What is "highway post office"

Due to the continual withdrawal of so many Railway Post Office (RPO) trains, the Post Office Department decided to experiment with the distribution of mail on large buses, equipped somewhat like RPO cars. 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 enmeshed with the RPO service, its value decreased when RPOs were abolished. The last service of this type to operate in the U.S. was the Cleveland, Ohio, & Cincinnati, Ohio HPO, which was discontinued in 1974.