Factors Affecting Braking Distance

Road Surface Conditions

The condition of the road surface has a major impact on braking distance because it affects the friction between the tyres and the road.

• Dry surfaces provide good friction, allowing tyres to grip the road well and brake efficiently.
• Wet surfaces reduce friction because water creates a slippery layer, increasing braking distance.
• Icy or slippery roads have very low friction, causing tyres to skid and greatly increasing braking distance.
• Surface texture matters: rough surfaces give better grip than smooth surfaces, helping reduce braking distance.

For example, braking on a wet road can double the braking distance compared to a dry road at the same speed.

Vehicle Condition

The state of the vehicle itself affects how quickly it can stop once the brakes are applied.

• Brake quality and wear: Worn or faulty brakes reduce the braking force that can be applied, increasing braking distance.
• Tyre tread and pressure: Good tread helps tyres grip the road. Bald tyres or incorrect tyre pressure reduce friction and increase braking distance.
• Vehicle mass: Heavier vehicles have more momentum and require more force to stop, increasing braking distance.

Maintaining brakes and tyres in good condition is essential for safe stopping.

Speed of Vehicle

Speed has a very strong effect on braking distance.

• Higher speed means the vehicle has more kinetic energy, which is given by the formula:

$$\text{Kinetic energy} = \frac{1}{2} m v^2$$

• Because kinetic energy increases with the square of speed, doubling the speed quadruples the kinetic energy.
• More kinetic energy means more work is needed to stop the vehicle, so braking distance increases rapidly with speed.
• The braking force applied by the brakes is limited, so the higher the speed, the longer the distance needed to stop.

For instance, a car travelling at 20 m/s has four times the kinetic energy of the same car at 10 m/s, so it needs roughly four times the braking distance.

Example: A car of mass 1000 kg is travelling at 20 m/s. Calculate its kinetic energy.

Using the formula:

$$KE = \frac{1}{2} \times 1000 \times 20^2 = 0.5 \times 1000 \times 400 = 200,000 \text{ J}$$

Driver Factors

The driver's actions and condition also affect braking distance.

Note: Braking distance is the distance travelled from the moment the brakes are applied until the vehicle stops. This is different from thinking distance, which is the distance travelled during the driver's reaction time before braking begins.

• Braking force applied: The harder the driver presses the brake pedal, the greater the braking force and the shorter the braking distance.
• Reaction time influence: While reaction time mainly affects thinking distance, it can indirectly affect braking distance if the driver brakes too late or not firmly enough.
• Driver alertness: A tired or distracted driver may not apply brakes effectively, increasing braking distance.