You are travelling at 50 km/h and have a reaction time of 1 second. What is the reaction distance according to the rule of thumb?

Reaction distance is the distance your car travels during the time it takes you to notice a hazard and start braking. The rule of thumb used in German theory is: reaction distance (in meters) equals speed in km/h divided by 10, then multiplied by 3.

At 50 km/h, 50 ÷ 10 = 5, and 5 × 3 = 15 meters. That’s why “15 m” is correct.

This rule assumes a normal reaction time of about 1 second, as stated in the question. If your reaction time is longer (tired, distracted, alcohol, phone), the reaction distance increases proportionally: at 50 km/h you travel about 14 meters per second, so even half a second more adds roughly 7 extra meters before you even begin to brake.