There are so many rear-end prangs these days I think it's because people drive too close to each other.

Part of the Highway Code in the UK is a table of Breaking Distances... the amount of space you should be behind a car if travelling at such and such a speed, the proper distance giving you enough time & space to break safely without crashing into the car.

But an easier way to do it, is by following the golden rule : "Only a fool breaks the 3 second rule!"
When the car in front passes a 'given point' eg a shadow on the road or a bridge or whatever, you say "Only a fool…" to yourself.

If you reach the 'given point' before finishing the 'rule' you are too close.
Happy safe driving :D