Not necessarily safer. Most cycling collisions occur at intersections, and even if you're on the sidewalk you still have to cross intersections. In fact sidewalk cycling may be more dangerous, because motorists at intersections and entering or exiting driveways may be less likely to notice you. Plus you may be endangering pedestrians.

Here in Toronto we had two sidewalk cyclists killed within days of each other a couple of years ago. I'll search for the link.