Braking Distance Calculator

It's the design standard's honest middle: laboratory simple-reaction is 0.7 s, but real roads add perception, decision and foot movement — AASHTO designs highways around 2.5 s to cover 90th-percentile drivers in unexpected scenarios. Alert and expecting trouble (following a brake-light chain) you might genuinely manage 1.0; glancing at a phone makes 2.5–3 s ordinary, which at 70 mph is 250–300 feet of un-braked travel. The slider is there to make that comparison personal.

On dry asphalt, modern tires and ABS achieve μ ≈ 0.7–0.9 (performance cars with warm summer tires exceed 1.0); wet pavement halves it (0.4–0.6, worse in the first minutes of rain when oils float); packed snow ≈ 0.2–0.3; ice 0.05–0.15, nearly frictionless near 0 °C. Note what's NOT in the formula: brake size — street stops are tire-limited, not brake-limited. Big brakes resist fade on repeated stops; they don't shorten a single cold stop.

Different assumptions, same physics: the UK Highway Code uses a short 0.67-s 'thinking time' with conservative braking; US charts vary reaction time and friction. This page exposes the assumptions as inputs instead of hiding them — set 0.67 s and μ 0.67 and you'll reproduce the Highway Code's 96 m at 70 mph almost exactly. The lesson survives every chart variant: past 50 mph, most of the stop is the part your reaction can't shorten.

The 3-second rule exists because it scales automatically with speed and roughly covers reaction plus braking-difference in dry conditions: at 70 mph, 3 seconds is 308 ft. Double it in rain (the calculator shows why: braking distance doubles), and on snow or ice the honest answer is 6–10 seconds plus reduced speed, since the square law means speed reduction is worth far more than any following-distance increase. If the gap feels embarrassingly large, it's approximately correct.