We used to go down to DC/Rockville Md.area and ride on an asphalt path. Well, the path had a few wooden bridges in shaded areas. The first time we rode there, went down and slid across one of them. Guess it was algae, or maybe moss, but you might as well have poured oil on them, it was that slick. We had matching, bloody knees, that day! Gotta watch those shady spots!!