When an idea or phenomenon gains enough popularity and/or following so as to spread from "coast-to-coast." Could also apply to the physical act of traveling from one coast to another.
A sunburn or suntan focused on the left arm. It is caused by the left side of the body being the only half that gets exposed to sunlight while driving. Presumably, it would be located on the right side of the body in countries where they drive on the left side of the road.