The Federal gov in the US has a "road legal" standard for commercial motor vehicles like trucks and buses. The feds also have minimum rules for headlights, brake lights and turn signals on passenger cars.

Everything else in terms of road legality is a state law in each of the 50 states.

The reason is the Constitution gives the feds power to regulate interstate commerce (i.e. big commercial vehicles that frequently cross state lines). The feds do not have the general "police power" that states have to pass laws on whatever.