[_delirium]

It's actually not illegal in most places. None of the 50 states have a blanket law prohibiting riding on sidewalks, so it's purely a matter of local law. It's illegal in some cities, not in others, and almost never in anything smaller than a metropolis.

Some states do have laws about what constitutes legally riding a bike on a sidewalk. For example, Oregon requires bicyclists to yield to pedestrians, and to generally go slowly, slowing down to walking speed when crossing a crosswalk or driveway: https://www.oregonlaws.org/ors/814.410

Common practice seems to vary a lot by area. Riding a bike on the sidewalk in SF would be unsafe, unnecessary, and against city law, but in suburban and rural areas with high-speed-limit roads (~50-55 mph) without bike lanes or paved shoulders, but which do have sidewalks, it's quite unusual for people to ride in the road. In suburban Houston, for example, I would be really surprised to see a bike in the traffic lanes on something like Texas Hwy-3, but it's not that uncommon to see bikes on the sidewalk.

[uxp]

Utah does have a blanket law for riding on sidewalks or pedestrian pathways, though it is not expressed deliberately.

The only exceptions to a person riding on the sidewalk is if the road does not have approved bicycle lanes, or the person is in a group with special permits, or the road is physically unfit for travel. As of the last 4 years, every paved road, with the exceptions of interstates, were approved for bicycle travel, even if they do not have designated bicycle lanes.

http://www.rules.utah.gov/publicat/code/r805/r805-001.htm

Granted, one may argue that any road is unfit for bicycle travel if the speed limit is 55 MPH or so, but you must prove in court how the entire road is unfit, which requires just as much effort as the DOT requires to gain permission to re-pave the surface from a graded dirt base.