Following Distance Calculator
Enter your speed and your headway in seconds. The calculator converts the two-second rule into metres and car lengths, shows what your reaction window alone consumes, and teaches the marker method that makes it stick.
Tailgating feels like progress and prices like a crash. This calculator shows the gap the physics wants - which grows with speed while the bumper-to-bumper illusion grows with courage - and why jams dissolve in the space you leave.
How to use
- Enter your real speed and pick headway: 3 s dry and awake, 4 s in rain or dark, 5 s for snow or towing.
- Practise the marker method: fixed point, count the seconds as the car ahead passes it.
- Compare the reaction-gap row with your following distance - the gap must swallow your reaction, not your hopes.
Frequently asked questions
What is the two-second rule?
Keep at least two seconds of time between you and the car ahead: pass a fixed roadside marker no sooner than two seconds after they do. Three seconds is the realistic minimum for alert driving; rain, night and towing each add a second.
Why do tailgaters not actually arrive sooner?
Traffic moves in waves, and close following amplifies every brake tap into a stop-go oscillation - the space you leave absorbs the wave and smooths your own journey. The queue paradox: gaps move traffic faster, not slower.
How does following distance relate to stopping distance?
The gap must cover your reaction distance at that speed - about 40 metres at 100 km/h - before your braking even begins, and ideally the car ahead's braking too. Time-based gaps scale with speed automatically; fixed metres do not.
What headway should I leave at night or in rain?
Add a second per degradation: night costs one for vision, rain one for grip, snow or towing two. Five seconds feels enormous and is merely survivable - the drivers behind can wait for their own safety margin.