In this fact page we will cover:
- How vehicle speed is related to stopping distances
- The difference between thinking and braking time
- Why official estimates for stopping distances may be wrong
Stopping distances include the distance travelled while the driver notices a hazard and applies the brakes (thinking distance), and while the vehicle comes to a full stop from its initial speed (braking distance). The government's official estimates of stopping distances for cars are shown below.
Are the official stopping distances wrong?
Brake asked TRL to provide evidence on the time taken by car drivers to perceive, recognise and react to emergency situations.
The thinking distances given in the government's estimate of stopping distances are based on a reaction time of 0.67 seconds, which assumes the driver is alert, concentrating and not impaired. Driving when tired, distracted or impaired significantly increases reaction times, so these thinking distances should be thought of as minimums.
TRL referred to academic literature and concluded that the average thinking time is 1.5 seconds − more than double the 0.67 seconds set out in the Highway Code.
This means that average total stopping distance − including thinking
and braking distance − is an extra 2.75 car lengths (11 metres) at 30mph
and an extra 3.75 car lengths (15 metres) at 40mph compared with the
distances used in the Code. This difference rises to an additional 6.25
car lengths (25 metres) at 70mph.
Cuerden, R. (2017). The mechanics of emergency braking. TRL
Thinking distance is the distance a vehicle travels during the time it takes for the driver to perceive a hazard, recognise that action needs to be taken and decide what the necessary action is, before applying pressure to the brakes. Thinking distance varies from driver to driver, and can be influenced by a number of factors, including driver fatigue, distraction and visual impairment.
Braking distance depends on how fast a vehicle is travelling before the brakes are applied, and is proportional to the square of the initial speed. This means that even small increases in speed mean significantly longer braking distances. Braking distances are much longer for larger and heavier vehicles, and in wet or icy conditions.
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