That might do, might not (depends a lot on state law and such). It's always better to have people sign something that makes clear that they know the dangers involved ("I realize I'm engaging in an activity that exposes me to a high risk of getting f'd up"). Also, organizers generally (that is, anyone providing people with some activity that might get them hurt) can usually get participants to waive liability for normal injuries resulting from the activity, but not for bizarre, unusual injuries that result from the organizer's negligence. For example, a ski mountain can require you to sign away your right to sue for a run-of-the-mill broken leg because you fell; but if the chair falls off the lift while you're on it and your leg gets broken, you can sue because it was clearly their fault (even if you waived your right to sue). I can't really think of how this would apply in an alleycat, unless the organizer supplied the bikes or something.