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

At 50 km/h, in 1 second of reaction time you continue rolling before you even start braking. The rule of thumb assumes 1 second and calculates the reaction distance as (speed ÷ 10) × 3. So 50 ÷ 10 = 5, and 5 × 3 = 15 m reaction distance.

With normal braking, the braking distance rule of thumb is (speed ÷ 10)². So again 50 ÷ 10 = 5, and 5² = 25 m braking distance.

Stopping distance is reaction distance + braking distance: 15 m + 25 m = 40 m. That’s why 40 m is correct.

Rule to remember for similar questions: stopping distance (normal) = (v/10 × 3) + (v/10)², with v in km/h and reaction time assumed to be about 1 second. If the reaction time is longer, the reaction part increases proportionally. If you brake harder (emergency braking), the braking part is roughly cut in half.