I hit a car while riding on the sidewalk as it was exiting an alley. The problem is that when you are riding on the sidewalk in a city, the corners are too blind. In order to guarantee avoiding an accident you essentially need to stop at each driveway and alley. Most cyclists and drivers aren’t going to do that. On the road you are far enough back that you can see cars exiting; they can see you as well.

They drive across the sidewalk when entering or leaving driveways, and drivers do not always see cyclists in those circumstances.