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

You answered correctly. In Germany, the rule of thumb for calculating reaction distance is: halve your speed (in km/h) and use that number as meters per second of reaction distance. So, at 100 km/h, halve to get 50. The reaction distance is therefore 50 meters—but the correct formula is actually: speed (in km/h) divided by 10, then multiplied by 3. So, 100 divided by 10 is 10; 10 times 3 is 30 meters. That means with 1 second of reaction time at 100 km/h, your reaction distance is 30 meters. This helps you quickly estimate how far you travel before you even start braking.