Emergency Braking Distance

Emergency braking distance by speed/road

Total stopping distance works out to d = v·t_reação + v²/(2·μ·g). Here v is speed (m/s), t_reação ≈ 1.0-1.5s, μ is the friction coefficient (dry asphalt 0.7-0.8, wet 0.4-0.5, ice 0.1-0.2), and g = 9.81 m/s². Run 80 km/h on dry asphalt and you get about 57 m in total, split into 22 m of reaction and 35 m of actual braking.

CONTRAN sets minimum stopping distances for each vehicle class. By stopping the wheels from locking, ABS shaves roughly 20% off the wet braking distance. Load matters too: a loaded truck against an empty one can differ by more than 50%. See SBC TI (Tecnologia Industrial) tests and SAE J2944.

Applications

You'll find it used by CONTRAN and SAE Brasil, in technical crash forensics, in professional driver training, at driving schools and in simulators, and in INMETRO's ABS tests.

FAQ

Why does wet asphalt double stopping distance? The friction coefficient falls from about 0.75 to around 0.4, since the film of water cuts tire grip by roughly half.

Does ABS shorten braking distance? Yes, mostly on wet or loose surfaces. On dry asphalt a skilled driver can come close to matching it.

Why is reaction time 1.5s? It's an empirical figure from AAA/NHTSA studies, and it covers perception, the decision to brake, and moving your foot to the pedal.