I can remember on our family vacations that we would see from time to time signs with funny sayings alongside the highway. Those signs were placed there by the Burma- Shave company. The company had been struggling before they used the signs to advertise Burma- Shave shaving cream.
These signs helped pass the time on long trips across the country and kept the passengers amused by the funny sayings on the signs. Wikipedia.com breaks down the signs into different time periods: http://en.wikipedia.org/wiki/Burma-Shave