How to Use This Tool
Set a speed, a surface and a reaction time. The two parts of the answer are shown separately because they behave completely differently.
One part is linear, the other is not
- Thinking distance is speed × reaction time. Double the speed and it doubles.
- Braking distance is speed² ÷ (2 × friction × g). Double the speed and it quadruples, because the energy to be dissipated goes with the square of speed.
On dry asphalt with a 1.5 second reaction, 50 km/h needs 20.8 m of thinking and 12.3 m of braking — 33.1 m in total. At 100 km/h it is 41.7 m of thinking and 49.2 m of braking, or 90.8 m. Twice the speed, nearly three times the total distance.
The comparison that actually lands
"Further to stop" understates it. At 100 km/h, your reaction alone consumes 41.7 m — further than the 33.1 m in which the 50 km/h car came to a complete halt.
So at the point where the slower car has stopped, you are still doing the full 100 km/h and your foot has not yet reached the brake pedal. That is why speed limits near schools and in residential streets are set where they are: the difference is not "a bit more distance", it is the difference between stopping and not having started.
The friction numbers are optimistic
The coefficient used here is a whole-vehicle figure covering tyres, road surface, temperature and the braking system together. The values offered are conventional:
- 0.8 — dry asphalt, tyres in good condition, warm.
- 0.5 — wet asphalt. Standing water and worn tread take it lower, and hydroplaning takes it close to zero.
- 0.25 — packed snow.
- 0.1 — ice, where braking distance is eight times the dry figure.
Real values vary a great deal. Cold tyres, a polished or contaminated surface, a heavy load, worn pads or brake fade all reduce grip below these, and the first rain after a dry spell is worse than steady rain because oil is lifted off the surface. Treat every number here as a best case.
Reaction time
1.5 seconds is the conventional figure for an alert driver responding to an unexpected event, and it covers perception as well as movement. It gets worse quickly: fatigue, alcohol, a phone conversation and searching for something in the cabin all add to it, and a distracted driver can be several seconds.
Because thinking distance is linear in speed, an extra second costs 13.9 m at 50 km/h and 27.8 m at 100 km/h — on top of braking that has already quadrupled.
What this leaves out
It assumes constant deceleration in a straight line on a level, uniform surface with the brakes at full effect immediately. Reality adds brake system lag, weight transfer, ABS cycling, camber, gradient and the fact that steering and braking share the same grip. This is physics for understanding the shape of the problem, not a reconstruction tool.
