Vehicle Braking Distance

Braking distance: physics behind the stop

Pure braking distance follows d = v² / (2 × μ × g), where μ is the tire-road friction coefficient and g ≈ 9.81 m/s². Run the numbers for 100 km/h = 27.8 m/s on dry asphalt (μ ≈ 0.7) and you get d ≈ 55 m. Wet asphalt (μ ≈ 0.4) pushes the same speed to about 97 m. Then there's the reaction distance, which is the driver's reaction time of 0.7-1.5 s times v. At 100 km/h that tacks on another 19-42 m before the brakes even engage, so the total stopping distance at 100 km/h in the wet can top 130 m.

Applications

Driving-school theory uses it, and so does the two-second rule for following distance. It shows up in accident reconstruction and forensic analysis, in road safety education aligned with the Brazilian traffic code (CTB), and when you want to check brake performance after maintenance.

FAQ

Why does doubling speed quadruple braking distance? Because d depends on v². Going from 50 to 100 km/h multiplies the distance by 4, not 2.

What friction coefficient should I use? Dry asphalt sits at 0.7-0.8, wet at 0.35-0.5, snow around 0.2 and ice near 0.1. Fresh tires land at the higher end of each range, worn ones at the lower end.

Does ABS reduce braking distance? Usually not on dry asphalt. What it does there is prevent lock-up so you keep steering control. On wet or loose surfaces it does shorten the distance.