You are travelling at 100 km/h, have a reaction time of 1 second, and brake normally. What is the stopping distance according to the rule of thumb?

With the German “rule of thumb” (Faustformel), the stopping distance is the sum of reaction distance and braking distance.

At 100 km/h, you cover about 28 meters per second in real life, but the rule of thumb uses an easy approximation: reaction distance = speed/10 × 3. With 100 km/h and 1 second reaction time, that is 100/10 × 3 = 10 × 3 = 30 m.

For “normal braking,” the rule of thumb is braking distance = (speed/10)². So at 100 km/h: (100/10)² = 10² = 100 m.

Stopping distance = 30 m + 100 m = 130 m, which is why your answer is correct.

Rule to remember for similar questions: reaction distance grows linearly with speed, but braking distance grows with the square of speed, so at higher speeds the braking part dominates quickly.