Photo taken by Shane Nelson, volunteer
It's true that a ball rolling quickly off a horizontal table will hit the ground at the same time and with the same vertical speed as a ball rolling slowly off that table at the same time. However, a bicycle is not a ball. A bicycle has two separate wheels and those wheels start to fall at different times. When you ride slowly off a "drop-off," the front wheel has considerable time to fall before the rear wheel leaves the ground and you find yourself spinning as you travel through the air. The front wheel is soon much lower than the rear wheel and it hits the ground first. You experience a jarring return to horizontal and wish you had a softer seat.
However, when you ride quickly off a "drop-off," the two wheels begin to drop almost simultaneously and you fly roughly horizontally. The landing is balanced between the front and rear wheels and is much less jarring. You don't hit the ground any harder when you ride faster, but you hit it more evenly.
Answered by Lou A. Bloomfield of the University of Virginia