Stopping Distance: Keeping a safe distance Behind other vehicles


Most rear end collisions are caused by drivers following too closely behind the vehicle travelling in front of them.
The gap between your vehicle and the vehicle in front is known as the ‘following distance’.
Maintaining a safe following distance gives you sufficient time to react to emergency situations. The safe following distance will vary with your alertness, the size, condition and speed of your vehicle, the road surface and the weather conditions.  

The total stopping distance of a vehicle is made up of 4 components.
Human Perception Time
Human Reaction Time
Vehicle Reaction Time
Vehicle Braking Capability

The human perception time; is how long the driver takes to see the hazard, and the brain realize it is a hazard requiring an immediate reaction. This perception time can be as long as ¼ to ½ a second.

Once the brain realizes danger, the human reaction time is how long the body takes to move the foot from accelerator to brake pedal. Again this reaction time can vary from ¼ – ¾ of a second.

These first 2 components of stopping distance are human factors and as such can be effected by tiredness, alcohol, fatigue and concentration levels. A perception and reaction time of 3 or 4 seconds is possible. 4 seconds at 100 km/hr means the car travels 110 metres before the brakes are applied.
Once the brake pedal is applied there is the vehicles reaction time which depends on the brake pedal free-play, hydraulic properties of the brake fluid and working order of the braking system.

This is why the tailgating car usually cannot stop, when the brake light come on in the car in front, this driver had already completed the perception, human and vehicle reaction periods. The following driver was perhaps 1 second too late in applying the brakes. At 100km/hr the car required 28 metres further to stop.

The last factor than determines the total stopping distance is the cars braking capability, which depends on factors such as;
the type of braking system, brake pad material, brake alignment, tyre pressures, tyre tread and grip, vehicle weight, suspension system, the co-efficient of friction of the road surface, wind speed, slope of road, surface smoothness
the braking technique applied by the driver.

Worth noting is that from 50 to 100 kph the braking distance of a car will increase from 10 metres to 40 metres. When you double the speed of a car braking distance quadruples.

This is based on the laws of physics. When a car is moving it has kinetic energy, ½mv2. When the velocity doubles the kinetic energy quadruples. The braking capability does not increase when driving faster, there are no reserves of friction. As such in any vehicle when your speed doubles braking distance is four times larger.  
The two-second rule 
The two-second rule, is a simple ‘time-lapse’ formula designed to help drivers maintain a safe following distance.
To check your following distance, pick an object (such as a light pole) that will soon be passed by the vehicle immediately ahead. As soon as the vehicle passes the object, say to yourself “one thousand and one, one thousand and two”.
If you reach the object before you have said “one thousand and two” it means that you are not following at a safe distance. Slow down in order to increase the following distance and apply the rule again.
 In adverse weather conditions and on poor road surfaces you will need to increase the time interval to three or four seconds.
In addition, if you are driving a heavy vehicle or towing a trailer, you will need to increase the time interval accordingly, i.e. by one second for every three metres in length that your vehicle (and trailer) is over six metres.  

Reduce the risk of a rear end crash by: 
• Recognizing potential hazards and observing the traffic ahead;
• Staying alert and being prepared to respond to situations as they arise; and
• Making sure your vehicle is always properly maintained, taking particular care with brakes, tyres and suspension.  

Long Vehicles
A long vehicle is any vehicle or combination (such as a towed caravan or trailer) that is 7.5m or longer. A long vehicle travelling outside a built up area must not travel closer than 200m to another long vehicle unless overtaking.

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