Space and Time: Reaction Times & Stopping Distances

In the first Highway Code, published in 1931, the first rule for drivers of motor vehicles concerned speed.  It simply said:

“Carefully regulate your speed at all times to suit the circumstances and weather prevailing. Always have your vehicle under full control and be ready and able to pull up well within the distance which you can see to be clear”.

Later editions of the Highway Code to this day, state driver reaction times and stopping distances, along with advice to keep at least a two second gap from the vehicle in front.

Most road safety practitioners would agree that these are the most important rules in the Highway Code.  If drivers were capable to giving these rules their full attention, this would quickly bring down the yearly death and casualty statistics.

The relation between speed and accidents is a straightforward, the faster the speed of a vehicle, the greater the risk of an RTA and the more serious the level of injury sustained on impact.

The popular slogan `Speed Kills' conjures up an image of driving at 100 mph on a motorway. In fact, just 5 mph too fast in the wrong place can be a more common problem. Most speed is lost just before we stop rather than as soon as we start braking.

This means, for instance, that where a pedestrian walks out in front of a car travelling at 30 mph. If the driver brakes when the pedestrian is still at least 45 feet away (14 metres), there will be enough space in which to stop.

If, however, the car is travelling at 35 mph and the driver brakes at the same point, the car will still be travelling at 18 mph when it hits the pedestrian. This impact can be sufficient to serious injury or even kill the pedestrian and damage the car to an extent where it has to be towed away.