In most cases you're more likely to be hit by a car if you're riding on the sidewalk. Last numbers I suggest that your risk of being killed by a car is about twice as high when you're on the sidewalk.

The reason is simple: Bikes move quickly, and bikes on the sidewalk are much harder for motorists to see than bikes on the street. That makes it extremely difficult - sometimes impossible - for drivers to see or anticipate when a bike is about to cross a street, alley, or driveway. Meanwhile, cyclists are relatively unaware of their surroundings compared to pedestrians, for the same reason that motorists are: They're moving faster. These two factors can easily combine to create a sudden use-of-space conflict that cyclists never win.