Measure a gap in seconds
The manual teaches at least a two-second following gap for motorcycles and light motor vehicles, and at least three seconds for heavy motor vehicles in suitable conditions. A time gap automatically grows in metres as speed rises. Pick a fixed point beside the road, begin counting when the vehicle ahead passes it, and check that you do not reach it before the interval has elapsed.
These are minimum study guidelines for favourable conditions, not a promise that every vehicle will stop within that distance. You still have to react before braking begins. If you count less than the interval, ease off smoothly and rebuild space; do not suddenly brake to create it.
When the minimum is insufficient
Increase the gap when visibility or grip is poor, the road is wet, the vehicle is heavily loaded, or another condition lengthens stopping or observation. Leave extra room when following a large vehicle that hides the road ahead. Do not copy its speed just because you can still see its rear lamps.
If a driver follows too closely behind you, build more space ahead so that any slowing can be gradual. Do not accelerate beyond a safe or lawful speed and do not brake sharply to punish the following driver. Create a lawful opportunity for that driver to pass when possible.
Worked situation
At a suitable fixed marker you count only “one” before your car reaches the place the leading vehicle passed. The road is wet. You need more than the ordinary two-second minimum, so reduce your speed smoothly until a larger gap opens. The task is to restore reaction and braking space, not merely to complete a count.
Keep the gap usable
Measure repeatedly when speed or traffic changes. If another vehicle fills the gap, rebuild it calmly. A safe following distance is space you can use to see, decide and slow while remaining in control.