๐Ÿ›‘ Stopping Distance & Following Gap

Where your truck actually stops — thinking, brake lag and rubber on the road, added up — and the following gap in seconds that goes with it. Change the surface, the grade or the load and watch the number move.

55
Friction coefficient the tool uses. Tyres and surface both matter more than this number suggests.
Positive = going downhill, negative = climbing.
Air brakes take about half a second to reach full power.
Defaults: 55 mph on dry pavement, flat, an alert driver at 1.5 seconds of reaction and half a second of air-brake lag.
Where the numbers come from (and the awkward bit about weight)

Total stopping distance has three parts: the distance rolled while you are still deciding (speed × reaction time), the distance rolled while the air reaches the chambers (speed × lag), and the braking distance itself, d = v² ÷ (2g(μ ± grade)). Because braking distance goes with the square of speed, 60 mph needs far more than 20% more road than 50 mph. A downhill grade subtracts from the grip you can use; a climb adds to it.

Why load barely changes it. A tyre's grip is close to proportional to the load on it, so weight cancels out of the friction calculation — the theoretical minimum stopping distance of a loaded and an empty tractor-trailer are nearly the same. What weight does change is everything around the physics: brake balance heats up and fades, an empty or lightly loaded set of drives locks easily, a bobtail can swap ends under a hard stop, and a loaded trailer pushes the tractor through a jackknife. Treat the empty truck as less forgiving, not quicker to stop.

The gap in seconds. Your stopping time is the reaction time plus the lag plus the time spent actually slowing down, which is v ÷ (g(μ ± grade)). A following gap equal to that time leaves you arriving exactly with the vehicle in front stationary — so the tool adds a second of margin and reports a gap you can count. The Highway Code's two seconds is a minimum for light vehicles at full alertness on a dry road; a heavy, air-braked combination in the wet needs several times that, and the arithmetic here shows you exactly how much.

Friction coefficients are typical engineering values, not promises: a smoothed, glazed or recently-wet surface can be far worse than the category suggests. Brake condition, fade and tyre choice are not in the equations.