Stopping distance has two parts
Thinking distance + braking distance.
Thinking distance grows with speed, tiredness and distraction. Braking distance grows with speed, weight and the surface. Both get worse together, which is why stopping distance rises far faster than speed does.
The two-second rule
Watch the vehicle ahead pass a fixed point — a sign, a bridge, a marking — and count two seconds before you reach it. If you get there sooner, you are too close.
It beats memorising distances because it scales automatically with speed. Double it to four seconds in the wet, and leave far more on ice.
Wet, ice and aquaplaning
A wet road can roughly double your braking distance. Ice can multiply it by ten.
Aquaplaning is when the tyres ride up on a film of water and lose contact with the road. The steering goes light and unresponsive. Ease off the accelerator, hold the wheel straight, and do not brake hard — wait for the tyres to find the surface again.
Skidding
With ABS: press the brake pedal firmly and keep it pressed, and steer where you want to go. ABS stops the wheels locking so you can still steer — that is the point of it. It does not always shorten the stopping distance, and on gravel or snow it can lengthen it.
Without ABS: release the brake to let the wheels turn again, then re-apply more gently. A locked wheel can neither steer nor brake efficiently.
Downhill
Select a lower gear so engine braking controls your speed. Riding the brakes down a long descent overheats them and fades them away exactly when you need them.
Loads
More weight means more energy to shed, so a laden car or one towing a trailer needs to brake earlier and leave a bigger gap.