During World War II, the government encouraged carpooling because they needed the gas and rubber for the war (source). Here is a sample of the propaganda:
This one is, as it appears, by Dr. Seuss:
The next two are from the State of Ohio:
In the comments, both Eoin and Toban B. (T.B.) pointed us to this poster (found here).Lisa Wade is a professor of sociology at Occidental College. You can follow her on Twitter and Facebook.