The Three-Second Rule
The three-second rule applies at every speed: count 'one-thousand-one, one-thousand-two, one-thousand-three' from when the car ahead passes a fixed point until you pass the same point. At 90 km/h = 75 metres. Halve the distance (1,5 sec) and you double the crash risk under hard braking. On winter roads or rain-wet tarmac, double the rule to 6 seconds.
