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Anyone on a running track going much faster or slower than a runner is a hazard to the runners on the track. This applies to dog walkers, bladers, cyclists, or cartwheelers. If you are using a track, you should also pay attention to lane markings and avoid moving counter to the flow of traffic.

If the track is deserted, a bicycle won't harm the surface. The reason for the sign is that tracks often become clogged with non-running users, so much so that they can't be used for running.
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Dan