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Stopping distance for cars

Many drivers have a false belief that if the car in front starts braking they can react, brake and come to a stop, still leaving the same distance between the two vehicles.

The total minimum stopping distance of your vehicle depends on four things:

 

image shows two cars decelerating as they approach a junction. one car stops safely , whilst the other impacts a vehicle at the junction

 

Your perception time is how long you take to see a hazard and your brain realising it is a hazard requiring you to take immediate action. This can be as long as 1/4 to 1/2 of a second or more.

Your reaction time is how long you take to move your foot from the accelerator to the brake pedal once your brain understands you are in danger. Your reaction time can vary from 1/4 to 3/4 of a second.

These first 2 components of stopping distance are down to you and can be affected by alcohol, drugs, tiredness, fatigue or lack of concentration. A perception and reaction time of 4 seconds at 100 km/h means the car travels 110 metres before the brakes are applied (this is more than the length of a football pitch).

Once you apply the brake pedal it will take time for your vehicle to react. This depends on the condition your vehicle is in and, in particular, the condition of the braking system.

The last factor that determines your total minimum stopping distance is the vehicle's braking capability. This depends on many things, for example:

Table 5: The RSA recommend you allow a minimum stopping distance under dry conditions of (see table below):
Speed (km/h) Minimum Reaction Distance (m) Minimum Braking Distance (m) Total Minimum Stopping Distance (m)
30 kilometers per hour speed-limit signal 6 6 12
40 km sign 8 10 18
50 kilometers per hour speed-limit signal 10 15 25
60 kilometers per hour speed-limit signal 12 21 33
80 kilometers per hour speed-limit signal 16 36 52
100 km per hour 20 50 70
120 kilometers per hour speed-limit signal 24 78 102
Source Transport Research Laboratory, UK, 2012, © Road Safety Authority, 2012.


Table 6: The RSA recommend you allow a minimum stopping distance under wet conditions of (see table below)
Speed (km/h) Minimum Reaction Distance (m) Minimum Braking Distance (m) Total Minimum Stopping Distance (m)
20 kmh 4 5 9
30 kilometers per hour speed-limit signal 6 10 16
40 kmh 8 17 25
50 kilometers per hour speed-limit signal 10 26 36
60 kilometers per hour speed-limit signal 12 37 49
70 kmh 14 50 64
80 kilometers per hour speed-limit signal 16 65 81
100 km per hour 20 101 121
120 kilometers per hour speed-limit signal 24 145 169
Source Transport Research Laboratory, UK, 2012, © Road Safety Authority, 2012

 

It is worth noting that from 50km/h to 100km/h the total braking distance of your car can increase from 26 metres to at least 101 metres. When you double the speed of your car you multiply the total braking distance nearly four times.

Remember a 5km/h difference in your speed could be the difference between life and death for a vulnerable road user like a pedestrian.

Source RoSPA UK

 

The RSA recommend you allow a minimum overall stopping distance of (see table below):

Total Minimum Stopping Distance

Source Transport Research Laboratory, UK, 2012, © Road Safety Authority 2012

 

Skidding

Any factor which reduces the grip of your tyres on the road is a possible source of skidding. Wet or greasy roads, overloading, worn or improperly inflated tyres, mud, leaves, ice, snow, harsh acceleration, sudden braking, or excessive speed for the conditions can all cause or contribute to a skid.

Aquaplaning occurs when a car is being driven on a wet road and a film of water builds up between the tyres and the road surface.

When that happens, the car loses contact with the road, and braking and steering are affected.