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.
Why two seconds and not twenty metres
The rule is counted in seconds and not in metres for a practical reason: metres cannot be judged by eye, seconds can. You pick a fixed point at the roadside (a sign, a bridge), watch the car ahead pass it and count: one thousand and one, one thousand and two.
The number works at any speed, and that is its virtue: two seconds at 50 km/h is 28 metres, at 130 km/h it is 72. The same rule gives the right gap on its own with nothing to recalculate.
Reaction time is half the problem
Stopping distance is made of two pieces: the stretch covered while you react and the one covered while you brake. The first lasts about a second for an alert driver, and in that second the car does not slow by a single km/h: at 130 km/h that is 36 metres at full speed.
It is why a phone in your hand is so dangerous: it does not weaken the braking, it lengthens the reaction. One extra second of distraction at 130 km/h is another 36 metres, more than two lorry lengths of queue.
When two seconds are not enough
In the wet the gap should be doubled, and on snow or ice tripled: grip drops and the braking distance grows far faster than the speed. At night it is worth stretching it too, because the obstacle appears later.
More seconds are needed when you brake worse than the vehicle ahead: with a loaded car, a trailer, worn tyres, or behind a motorbike, which stops in far less space than a car. On motorways the roadside marker posts help: they stand at known intervals, so two of them are already a measurement.
Nearby tools
The space you actually need to stop is worked out with Braking distance. If the tyres are the weak point, Tyre sizes and Read the tyre code say what you are running on.