The other (and much more dangerous to you) threat when it comes to riding on the sidewalk is cars. Yup, you read that right, I said cars. Cars aren't expecting fast moving (even 10-12 mph is fast compared to a pedestrian) bicycles. So they may stop at an intersection and look at what may come in the crosswalk, turn and look the other way, and then pull out. Meanwhile you came from the first direction they looked and have pulled into the crosswalk. They didn't expect you to come that fast and now you're under their tires. Same thing with cars backing out of driveways. The drivers typically only look the short distance a pedestrian would be that could get behind them and don't look the further distance you'd be on your bike that could still result in you getting behind them. Again, you're hit. Being out on the road where you're expected is the safest place to be.