Welcome! Why does a car need so much room to stop? Today you’ll use real stopping-distance data. Then you’ll design a road safety campaign for the road outside a school.
🔑 Key words for today
- Reaction time – how long it takes the driver to react and press the brake.
- Thinking distance – how far the car goes during the reaction time.
- Braking distance – how far the car goes after the brakes are pressed.
- Stopping distance – thinking distance + braking distance.
- Friction – the force between the tyres and the road that slows the car.
- Kinetic energy – the energy a moving object has.
Before you start
- You need: your exercise book or paper, a pen and a calculator.
- Write the title and date: Stopping distances – Road Safety Campaign.
- ⭐ Core = everyone does this (about 40 minutes). 🚀 Stretch = extra challenge if you finish.
- Write your answer first, then tap Show answer. Copy the blue 📝 boxes into your book.
📝 Copy into your book
By the end of this lesson I will be able to:
1. explain thinking distance and braking distance
2. list what affects each one
3. use data to show how speed changes stopping distance
4. (🚀) calculate thinking distance.
⭐ Step 1: Starter (5 minutes)
A car is going at 30 mph. A child runs out. Guess: how many car lengths does it travel before it stops? (A car is about 4 m long.)
Show answer
About 23 m – roughly 6 car lengths! The car keeps going at full speed while the driver reacts, before the brakes even start working.
📝 Copy into your book
Stopping distance = thinking distance + braking distance
⭐ Step 2: Read and write (8 minutes)
Copy these three questions. Then read the text and answer them.
- What two distances add up to the stopping distance?
- Give three things that make reaction time longer.
- Give three things that make braking distance longer.
📖 Read: Why can’t a car stop straight away?
Thinking. When a driver sees danger, they need time to react. During this reaction time, the car keeps going at full speed. The distance it travels is the thinking distance.
Braking. Then the driver brakes. The car still travels a bit further while it slows down. This is the braking distance. Add the two together to get the stopping distance.
What makes it worse? Reaction time is longer if the driver is tired, has drunk alcohol, has taken drugs, or is distracted (like using a phone). Braking distance is longer if the car is going faster, the road is wet or icy, or the tyres or brakes are worn.
Show answers
(a) Thinking distance + braking distance.
(b) Any three: tiredness, alcohol, drugs, distractions such as a phone.
(c) Any three: higher speed, wet or icy roads, worn tyres, worn brakes.
| Makes THINKING distance longer | Makes BRAKING distance longer |
|---|---|
| Tiredness | Higher speed |
| Alcohol or drugs | Wet or icy road |
| Distractions (phone) | Worn tyres or brakes |
| Higher speed | Heavier vehicle |
📝 Copy this table into your book.
⭐ Step 3: Read the data (8 minutes)
Typical stopping distances for a car on a dry road (Highway Code):
| Speed | Thinking | Braking | Stopping |
|---|---|---|---|
| 20 mph | 6 m | 6 m | 12 m |
| 30 mph | 9 m | 14 m | 23 m |
| 40 mph | 12 m | 24 m | 36 m |
| 50 mph | 15 m | 38 m | 53 m |
| 60 mph | 18 m | 55 m | 73 m |
| 70 mph | 21 m | 75 m | 96 m |
⭐ Q1 What is the stopping distance at 20 mph? At 30 mph? How much further is it at 30 mph?
⭐ Q2 When the speed doubles from 20 to 40 mph, what happens to the thinking distance? And the braking distance?
Show answers
Q1: 12 m at 20 mph, 23 m at 30 mph – 11 m further (almost 3 car lengths).
Q2: Thinking distance doubles (6 → 12 m). Braking distance goes up 4 times (6 → 24 m).
📝 Copy into your book
If the speed doubles: thinking distance doubles, but braking distance goes up about 4 TIMES.
🚀 Q3 A driver on their phone has double the thinking distance. What is their stopping distance at 30 mph?
Show answer
Thinking: 9 × 2 = 18 m. Stopping: 18 + 14 = 32 m.
⭐ Step 4: Main task – Road Safety Campaign (12 minutes)
The council wants a poster to persuade drivers to slow to 20 mph outside a primary school. Plan your poster. It must include:
- ⭐ A headline that grabs drivers’ attention.
- ⭐ A fact from the table comparing 20 mph and 30 mph.
- ⭐ One warning about reaction time (phones, tiredness or alcohol).
- 🚀 A warning about wet or icy roads, explaining why.
- 🚀 A physics explanation using the words kinetic energy and braking force.
✏️ Sentence starters
At 20 mph, a car stops in ______ m. At 30 mph, it needs ______ m.
Using your phone makes your reaction time ______, so the car travels ______ before you brake.
On a wet road there is less ______, so the car takes ______ to stop.
Show a model poster plan
Headline: “At 30, you need 11 more metres to stop. Our children don’t have them.”
Data: At 20 mph, a car stops in 12 m. At 30 mph, it needs 23 m.
Reaction time: Using your phone makes your reaction time longer, so the car travels further before you even brake.
(🚀) Road conditions: On a wet or icy road there is less friction, so the car takes further to stop.
(🚀) Physics: A faster car has much more kinetic energy, so the brakes have to do more work. With the same braking force, that takes a longer distance.
Step 5: Exam practice (10 minutes)
⭐ Q1 (3 marks) Give three things that make a driver’s thinking distance longer.
Show model answer
Any three: tiredness (1); alcohol or drugs (1); distractions such as a phone (1); higher speed.
⭐ Q2 (3 marks) Explain why a car’s braking distance is longer on a wet road.
✏️ Start with: “A wet road means less…” then “so the braking force…” then “so the car…”
Show model answer
A wet road means less friction between the tyres and the road (1), so the braking force is smaller / the car slows down more slowly (1), so the car travels further before it stops (1).
👀 Watch me first (for Q3)
A car travels at 20 m/s. The driver’s reaction time is 0.5 s. What is the thinking distance?
1. Equation: distance = speed × time
2. Numbers in: distance = 20 × 0.5
3. Answer: 10 m
🚀 Q3 (3 marks) A car travels at 13 m/s. The driver’s reaction time is 0.7 s. Calculate the thinking distance.
Show model answer
s = v × t (1)
s = 13 × 0.7 (1)
s = 9.1 m (1)
🚀 Q4 (4 marks) Describe how to measure someone’s reaction time using a ruler, and how to make the results reliable.
Show model answer
The person rests their arm on a table with their hand over the edge. A partner holds a ruler with 0 level with their thumb (1). The partner drops it without warning and the person catches it (1). Record how far the ruler fell – a longer distance means a longer reaction time (1). Repeat at least 5 times and calculate a mean (1).
✅ Core score out of 6 (Q1–Q2). Stretch: out of 13.
⭐ Step 6: Exit ticket (5 minutes)
If you have a ruler and a partner, try the ruler-drop test three times each. Then finish:
- The most surprising thing I learned today is…
- Doubling the speed makes the braking distance about… times longer.
- One thing I’m still not sure about is…
🎉 Well done – lesson complete! Your 📝 boxes are your revision notes. Want more? Try the Stopping distances questions or the Crash Test Engineer lesson.
▶ Want to watch a video? (optional)
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🖨️ Want this lesson on paper? Download the whole lesson as a printable worksheet – answers are on the last page.
