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Possibly one of the major problems contributing to South Africa's appalling road accident record is the failure of so many South African drivers to maintain a safe following distance. This problem is aggravated by the unanswered question: "What is a safe following distance?" Here FleetWatch legislation correspondent Jack Webster throws some light on the subject.

Several very clever experts in mathematical calculations have tried to discover a general formula for calculating a safe following distance without success, the reason being that to establish a formula, there must be variable factors and one of these factors cannot be determined!

In discussing following distance, a very important consideration must be addressed - that is the minimum stopping distance of the leading and following vehicles in the consideration. Stopping distance is dealt with in Road Traffic Regulation 155 and comprehensively in the FleetWatch A - Z of Road Transport 2006, pages 92 and 93, but these two documents deal with the distance travelled from the time the brakes are ACTUALLY applied until the vehicle stops, starting from an initial given road speed x km/h.

The formula used to calculate stopping distance is given in Table 1.

Missing factor
However, the formula for stopping distance and attempts to develop any formula for following distance miss out on an important factor which, as mentioned above, cannot be either determined or calculated. The missing factor is:

The actual reaction time in seconds of the driver from the time the driver's brain gets a message that there is a need to apply the brakes until the brakes are actually applied. The driver's reaction time can vary considerably in the same person according to his state of mind and health at a particular time.

The importance of recognising the driver reaction time cannot be over-emphasised when it is understood that a motor vehicle travelling at one km/h travels 0.2778 m/sec and at 80 km/h travels 22.2 m in one second and at 120 km/h, travels 33.3 m in one second. And remember, these distances are travelled during driver reaction time before the brakes are applied.

Let's take an example involving Driver A driving a truck and travelling on a freeway at 80 km/h. He is followed by Driver B driving an identical truck at 80 km/h and following 12 m behind Driver A.

Driver A is alert and is not fatigued but driver B is tired and not too alert. Driver A has a reason to apply his brakes and stop. The brake light comes on at the rear of his vehicle immediately he applies his brakes. Driver A's vehicle has brakes which will give a retardation rate of 4.4 m/sec˛ and will stop in 68.0561 m.

Driver B sees the brake light of the vehicle in front come on. Driver A will stop in 68.0561m but driver B is tired and his reaction time is extended by one second. His vehicle will therefore travel for one second at 80 km/h before he applies his brakes during which time he will travel a distance of 22.2 m. (One second at 80 km/h = 22.2 m.)

This means Truck B will travel 68.0561 + 22.2 = 90.2561 m before stopping. But Driver B started only 12 m behind Driver A therefore 10.2 m before stopping, Truck B will collide with Truck A. 

It is so often not realised that driver fatigue is one of the most important factors in the determination of a safe following distance. Road and weather conditions usually apply to both the leading vehicle and the following vehicle.

A further point related to driver reaction time is the affect of alcohol. A driver may well be below the maximum legally permitted blood/alcohol limit but a couple of drinks may well extend a driver's reaction time to a dangerous limit. 

Finally, a rule of thumb to determine a safe following distance when a driver is tired, frustrated or in any way fatigued (mentally or physically) is to check your road speed and divide the km/h by three. The answer gives you the number of metres you should be behind the vehicle you are following - assuming that the two vehicles involved are similar in specification.

Of course, if you are really tired or fatigued, you should not be driving at all!!