Lesson focus: stopping distanceThe total distance a vehicle travels to stop: thinking distance plus braking distance. combines reaction and braking phases, while restraint systems reduce injury by increasing the time over which momentumThe product of an object's mass and velocity (momentum = mass × velocity), measured in kg m/s. changes.
Explain factors affecting thinking and braking distanceThe distance a vehicle travels after the brakes are pressed until it stops. and use force, energy and momentum ideas to evaluate safety features.
Build on earlier learning
Acceleration and SUVATThe earlier lesson calculated constant-deceleration displacement. This lesson applies the model carefully to vehicle safety.
You will be successful when you can…
- Separate thinking and braking distance.
- Explain key influencing factors.
- Apply speed-squared reasoning.
- Explain impact-time safety features.
Words that carry the science
- thinking distance
- Distance travelled during the driver’s reaction time before braking begins.
- braking distance
- Distance travelled after braking begins until the vehicle stops.
- stopping distance
- Thinking distance plus braking distance.
- reaction time
- Time between detecting a hazard and initiating a response.
- momentum
- Mass multiplied by velocity, measured in kg m s⁻¹.
- crumple zone
- A vehicle region designed to deform and lengthen collision time while absorbing energy.
Stopping has two distinct phases
01Stopping has two distinct phases
Stopping distance = thinking distance + braking distance. thinking distanceThe distance a vehicle travels while the driver is reacting, before the brakes are pressed. depends on speed and reaction timeThe time it takes you to react to something, such as a driver seeing danger and pressing the brake.; fatigue, distraction, alcohol and some drugs can increase reaction time.
braking distanceThe distance a vehicle travels after the brakes are pressed until it stops. depends on initial speed, mass, brake and tyre condition, road grip and gradient. Separate the phases when explaining a factor.
02Speed strongly affects braking
Kinetic energy is proportional to speed squared, so doubling speed gives four times the kinetic energy at the same mass. Brakes and road forces must transfer this extra energy.
Under an ideal constant braking force, v² = u² + 2as predicts braking distance proportional to u². Real braking also depends on grip, heating and control systems.
03Safety features change force and energy pathways
momentumThe product of an object's mass and velocity (momentum = mass × velocity), measured in kg m/s. p = mv must change during a stop. For the same momentum change, increasing collision time reduces average force because F = change in momentum ÷ time.
Seat belts, airbags and crumple zoneA part of a car designed to crumple in a crash, increasing the collision time so the force on the people inside is smaller. lengthen stopping time or distribute force. Crumple zones also transfer kinetic energy into deformation and heating; they do not remove the need for restraints.
Stopping distance
Stopping distance = thinking distance + braking distance. Thinking distance is covered before the brakes go on; braking distance grows with the square of the speed.
Controls
With a 0.67 s reaction time on a dry road these match the Highway Code: 23 m at 30 mph, 96 m at 70 mph. Wet and icy braking distances are about 2 and 10 times longer.
Combine stopping components
A driver reacts in 0.80 s at 20 m s⁻¹, then brakes for 40 m.
- Thinking distance = speed × reaction time.
- Thinking distance = 20 × 0.80 = 16 m.
- Stopping distance = thinking + braking.
- Stopping distance = 16 + 40 = 56 m.
How to make the reasoning visible: A faster speed increases distance during the same reaction time and usually increases braking distance more strongly.
Analyse road-safety evidence
Compare model stopping components under different conditions.
| Condition | Speed / m s⁻¹ | Thinking / m | Braking / m |
|---|---|---|---|
| alert, dry road | 15 | 10.5 | 18 |
| tired, dry road | 15 | 18 | 18 |
| alert, wet road | 15 | 10.5 | 31 |
| alert, dry road | 30 | 21 | 72 |
Which condition changes thinking but not braking distance?
How does doubling speed affect the model braking distance?
Check your understanding
Answer from memory, then check against the suggested answer.
State the stopping-distance relationship.
Name two factors increasing reaction time.
How does increasing collision time affect average force?
Why can worn tyres increase braking distance without changing thinking distance?
Why does an airbag not make momentum change smaller if the passenger still comes to rest?
Exam Question Practice
Exam Question Practice
Mark points shown after marking
Driving faster increases stopping distance. Seat belts reduce injury in a collision.
Explain why driving faster increases stopping distance.
(3)Explain how a seat belt reduces injury.
(3)Total for this question: 6 marks
Command word: For explainA command word: say how or why something happens, giving reasons with words like "because" or "so"., separate the pre-braking, braking and collision-force arguments.