A good way to judge a safe distance behind another vehicle is the two-second rule. In good dry conditions, stay at least two seconds behind; in poor conditions, double the gap to four seconds or more.
How to measure
Choose a fixed point such as a tree or sign. When the vehicle ahead passes it, say “only a fool breaks the two-second rule”. If you reach the point before finishing, you are too close.
Defensive driving
If a vehicle behind is too close, ease off gradually and increase the gap ahead so you have more time to react.