During WWII, the United States Office of Price Administration printed a great deal of propaganda meant to encourage the American public to support the war effort by conserving resources. Much of this propaganda centers on the idea that even the smallest failure (not carpooling, for example) directly helps the enemy.
This poster, created in 1943 by Weimer Pursell, depicts a well-to-do man riding in his 1940s convertible alone, save for a ghostly visage of Adolf Hitler riding next to him. The text, "When you ride ALONE you ride with Hitler!" is emblazoned above him, while below reads, "Join a Car-Sharing Club TODAY!" The effect today, in peacetime, is rather amusing.
An image of the poster can be found at:
http://www.archives.gov/exhibits/powers_of_persuasion/use_it_up/images_html/ride_with_hitler.html
Please let me know if this link goes out of date. The poster is part of the National Archives, Still Pictures Branch. Its accession number is NWDNS-188-PP-42.
ed. note: updated link on 7.30.06