I believe there is a study that shows you are twice as likely to get hit by a car riding on sidewalks than you are riding in the street. There are some very rare occasions where I assess it prudent to ride on the sidewalk, but generally, it makes it a lot more likely to get hit at the intersections and where the sidewalk crosses a driveway.
Not to mention, I don't like having to dodge pedestrians unnecessarily. It's unpleasant for them and for me.
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Ironfish653 has an interesting point in an excellent post about fast arterial roads. I find that the relevant variables for me are the widths of the shoulder and whether the intersections are signaled and/or stop signed. Some of the best riding I do around these parts is a road that has a 55 mph speed limit and really broad smooth shoulders.