I guess I should make a little box with a long range distance finder that saves a number into memory every time a car passes, then compare results with/without a helmet. (My outer appearance doesn't change much from day to day.)

And I'd have to attempt to ride the EXACT same line every day.

Honestly, the whole idea is easy enough that I may just try it.

EDIT: Even though I mentioned an article that a few of you here have shot down (it's fine, I didn't even read it.), there are many other pieces of evidence on that page I posted from bicyclesafe.com (or whatever the link was.)

Another argument is that a helmet makes your head effectively bigger, meaning you're more likely to hit your (now bigger) head on something.

Another argument is that helmets reduce your ability to hear cars approaching from the rear. I can attest to this problem. I can hear much better without a helmet on.

Lastly, on that same page, they have a paper written (not peer reviewed) by someone at the "Bicycle Helmet Research Foundation". This paper brings a lot of data together regaurding injuries before and after helmet laws were introduced. It's an interesting read, and seems pretty unbiased. It seems like the main point is to disprove claims like "Helmets make cycling 85% more safe." Claims born from idealistic interpretations of scientific results.