Stopping distances

One common cause of crashes in South Canterbury is drivers not keeping safe following distances. It is interesting to note that the majority of these crashes happened on a clear day on dry roads where conditions were very good.

Over the past five years rear end/obstruction crashes made out 43% of all reported crashes in South Canterbury. Apart from the common it wouldn’t happen to me mind-set, there are other unique factors involved here.

Most people subconsciously weigh up the risk of being involved in a crash as being very small. Unfortunately it exists – just ask those people it happened to!

The best way to reduce risk is to alter driving habits and always anticipate the possibility of a crash. Think for a change that while you are out there, everybody on the road is trying to crash into you and you have to take evasive steps.

Reaction Distance + Braking Distance = Stopping Distance (Crash or no crash)

Driver A drives a car at 50 km/h, (that is 13.89 meter per second), during the day on a dry, level road. The car in front of him, driver B, suddenly brakes hard. Driver A sees this and applies the brakes. What sort of stopping distance can he reasonably expect?

Total stopping distance consists of two components:

  1. Reaction Distance - Suppose the reaction time is 0.65 seconds. This means that the car will travel 0.65 seconds x 13.89 m/s and that equals 9.02 meter even before the brakes are applied.
  2. Braking Distance -  This is the distance it takes from the time you start braking until you are stationary. An average response time to move a foot from the accelerator pedal to the brake pedal is around 0.3 seconds. At 13.89 m/s x 0.3 = 4.17 meter- one car length! It then takes another 10 meters to come to a complete standstill and that depends heavily on the following factors:
  • Brakes – condition of the brake pads and brake system;
  • Road surface – is it wet or dry, free of loose material like sand, diesel, oil, etc.?
  • Tyres - condition of the tyres – depth of thread, tyre pressure, tyre pattern, mixed tyre patterns on wheels, etc.
  • Weight of the vehicle – the heavier it is the longer it will take to stop.

Level of alertness:

As we can see, almost half of the stopping distance is created by driver reaction time. A driver who is not alert will take longer to identify a risk and subsequently the stopping distance will increase. Here we could put drivers in three levels of alertness:

  1. Expected: The traffic light changes to orange and the car in front of you brake lights go on. Drivers expect vehicles to stop for orange or red lights. Reaction time should be about 0.5 seconds plus the 0.2 seconds to start to brake.
  2. Unexpected: Drivers become aware of potential risks like when a young child starts to wander away from the footpath towards the road. An alert driver would anticipate that the child could possibly enter the road and will take action to slow down or even stop if necessary. Here the reaction time is a bit slower around 1 second plus the 0.2 seconds to start braking.
  3. Surprise: - A child suddenly appears as he comes out running out of a driveway close to the oncoming driver, straight towards the road. This is the worst case scenario. Driver reaction is about 1.2 seconds plus a slower 0.3 seconds before braking.

Factors that could reduce reaction time:

  1. Intoxication (alcohol, drugs or medicine). You do not necessarily have to be over the legal limit for it to have an effect on your reaction time.
  2. Fatigue. Recent studies have shown that a fatigued driver is just as dangerous as an intoxicated driver!
  3. Driver distractions like talking on a mobile phone, texting, changing CD’s or radio stations or talking to someone in the vehicle.

Try to avoid all of these situations as much as possible and focus on your driving. If you lose concentration, maybe it is a good time to stop and take a rest. People may say they don’t have time to stop because of busy schedules, but they are going to be late in any case if they are involved in a crash.

Some may never make it to the appointment.