Bicycles don't ride on sidewalks because you want to maximize their visibility. Most deadly accidents between bikes and cars happen at intersections where bikes are getting right-hooked by drivers who forget to check their blind spot. Drivers don't expect objects traveling at 30km/h on the sidewalk, so they're even less likely to look. Sharing the road between bikes and cars is much safer. There would be almost no problem if cars respected safe overtaking distances and switched lanes to overtake bikes.