You are driving on a narrow road. About 20 m ahead of you a child suddenly runs onto the roadway. When is a collision unavoidable despite emergency braking?

A collision can be unavoidable at 50 km/h because 20 m is often not enough distance once you include both reaction distance and braking distance.

Even with “emergency braking,” the car does not start braking instantly. You first need a short moment to perceive the danger and hit the brake (reaction time). A common rule of thumb in German theory questions is about 1 second for reaction time. At 50 km/h you travel roughly 14 m in that one second (50 km/h ≈ 14 m/s). That means before the brakes even fully bite, most of the 20 m can already be gone.

After that, you still need braking distance. Even with strong braking on a normal road, you typically need additional meters to stop from 50 km/h. So reaction distance (already close to the full 20 m) plus braking distance usually exceeds 20 m, making a collision possible or even likely despite emergency braking.

Why the other speeds are not the “unavoidable” answer: At 30 km/h (≈ 8 m/s) or 20 km/h (≈ 6 m/s), your reaction distance is much shorter, and the braking distance is also far shorter, so stopping within 20 m is much more realistic. The key learning rule: stopping distance is reaction distance + braking distance, and it rises sharply with speed, so “a few km/h more” can make the difference between stopping and hitting.