Stopping Distances

Over a total stopping distance of say 40-metres, more than half the speed is wiped off in the last 10-metres, or two and a half car lengths. Thirty metres to get from 60- to 30mph; just 10-metres to get from 30mph to zero. That’s scary when you put it in context with thinking distance.

Stopping Distances

When you want to stop in a hurry there are several factors that will define the outcome. Among the most important is the Thinking Distance. This is the distance you travel during the time it takes to spot a hazard worth braking for and then do something about it. Thinking distance is entirely linked to your powers of observation and your speed.

The Highway Code suggests thinking and braking distances for bringing a vehicle to a stop from different speeds. With modern tyres and a trained brain, you can beat the distances it quotes. At 60mph the average driver’s reaction time is 0.6 seconds, which equals 18-metres covered, or about four car lengths. If you double your speed it naturally doubles your reaction distance, which is why you extend your vision at high speed in order to see things coming from further away.

After a gut-busting meal or when you’re tired, your thinking distance edges closer to 1 second or seven car lengths, maybe a lot longer. The trick is to reduce the thinking distance closer to zero by anticipating the situation and covering the brake pedal with your foot, so you’re ready the moment you need to use it.

Expect the confused individual looking for a parking space in front of you to anchor up without warning and you can’t go wrong. Anticipation replaces a reaction to an event with a true action. As for the unexpected, it pays to be decisive and measured.

Next we come to the Stopping Distance, which is the battle between speed and grip. The important thing to comprehend about deceleration is this process is not a constant. Our perception of the slowing process is linear because we tend to think in terms of time and we generally shed around 20 mph for every second of braking. A car that doubles its speed will take twice the time to stop. And that sounds OK.

However, during that extra time it travels twice as fast and covers four times the distance. You might also think a vehicle travelling at 60mph would shed a larger proportion of its speed at the onset of braking. The opposite is true.

Over a total stopping distance of say 40-metres, more than half the speed is wiped off in the last 10-metres, or two and a half car lengths. Thirty metres to get from 60- to 30mph; just 10-metres to get from 30mph to zero. That’s scary when you put it in context with thinking distance.

You’re on a motorway where everyone is travelling at 70mph, then you remember the chocolate bar you hid in the glovebox and go for the grab. When you look back up, there’s a sea of brake lights across all three lanes. In the second that passed, the traffic ahead slowed to 50. By the time you react they are doing 30mph with a rate of deceleration that is a hell of a lot faster than yours. You can’t match them, so unless you have the distance you find yourself at the wrong end of the braking curve, and the last one to collide in the concertina.

This is Newton’s fault. A vehicle builds up kinetic energy by a factor of its weight multiplied by the square of its speed, and as a result the braking distance increases exponentially. So, if you double the speed, you quadruple the braking distance. A Fiat 500 moving at 40mph hits with the impact energy of a lorry doing 10mph.

This physics lesson matters for several reasons. Anything that postpones the onset of braking poses a similar problem, such as locking a tyre. At higher speeds when you first brake, a locked tyre creates immense heat and reduces the tyre’s friction just when you need it most. A lock-up at lower speed is far less punitive.