I second this. I few years ago I jumped on the sidewalk just for a block to get to an intersection with lights. I made eye contact with a guy stopped at a parking lot exit, and presumed it was safe to continue. I guess he felt the same way and knocked me right into traffic... Thankfully that lane was clear, but my light bounced off and got taken out by a car in the next lane over. Kind of shocking.

As if that hadn't taught me well enough, a few months later I decided to save a little time by going down the wrong sidewalk of a fairly large hill. It was in an industrial area and knew it would be free of pedestrians, so I got going pretty good (a little ways back I had 25mph indicated and kept accelerating, as stupid as that is ). The sidewalk has a slight curve to it and a rough-hewn rock wall bordering it. Low and behold, the rock wall had collapsed over the sidewalk. Due to the curve I didn't see it soon enough to totally stop, and I couldn't go into the road because of oncoming traffic. I figure I got down to about 15mph before I hit the small boulder pile and went down, sliding a good ways. I managed to put a hole through a surplus military jacket, sweater, shirt, and undershirt, but stopped in time to avoid any bad road rash. That incident also cost me a nice pair of jeans, a helmet and a new front tube, but amazingly the front rim was alright!

Now I only bike in the road... I feel much more comfortable there

BTW... don't try to hold on to cars.