You are on a hill, about 15 miles outside of a small town that is your home. Between you and the town is a long stretch of wilderness, a housing district or two, and some roads. It's all more or less downhill, although it's a pretty gradual slope.
You just accidentally started a very large boulder rolling down the hill towards your town. You don't know any way to stop it; it seems to be breaking through trees and such without deviating. Fortunately, you can easily outrun it, and you have a bicycle. You guess it might take an hour or two to get to the town, maybe half an hour to hit the first houses.
What do you do?