Vehicle Braking Time

Vehicle braking time and stopping distance

Braking time comes from t = v / a and distance from d = v² / (2 · μ · g). Here v is in m/s, a in m/s², and μ is the tire-road friction coefficient (0.7-0.9 on dry asphalt, 0.4-0.6 wet, 0.1-0.3 on ice). Run the numbers for 100 km/h, which is 27.8 m/s. With a = −7 m/s² the car takes roughly 4 seconds to stop and covers around 55 m of pure braking. You still have to add about 1 second of reaction time, another 28 m at this speed, which pushes the total stopping distance past 80 m. ABS holds μ near its peak by keeping the wheels from locking. Lock the tires up without ABS and μ falls by 20-30%.

Applications

Traffic safety classes lean on it, and so do forensic experts reconstructing accidents. It shows up in driving school theory, behind the 2-second following rule, when sizing safe gaps on highways, and while tuning AEB (autonomous emergency braking) systems.

FAQ

Why does distance grow with the square of speed? Because kinetic energy scales with v². Double the speed and the brakes suddenly have four times as much energy to bleed off.

Does ABS shorten braking distance? On dry pavement the gain is usually small. On wet roads, gravel or mixed surfaces it clearly helps, and either way it lets you keep steering while you brake.

What is the 2-second rule? Leave at least 2 seconds of gap to the car in front. That covers your reaction time plus most of the braking on dry roads. When it rains, make it four.