The United States Postal Service employed highway post offices (HPO) in the back of large buses as a means of sorting mail between offices, mostly in remote areas. The first run of the HPO was from Washington, DC to Harrisburg, VA on February 10, 1941. The HPO were designed to replace Rail Post Offices (RPO) with declining rail traffic. Postal service was reorganized with Post Office Sectional Sorting Centers reducing the need for sorting mail enroute in the early 1970s. The last Highway Post Office run was made on June 30, 1974, although there was a first run reenactment in 1988.
US #974 on HPO between Utica, Oneonta, & Binghamton, NY
Occasionally Scout commemorative stamps were used on these covers. Commercial use covers can be distinguished as highway post by the initials HPO in the cancel or by a backstamp.
US #974 on HPO from Coffeyville to Wichita, KS
US #995 on HPO from Florence, SC to Fayetteville, NC
These three covers are further distinguished as highway mail by their commemorative slogan and cachet for first route trips. The only mail canceled on the inaugural trip of any highway route was philatelic mail.
US #1145 on HPO from Milwaukee to Green Bay, WI
Many HPO routes were one way trips on one day with a return trip on the next day. HPO collectibles include: First Trips, Second (return) Trips, Last Trips, Steel Cancels, Provisional Cancels, Event Cancels (State Fairs, Anniversary Trips, etc), Commercial Use. Variations can include: matching stamps, addressed, unaddressed, with/without cachets, and airmail envelopes.