"In 1920, the United States opened its first coast-to-coast airmail delivery route. Before radio navigation was common in airplanes, pilots had to navigate by landmarks. Therefore, bad weather and nighttime essentially grounded the airplanes. The solution was to build a system of concrete arrows — stretching 70 feet along the ground — every ten miles from New York to San Francisco. Each arrow had at its center a steel tower with a beacon on top. As long as the pilots followed the markers they would reach their destination. But if they veered off, they might have difficulty finding the route again". (LINK TO MORE)