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Many drivers, drive in a false belief that if the car in front suddenly started braking, they would react and brake and end up stopped the same distance apart.

The total stopping distance of a vehicle is made up of 4 components.

    • Human Perception Time
    • Human Reaction Time
    • Vehicle Reaction Time
    • Vehicle Braking Capability

The human perception time is how long the driver takes to see the hazard, and the brain realize it is a hazard requiring an immediate reaction. This perception time can be as long as ¼ to ½ a second.

Once the brain realizes danger, the human reaction time is how long the body takes to move the foot from accelerator to brake pedal. Again this reaction time can vary from ¼ - ¾ of a second.

These first 2 components of stopping distance are human factors and as such can be effected by tiredness, alcohol, fatigue and concentration levels. A perception and reaction time of 3 or 4 seconds is possible. 4 seconds at 100 km/hr means the car travels 110 metres before the brakes are applied.

Once the brake pedal is applied there is the vehicles reaction time which depends on the brake pedal free-play, hydraulic properties of the brake fluid and working order of the braking system.

This is why the tailgating car usually cannot stop, when the brake light came on in the car in front, this driver had already completed the perception, human and vehicle reaction periods. The following driver was perhaps 1 second to late in applying the brakes. At 100km/hr the car required 28 metres further to stop.

The last factor than determines the total stopping distance is the cars braking capability which depends on factors such as:
The type of braking system
Brake pad material
Brake alignment
Tyre pressures
Tyre tread and grip
Vehicle weight
Suspension system
The co-efficient of friction of the road surface
Wind speed
Slope of road
Surface smoothness
The braking technique applied by the driver (and/or ABS systems)
Worth noting is that from 50 to 100 kph the braking distance of a car will increase from 10 metres to 40 metres. When you double the speed of a car braking distance quadruples.

This is based on the laws of physics. When a car is moving it has kinetic energy, ½mv2. When the velocity doubles the kinetic energy quadruples. The braking capability does not increase when driving faster, there are no reserves of friction. As such in any vehicle when your speed doubles braking distance is four times larger.

Thinking Distances
If the driver is aware of the hazard ahead and knows exactly what to do and when to do it, then The "thinking distance" that the Highway Code talks about is actually the time taken to transfer your foot (or on a motorcycle fingers and foot) to the brakes and begin to apply them. So in reality it's not "thinking distance", it's REACTION distance - and it's this reaction time that is commonly measured as being approximately 0.7 second.

However, to react promptly, the driver has to be already aware of the hazard and ready to brake, just like when doing the emergency stop for the examiner on test! You have to be thinking "I'm going to need to brake" and just be waiting for the correct moment - in the case of the driving test, your cue is the examiner banging his clipboard on the dash!

However, in real driving, the driver needs to SEE the developing situation, IDENTIFY the fact there is a threat and then DECIDE if the required RESPONSE is to brake.

Vehicle weight
Weight is something that has to be considered during towing. If the trailer being towed does not have any braking system then the total weight of car and trailer is added together and spread over just the four braking tyres of the towing vehicle. If the trailer has a braking system then the weight is dispersed over all braking tyres and will give better stopping distances. On a wet or slippery surface the lack of brakes on a trailer will have much more effect on total stopping distances than in the dry.