2.27–2.31: Reaction time, stopping distance and road safety

What is human reaction time?

The time between detecting a stimulus and responding to it.

How can human reaction time be measured in a laboratory?

Use a ruler-drop test or an electronic reaction timer.

What is a typical human reaction time?

About 0.2–0.3 s, although it varies between individuals and circumstances.

What is thinking distance?

The distance travelled by a vehicle during the driver’s reaction time.

What is braking distance?

The distance travelled from the moment the brakes are applied until the vehicle stops.

What is the equation for stopping distance?

Stopping distance = thinking distance + braking distance.

What factors affect thinking distance?

Vehicle speed, driver reaction time, tiredness, alcohol, drugs and distractions.

What factors affect braking distance?

Vehicle speed, vehicle mass, brake condition, tyre condition, road surface, weather and friction.

How does increasing vehicle speed affect thinking distance?

Thinking distance increases because more distance is travelled during the same reaction time.

How does increasing vehicle speed affect braking distance?

Braking distance increases; under otherwise identical conditions it increases approximately with the square of speed.

How does increasing vehicle mass affect stopping distance?

A greater mass can increase braking demands and may increase braking distance under the same braking conditions.

How do worn brakes affect braking distance?

They can reduce braking effectiveness and increase braking distance.

How does a wet or icy road affect braking distance?

It reduces tyre-road grip, increasing braking distance.

How does low tyre-road friction affect stopping distance?

It makes it harder to produce a large braking force, so braking distance increases.

How do alcohol and recreational drugs affect reaction time?

They can impair judgement, coordination and reaction time, increasing the risk of a collision.

How can distractions affect reaction time?

They divert attention from the road and can delay the driver’s response.

Why does tiredness increase stopping distance?

It can increase reaction time, increasing thinking distance.

What is deceleration?

Acceleration that reduces an object’s speed.

Why are large decelerations dangerous?

They cause large forces on vehicle occupants and can cause serious injury.

How can the force during a collision be estimated?

Use resultant force = mass × acceleration, or force = change in momentum ÷ time.

A 1,000 kg car decelerates at 5 m/s². Calculate the magnitude of the resultant force.

F = ma = 1,000 × 5 = 5,000 N.

A 60 kg passenger decelerates at 20 m/s². Estimate the force on the passenger.

F = ma = 60 × 20 = 1,200 N.

Why do seat belts reduce injury?

They increase the time over which the passenger’s momentum changes and spread the force across stronger parts of the body.

Why are high-speed collisions more dangerous?

They involve greater momentum and kinetic energy, and stopping may require larger forces or longer distances.

What is the difference between thinking distance and braking distance?

Thinking distance occurs before the brakes are applied; braking distance occurs after braking begins.