By how much does the braking distance increase, if you double the speed?

Why?

Under the same braking conditions, braking distance increases approximately with the square of speed. Doubling your speed therefore makes the braking distance about four times as long, because 2 × 2 = 4, rather than twice or three times as long. The brake lights and wet road in the picture highlight the need for extra space, but the question concerns braking distance alone, excluding the distance travelled during reaction time.