5.4 Factors affecting the braking distance (ultimately affecting stopping distance too)

What factors affect braking distance?

Again, speed is the first obvious factor whatever the road vehicle - car, lorry, wagon, bus etc.

The faster you are going the more kinetic energy has to removed from the car's kinetic energy store. At a constant rate of braking force, it will take longer the greater the speed, because more kinetic energy has to be converted to heat energy in the brake pad and disc system.

This is shown on the right (brake pads P in contact with disc D).

All of the factors discussed here become particularly crucial in an emergency braking situation or you suddenly find yourself too close to the car in front.

The greater your speed, the greater the stopping distance and the greater distance you should allow between one vehicle and another e.g. the two chevron distance for 70 mph you see on some sections of a motorway.

However good your brakes are, its no good being too close to another vehicle i.e. well within the stopping distance, if you are to avoid an accident if the vehicle in front does an emergency brake or the traffic head rapidly comes to halt!

Speed limits aren't simply about speed reduction, they are also about reducing both the stopping distance where higher speeds are considered hazardous for a particular section of road. This for the safety of road users and pedestrians e.g. 20 mph in narrow streets in built up areas where there are likely to be many people walking and crossing roads.

 

Road condition and the weather:

The adverse condition of a road has already been mentioned. With a dry road (and tyres in good condition) you will get the maximum friction grip from the tyre-road surface contact on braking, giving you the minimum distance travelled - the minimum thinking distance. If the road is wet from rain, coated in snow or ice, the friction grip is reduced (ice > snow >> standing water all leading to skidding on braking). Modern tyres do very well on braking if the road is just a bit wet and no obvious standing water - where can get 'aquaplaning'/'hydroplaning' as you skid over a layer of water on the road surface. Leaves and split oil also reduce the friction grip between tyre and road. All of these conditions reduce the friction grip of tyre on road and increase the braking time and braking distance

 

The condition of the tyres:

Tyres are designed to give the maximum road grip and expel water from under the tyres on wet roads. If the tyres are worn (bald or little tread left), there is less grip and the vital friction and water expelling function to slow the vehicle down are reduced and so increasing the braking distance and the chance of skidding. Also, the tyres should contain enough air to give the correct operating pressure.

 

The effectiveness of the brakes

If the brakes are not well maintained, braking function may be impaired. Brake pads might be worn or a leak in the hydraulic brake system can be a source braking impairment. Are the brakes balanced so that you slow down in a straight line - this is point applies to the condition of the tyres too.