I too have rolling billboards. In addition to my car, I even fixed up my semi. (The truck actually belongs to the company, I just drive it.) I got permission from the boss to put signs on the truck, so I took a couple of the vinyl yard signs, spit them and taped one on the top fairing facing forward, and one on each side of the truck. Unfortunately, yesterday morning she calls me up and tells me I have to remove them. The company I pull for (I do a dedicated run for Lewis Bakery/Bunny bread and pull their trailers with their company markings) called her up and told her they want them removed.
Driving by Ft Campbell 10 times a week, I was getting all kinds of waves and honks of approval. I have only been flipped off once. But since Lewis is afraid of pissing off the libs, I had to remove them
At least I had several weeks of advertising I got done...