About this tool
The stopping distance is the distance from noticing a hazard to standstill: reaction distance plus braking distance. The reaction distance grows linearly with speed, the braking distance with its square – twice as fast means four times the braking distance.
The calculator shows both: the rule of thumb taught in German driving schools and a physical calculation with reaction time and deceleration depending on the road surface. Enter the distance to an obstacle to see whether you stop in time or at which speed you hit it.
How it works
- Enter the speed and choose the road surface: dry, wet, snow or ice.
- Adjust reaction time and deceleration if needed.
- Optionally enter the distance to an obstacle to see the impact speed.
Frequently asked questions
What is the rule of thumb for stopping distance?
Reaction distance = speed ÷ 10 × 3, braking distance = (speed ÷ 10)². At 50 km/h that is 15 m + 25 m = 40 m. For emergency braking the rule of thumb halves the braking distance.
Why is the physical result shorter than the rule of thumb?
The rule of thumb is deliberately cautious. Modern cars with ABS decelerate at about 8 m/s² or more on a dry road, which gives much shorter braking distances. The rule of thumb is mainly meant for the driving test.
What reaction time is realistic?
About one second is a guide value for attentive drivers. Distraction, fatigue or alcohol make it much longer – a glance at the phone alone can cost one to two seconds.
Why does 30 instead of 50 km/h make such a difference?
Where a car at 30 km/h just comes to a stop, a car at 50 km/h has barely slowed down because it is still within its reaction distance. Try it with the obstacle distance. How fast you really drive is measured by the GPS Speedometer.