You need about four times as much braking distance at 40 mph as at 20 mph.
When you increase your speed, your braking distance increases as the square of the speed increase. If you double your speed, your braking distance will be four times as great.
B The posted speed limit will always allow you to stop safely.
C The total stopping distance of a bus is the distance it takes to stop once the brakes are applied.
D Stopping time increases by one second for each 10 mph over 20 mph.