Facts
If you reduce the speed from 80 km/h to 40 km/h, you reduce the speed by half. We know that the braking distance changes squarely with the speed, in the event of a reduction like this, it means that the braking distance decreases to a quarter. Calculation example: At 80 km/h, the braking distance is 25.6 meters. When you halve the speed, the braking distance decreases by 3/4, so the new braking distance is 6.4 meters (25.6 / 4 = 6.4).
