I actually own a website , where I have thousands of people waiting to be contacted to advertise your website , via car , tattoo, bumper sticker, tee shirt and so on , Let me know if this interests you or anyone else . I wasnt trying to thread jack, Just trying to help
One way to get almost the same effect as a billboard but for the cost of a cheap little sign on a car is to park the car in a very high traffic area, for example I take my can and park it right next to the state fair entrance and hang a sign over each side of my van, I get tens of thousands of eyeballs every day for two weeks as people walk past my van to enter the fair, plus they do live remotes for the local news lots of nights and sometimes my sign makes it on the news.
I do contract web design for a local sign company, and I have heard first hand from our customers that lettering n the cars work. Also, the yard/real estate type signs are effective as well. And the cost is reasonable (shop around). I am thinking very seriously of having a 4x8 sign made, mount it in the back of my small, utility trailer and tow it around town when I am running errands, etc.
I have the url of my main website on both sides and the rear of my van. I bought the lettering from Ebay. It's hard to quantify exactly how many people see it and then type in the url, but I guess if I was parked outside someones house then neighbours in the street may be curious and check out my site. You can get some extremely smart vinyl signs made now for vehicles, in the UK it is known as vehicle wrapping, not sure what it is called in the US. Like someone said before it can't harm, unless you are a really bad driver and pi** people off