my brother used to do that with a skateboard.....the day before school started (he was in 6th grade I think), he was pulling his buddy. His buddy fell and my brother didn't know, the rope wedged under a car tire. Needless to say, the bike immediately stopped, and my brother didn't. He was unconcious untill ambulance arrived, had scrapped the entire right side of his face, had a huge scab for the next three weeks!
The funny part to the story, is the next summer a piece of gravel got pushed to the surface and he had a hole in his skin for a couple weeks!