Tens of millions of Americans have hit the road this summer. The all-American road trip has long been a signature adventure, but once upon a time the notion of your own motorized excursion of any length would have seemed impossible.

In 1900, Americans were hampered by wretched roads and limited by the speed and endurance of the horses that powered buckboards, coaches, and wagons. If they had an urge to travel far distances, they had to rely upon the steam …