Comment by WorldMaker

Not just informal norms, in many states formal laws, just ones that are hardly ever enforced this century. (Many of them on are on the written part of your driving test, though people don't take those often enough to remember their details.)

For example, Kentucky law requires bicyclists over the age of 11 to use the streets. (Prohibits sidewalk usage.)

Washington state law allows sidewalk usage, but also requires wider sidewalks than the Kentucky minimum, and includes a lot of very specific wording about pedestrian right of way on sidewalks (sidewalks are theirs first) and mandate being very careful in overtaking pedestrians and other cyclists.