Back in the 60's and 70's my family had Volkswagon buses and they were such a rare item that anytime you saw one coming toward you, everybody sat up and waved. It was like meeting a long lost relative even though you didn't know those people from Adam & Eve.
I know about half the states have required front license plates, and maybe that could be addressed somehow as well, but if I saw another front license plate sporting the geocaching logo coming at me, you can be sure I'd give a wave.
I see that there are license plate frames for sale, but let's look at the numbers. I may pass hundreds of vehicles head on while I'm stuck behind one vehicle. And if I follow the safe following distance rules, other geocachers would surely see my geocache logo.
I don't have one, but by God I can make one, and I'm not trying to make any money off of this, but I probably should. The point is, I could say, "Hey!" to a fellow geocacher, if only for a fleeting moment.
--Joe