A. To allow overtaking vehicles to move back into the line of traffic safely if required.
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A. To allow overtaking vehicles to move back into the line of traffic safely if required.
B. To give you enough time to stop if the vehicle in front stops.
C. Two seconds is enough time to stop in any weather conditions, if required.
Your total stopping distance consists of your thinking distance and your braking distance. It can take a driver a second to realise that the vehicle in front has stopped, so that leaves only one second to either bring the vehicle to a halt, or take evasive action around the other vehicle. Technically, even two seconds can be insufficient to stop when you factor in reaction times. In drag racing a 0.5-second reaction time to the lights is considered perfect; older drivers can have reaction times that exceed 1.5 seconds.
Why don't we have more nose-to-tail accidents, then?
It's not often that a vehicle in front would stop immediately as it has its own momentum. It would have to hit something enormous and solid to stop - e.g. a large boulder in the road, or another very large vehicle head-on. In a lot of these cases, the driver ahead would be aware and will start braking, and that will give the driver behind some time to start reacting.
When following using very short distances the main problem is that if the car in front brakes heavily, the driver following might not even have time to react before hitting it.
You should also consider your vehicle vs other vehicles. In general:
If it's wet or icy then increase your following distance to four seconds. As well as giving you more room to stop, it also improves your view of the road ahead, giving you more time to react.