Safe following distance
Enter the speed: find how many meters to keep from the vehicle ahead with the 2 and 3 second rule.
The seconds rule
A simple way to keep the right distance: when the car ahead passes a fixed point, you should pass it at least 2 seconds later (3 in rain or poor visibility). These meters grow with speed.
Note
The safe distance must let you stop in time: at high speeds you need dozens of meters.