Crash Avoidance Space -  Three Second Rule

A safe low risk driver maintains a Crash Avoidance Space completely around the vehicle by adjusting the vehicle’s speed and road position.

To determine the Crash Avoidance Space to the front of the vehicle you need to take into account two key factors – Brake Reaction Time and Brake Response Time.

Brake Reaction Time is the time the driver needs to:

• See the information.

• Perceive what it means.

• Decide on a response.

• Instigate that response.

A driver who is fit, concentrating, alert and not affected by alcohol, drugs, fatigue or a distraction, will require about one and a half seconds to react to a hazard.

Brake Response Time is the time required to take action. Generally a minimum of one and a half seconds is needed to respond when driving a car.  In many situations braking may be the only possible response. Swerving is rarely appropriate and can result in a more severe crash, for example a head-on collision.

A total of three seconds crash avoidance space is needed to react and respond to a situation in front of you when driving a car. You may need even longer in poor conditions such as in rain or darkness.

The three-second gap can be used when following another vehicle or if there is potential for something to move into your crash avoidance space.

Following another vehicle

To calculate a three-second crash avoidance space when following another vehicle use this basic technique.

As the rear of the vehicle in front of you passes an object at the side of the road such as a power pole, tree or sign, start a three-second count ‘one thousand and one, one thousand and two, one thousand and three’.

If your car passes the object you picked before you finish the three-second count, you are following too closely. Your crash avoidance space is not large enough. Slow down, and repeat the count again until the three-second crash avoidance space is achieved.

In poor driving conditions, such as rain, night and gravel roads, it may be necessary to increase 'your crash avoidance space' to four or more seconds.

To reduce the risk of driving into the rear of a vehicle, the three-second crash avoidance space is essential, as the vehicle in front has the potential to stop very quickly if it collides with another vehicle or stationary object.