Three-second Following Rule
A safe following distance varies with speed. To work out safe following distances with these variables would require a calculator. The problem is solved by working from another factor that is not variable: time. If time is the cushion, the following distance will automatically increase as the speed increases.
For safe driving, keep a cushion of 3 seconds between vehicles. This applies at all speeds. From the moment the leading vehicle passes a fixed object (a parked car, a light pole, etc.) 3 seconds should elapse before the following vehicle passes the same point. One easy way to calculate this period is to say “one thousand and one (1 second), one thousand and two (2 seconds), one thousand and three (3 seconds)”.
The following distance should be practiced until it becomes automatic at all speeds.