Stopping distance is reaction distance plus braking distance. This is how you calculate it using classical kinematics — why μ (friction) and reaction time are estimates that vary with load, tyres and road conditions, and how the 'distance rule' translates the physics into a usable driving rule.
Introduction
The stopping distance is the total distance from the moment you spot the hazard until the vehicle comes to a stop. It consists of two parts: the reaction distance (you keep moving while you react) and the braking distance (the vehicle slows down).
The formula
The reaction distance grows linearly with speed, but the braking distance grows with the square of the speed — so the stopping distance becomes very long very quickly as the speed rises.
Friction is the uncertain figure
- 01Dry asphalt, new truck: µ ≈ 0.8 (sources give 0.8–0.85).
- 02Conservative calculation recommendation: µ ≈ 0.4.
- 03Wet or slippery road: lower µ — and therefore a longer braking distance.
The truck's air brake
A car has hydraulic brakes; a truck brakes with compressed air. The system is divided into at least two independent circuits (dual circuit) so failure in one circuit does not take all braking power. There is a supply section (compressor + pressure vessels) and a control section (brake valve, you step on). The spring brake (parking brake) holds automatically when pressure drops — therefore a truck 'locks' itself when pressure is lost.
Because the brake works with air, there is a brief moment (fractions of a second) from when you press until pressure is built up in all brake cylinders. This build-up time adds to your own reaction time and extends the total stopping distance. ECE regulation no. 13 (R13) sets limits for precisely this response time and for the minimum braking performance.
ABS, EBS and load-dependent braking force
- 01ABS prevents the wheels from locking so you retain steering under hard braking.
- 02EBS (electronic braking system) controls pressure electronically per axle and responds faster than a purely pneumatic system.
- 03Load-dependent brake force regulation adjusts brake pressure to how heavily the vehicle is loaded — an empty car must be braked with less force than a fully loaded one, otherwise the wheels lock.
Engine brake, retarder and fade
On long downhill stretches the service brake (footbrake) alone is not enough: if the brakes get too hot they lose effect — this is called fade (brake fade). That is why heavy vehicles use retarder brakes, which do not wear on the brake pads: engine brake (exhaust brake) and retarder (hydraulic or electric). The rule on long descents is to select a low gear and let the retarder brake hold the speed, so the footbrake stays 'fresh' in case hard braking is suddenly needed.
Brake distribution between tractor and trailer
In an articulated vehicle, the tractor and trailer brakes must be coordinated. If the trailer brakes too little, it 'pushes' the tractor (risk of jackknifing); if it brakes too much, it can lock. The EBS coupling between the units ensures that brake pressure is distributed correctly so the articulated vehicle remains straight and stable during braking.
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