HIGHWAY POST OFFICE
On February 10, 1941, experimental service started on the Washington, DC & Harrisonburg, Virginia HPO. It was a success from the start, but, due to WORLD WAR II, expansion of this service was delayed for several years. After the war, the service increased rapidly, with more than 130 routes established between 1948 and 1955.
As this service was somewhat dependent on the RPO service, its value decreased when RPOs were abolished. The last service of this type in the U.S. was the Cleveland, Ohio, & Cincinnati, Ohio HPO. This was discontinued in 1974.