I think you're on to something there. Helmets do seem to signal higher speeds to drivers.Placidscene, I have found a big ol' not bicycle helmet works great! No kidding, I can tell the difference when on very rare occasion, I forget to put it on.
Seems to be a subtle indicator that your on some thing going at motor speed and not on a regular bicycle.
I really, really hope that helps you. I've had a few close calls with the "pass & turn right" and the "pull out forcing an oh dang* moment" folks.
* not actually what I was yelling but close. Does begin with "OH"