I spend a lot of time sitting on the freeway (uggh) commuting to work.
I use magenetic signs and cling window decals. I can't attest to their effectiveness, but people do notice them. Plus, they are a great tax right off for your vehicles.
Here are a couple of good places to look.
Spider Woman Graphics
You can see mine on this page if you scroll down. She does very good work. (no, this is not an ad, just a satisfied customer)
And try here too
Web Decals

Gayle