From the BicycleDriving Google Group:
"People have a widely affirmed right to travel in public in this country. People do not need a license to leave their house and travel on public roads. People are free to walk, ride a horse, bike, and travel by public conveyance in this country, free of scrutiny or license to travel in public on public rights of way.
People DO, however, need to be licensed and have motor vehicles licensed in all 50 states. The supreme court ruled in several cases over the last century that methods of travel damaging to roads and hazardous to the public merit licensing by states. Hendrick V Maryland, 1915 was one of the first. The supreme court has upheld licensing of motorists and motor vehicles because of the attendant damages, dangers and hazards associated with motor vehicle use."