braking distance, and factors must be assigned to the correct component.
• Thinking distance increases with speed and with reaction time, including effects of tiredness, alcohol or distraction.
• Braking distance increases with speed because kinetic energy is proportional to speed², so more work must be done by the braking force.
• Reduced friction from wet, icy or loose surfaces lowers the braking force and increases braking distance.
• Poor tyre tread, worn brakes or extra mass can increase braking distance by reducing braking force or increasing the energy that must be transferred.
• Safety implications include reducing speed, increasing the gap to the vehicle in front, and maintaining tyres and brakes so that stopping distance stays as short as possible.

Common mistakes and how to put them right

• Confusing thinking distance with braking distance.

  Put it right by stating that thinking distance is the distance travelled during the driver's reaction time.

• Assuming braking distance is directly proportional to speed.

  Put it right by linking kinetic energy to speed² and explaining the steeper increase.

• Ignoring the effect of mass.

  Put it right by noting that a heavier vehicle has more kinetic energy at the same speed, so more work is needed to stop it.

Exam tips

• Structure each explanation as factor, affected distance, mechanism, and safety consequence.
• Use the kinetic energy equation and the work done by the braking force to support the speed-squared relationship.
• Compare two conditions, such as dry versus icy roads, to make the effect on stopping distance clear.