Riding shotgun was used to describe the guard who rode alongside a stagecoach driver, ready to use his shotgun to ward off bandits or hostile Native Americans.
In modern use, it refers to the practice of sitting alongside the driver in a moving vehicle; a perfect spot for a photographer.
Years ago I started this ever expanding collection on journeys around America & Europe. I love the sense of motion that is in almost all of these pictures. This is just a small selection.