4.5.6.3.3 - Factors Affecting Braking Distance

What affects braking distance

In an emergency stop, the driver first reacts and then the brakes slow the vehicle. The distance travelled while the brakes are acting is the braking distance.

This specification point is about factors that make the vehicle travel further while braking. The named factors are:

• adverse road and weather conditions, especially wet or icy roads
• poor brakes
• poor tyres

These factors matter because they reduce either the available grip with the road or the braking effect of the vehicle. If a vehicle slows down less quickly after the brakes are applied, it travels further before it comes to rest.

Do not make this lesson about the driver's reaction time. Wet weather or ice may also make driving harder in other ways, but this lesson's braking-distance chain is about the distance after braking begins.

Wet and icy roads

Tyres need grip with the road surface so the vehicle can slow down without sliding. On a dry road there is usually more grip. On a wet road, water reduces the grip between the tyres and the road. On an icy road, the grip can be much lower again.

Less grip means the vehicle cannot decelerate as much before the tyres begin to slide. The vehicle therefore travels a greater distance while braking.

The exam explanation should link the factor to the mechanism and then to the effect:

Wet road -> less grip between tyres and road -> vehicle slows down less quickly -> braking distance increases.

Icy road -> much less grip -> much longer braking distance -> greater risk that the vehicle will not stop before a hazard.

If a question gives a multiplier for the road condition, use it as an estimate. For example, if a dry-road stopping distance is $36\text{ m}$ and a wet-road distance is at least double, the wet-road estimate is:

That is an estimate, not a guarantee. Real stopping distances depend on the vehicle, driver, road surface, and conditions.

Brakes and tyres

Poor vehicle condition is limited here to the vehicle's brakes or tyres.

Poor brakes increase braking distance because the braking system cannot slow the wheels as effectively. The vehicle then takes longer distance to come to rest after the brakes are applied.

Poor tyres increase braking distance because they give less effective grip with the road. Worn tread is especially risky on wet roads because the tyre is less able to maintain good contact with the road surface. Incorrect tyre pressure can also reduce how well the tyre grips and how well the vehicle handles while braking.

Brakes and tyres are different causes, so keep them separate in explanations:

• poor brakes: weaker braking effect on the wheels
• poor tyres: less grip between tyres and road

Both can lead to the same result: a longer braking distance and a larger clear space needed to stop safely.