A biker is suspended above the ground, in a very unstable position. In contrast, a hiker can control his/her movement in all directions. (With my 56kb modem, this video took maybe half an hour to download, but it was really worth it, because it shows exactly what can happen, and why.)
Subject: [ROMP] Be careful out there.
Here is a nightmarish video of a friend of a friend of a friend.