During World War II, the government encouraged carpooling. If I understand it correctly, it is because they needed the gas and rubber for the war. I find it interesting that we are, today, in a war largely over oil and, yet, there is no government propaganda (that I am aware of) that seriously encourages conservation through carpooling or any other means.
Compare the propaganda posters below (found here) with the “Drill baby, drill” t-shirt (which, I understand, are not really comparable… but are fun to compare nonetheless):
This one is, as it appears, by Dr. Seuss:
The next two are from the State of Ohio:
NEW: In the comments, both Eoin and Toban B. (T.B.) pointed us to this poster (found here).
