Young
DriversDriver Education & Road Safety
Following Distance (3-second rule)
One of THE biggest problems on our roads is the failure to achieve and maintain an adequate following distance.
There are numerous sites that talk about the 3-second rule. Thats 3 not 2! But why do we have to keep a three second distance? Those sites explain how to achieve the distance by picking a stationary object that the vehicle in front passes, then you count 'one thousand and one, one thousand and two, one thousand and three'. If you pass that object prior to counting to three, you are too close.
That was easy wasn't it? Great!
But why do it?
At most, the thing drivers are taught is all about the braking distance. This will vary with the changes in speed.
Let us look at the average stopping distance per vehicle speed:
at 40 km/h… 10 metres
at 60 km/h… 19 metres
at 80 km/h… 38 metres
at 100 km/h… 55 metres
at 120 km/h… 88 metres
Again, that was easy wasn't it? BUT
It is not the braking distance that is the problem. It is your reaction time!
REACTION TIME?
Let us look at the physics of driving with regard to stopping distances.
Depending on the speed the vehicle is travelling, distance travelled each second is:
at 40 km/h… 11 metres
at 60 km/h… 16.5 metres
at 80 km/h… 22 metres
at 100 km/h… 27.8 metres
at 120 km/h… 82.5 metres
Not averages, FACT!
Now back to reaction times.
To a predictable hazard such as the vehicle in front of you puts his foot on the brakes causing the brake lights to come on, reaction times vary between .5 and 1.0 second. That is very good. As humans, we have a very good reaction time to something that is predictable.
But when do we have crashes?
When it is UNPREDICTABLE!!!
That might be the vehicle in front of you jumps on the brakes and his tail lights are not working!
Unpredictable? You bet.
As humans, we have a bad reaction time to something that is unpredictable.
Summala (1981) measured the reaction times of 1326 drivers to an unpredictable hazard in actual driving conditions. What he found was that the reaction times varied between 1.5 and 4.0 seconds, with the average being 2.5 seconds.
This equates to:
at 40 km/h… 27.5 metres
at 60 km/h… 41 metres
at 80 km/h… 55 metres
at 100 km/h… 68.5 metres
at 120 km/h… 82.5 metres
Now when we put all this together, braking distance, distance travelled every second and average reaction times, the total distances from seeing the hazard to actually stopping now looks like this:
at 40 km/h… 37.5 metres
at 60 km/h… 60 metres
at 80 km/h… 93 metres
at 100 km/h… 123.5 metres
at 120 km/h… 170.5 metres
Of course everyone has a distorted belief that we are better than average drivers and have better braking and reaction times. But this is the reality of the situation.
It should also be stated that these figures assume that the road surface is good, tyres and brakes are good, the weather is good and the driver is good (refer to fatigue page).