Most knowledgeable cyclists ride on the road all the time. Riding on sidewalks is dangerous, mainly because cars frequently pull out of driveways and stop at the edge of the road, not the edge of the sidewalk. A cyclist on the sidewalk is usually moving faster than the pedestrians that cars are used to, so they don't think to stop at the sidewalk and watch for cyclists.

Makes sense, but what if it's the kind of street that doesn't have driveways? I always wonder why the bicyclists ride on the road, apparently taking serious risks, when the sidewalk is right there and almost entirely empty, as is usually the case in suburbia.