Originally Posted by tarwheel

It's dangerous to ride on sidewalks for several reasons:
-- Cars entering from side streets often won't see you because they're focused on looking for traffic on the road, not the sidewalk.
-- Cars will entering from sidestreets, homes, etc, will block your path, forcing you to stop and wait all the time.
-- Many pedestrians don't expect to see bikes on sidewalks, particularly if you are approaching from behind and they are wearing an I-pod, talking on the phone, etc.
-- Cars turning into sidestreets, driveways, etc. often won't see you, again because they are focused on traffic on the road.

That said, there are occasions where riding a sidewalk is prudent. Eg, on my commute one of the streets deadends into a busy 4-lane road. I need to turn right on this busy road and then make a quick left at a traffic light. In this case, I go ahead and turn right onto the sidewalk, ride about 50 yards, and get in line at the traffic light to cross the busy street. There aren't any sidestreets or driveways during this short stretch.