Other threads have discussed the fact that many drivers are unaware of cyclists' rights to be on the road. They have also discussed how some drivers go through a real change of attitude once they are educated a little (about laws pertaining to bikes belonging on the roads).
So there's this possibility: Why not have jerseys or shirts or safety vests with concise educational messages for drivers?
[[side note: this other possibility just occurred: the message could be selectable -- one could communicate with drivers this way (I'm thinking mainly of drivers coming from behind) -- one could select from several messages....]]
The first idea was just to have a single message printed -- something like LEGAL ON THE ROAD, or
on the road
There are other possibilities as well.
This idea just came up a few minutes ago, and I haven't thought about it much. It does seem, though, that some drivers (a fair percentage of drivers) might be learning something new, and may very well change their perspective on (and have more respect for) the bike on the road in front of them.
(It might also serve to make them more attentive to what is in front of them. Many people are drawn to read what something says. Once they read it, a sort of relationship is established. They might be a bit more considerate and thoughtful, and less likely to forget, ignore, abuse, drift off toward something else, treat the bike as a mere annoyance, or as something that doesn't belong, and give little room, etc.)