Stopping Distances
In this lesson we learn about factors that affect stopping distance.
IN THIS LESSON:
Stopping distance factors
Why factors affect stopping distance
What affects thinking distance
What affects braking distance
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Practice Questions
Question 1: List the key factors that can increase a driver's thinking distance.
Answer: The main factors are:
- Driver's reaction time.
- Initial speed of the vehicle.
- Driver's state, such as tiredness, distraction, or being under the influence of alcohol or drugs.
Question 2: Explain why a tired driver will have a longer thinking distance than an alert driver at the same speed.
Answer: A tired driver's reaction time is slower. It takes them longer to process the information (the hazard) and react by moving their foot to the brake pedal. During this longer reaction time, the vehicle continues to travel at its original speed, covering more ground, which increases the thinking distance.
Question 3: Explain how driving under the influence of alcohol or drugs affects a driver's thinking distance.
Answer: Alcohol and many drugs slow down a person's reaction time, impairing their judgment and coordination. This increases the time it takes for the driver's brain to process the need to brake, meaning the vehicle travels a greater distance before the brakes are even applied.
Question 4: Explain the relationship between a vehicle's speed and its thinking distance.
Answer: Thinking distance is directly proportional to the vehicle's speed. This means that if you double your speed, your thinking distance will also double. If you triple your speed, your thinking distance will triple.
Question 5: List the key factors that can increase a vehicle's braking distance.
Answer: The main factors are:
- Initial speed of the vehicle.
- Condition of the vehicle (e.g., worn brakes, worn tyres).
- Road conditions (e.g., wet, icy, or muddy surfaces).
- Mass of the vehicle.
- Gradient of the road (e.g., driving downhill).
Question 6: Explain why a wet or icy road surface increases a vehicle's braking distance.
Answer: Wet or icy road surfaces have a lower coefficient of friction than dry tarmac. This means that when the brakes are applied, the friction force between the tyres and the road is smaller. A smaller braking force results in a lower deceleration, so it takes longer and further for the vehicle to stop.
Question 7: How does having worn brakes or bald tyres affect a vehicle's braking distance?
Answer: Both worn brakes and bald tyres reduce the maximum braking force that can be applied. Worn brakes are less effective at creating friction, and bald tyres have less grip on the road, reducing the friction between the tyres and the road surface. A lower braking force leads to a smaller deceleration and therefore a longer braking distance.
Question 8: Explain why a heavier vehicle, such as a lorry, has a longer braking distance than a lighter car, assuming they have equally effective brakes and are travelling at the same speed.
Answer: According to Newton's Second Law (F=ma), for a given braking force (F), a larger mass (m) will result in a smaller deceleration (a). Since a heavier lorry has a greater kinetic energy to lose, it requires more work done to stop it. If the braking force is the same, this will happen over a longer distance.
Question 9: Explain the relationship between a vehicle's speed and its braking distance.
Answer: Braking distance is proportional to the square of the speed. This means that if you double your speed, your braking distance will increase by a factor of four (22=4). If you triple your speed, the braking distance will be nine times greater (32=9).
Question 10: Why does driving a vehicle downhill increase its braking distance?
Answer: When a vehicle travels downhill, a component of the gravitational force acts to pull it down the slope, which counteracts the braking force. This reduces the resultant braking force, leading to a smaller deceleration and a longer braking distance.
Question 11: Compare the effect of doubling a vehicle's speed on its thinking distance versus its braking distance.
Answer: Doubling the speed will double the thinking distance (a linear relationship). However, doubling the speed will quadruple the braking distance (a squared relationship). Therefore, the increase in braking distance has a much greater impact on the total stopping distance when speed increases.
Question 12: A driver is distracted by their mobile phone. Which component of the total stopping distance is most affected by this distraction, and why?
Answer: The thinking distance is most affected. Distraction increases the driver's reaction time, as their brain is not focused on the road. The vehicle will travel a greater distance before the driver even perceives the need to brake, let alone applies them.
Question 13: A car has worn brake pads, while a different car has a distracted driver. Assuming they are travelling at the same speed, which vehicle will likely have the longer stopping distance in an emergency stop?
Answer: It depends on the severity, but a distracted driver will likely have a more significant impact. Worn brake pads affect braking distance, which is proportional to speed squared. However, a severely distracted driver could have a very long reaction time, during which the car travels at full speed. At high speeds, the effect of reaction time can be very large. For example, a reaction time of just 1.5 s at 30 m/s adds 45 m to the thinking distance. This is often larger than the increase from worn brakes alone.
Question 14: A lorry is fully loaded with cargo, making it much heavier. A cyclist is also on the road. The lorry driver and the cyclist have the same reaction time. Compare their braking distances in an emergency stop from the same speed.
Answer: The lorry will have a much longer braking distance than the cyclist. The lorry's greater mass means it has significantly more kinetic energy. To stop, the brakes must do much more work over a longer distance to dissipate that energy. For the same braking force, the lorry's deceleration will be much smaller.
Question 15: Explain why it is safer to increase the distance between your vehicle and the one in front of you when driving at a higher speed.
Answer: When you increase your speed, your total stopping distance increases dramatically because braking distance is proportional to speed squared. By increasing your distance from the car in front, you are providing a larger buffer zone, which gives you more space and time to stop safely in an emergency, especially given the significant increase in braking distance at higher speeds.