[old_tv_nut]

I can't say that I have observed car wheels appearing to go backwards, but I have seen some sort of strobing that I think was due to the shadows of the spokes falling on the spokes themselves. Strobing in the eye/visual system might be possible under certain conditions due to short-duration after images, and it might depend critically on the level of illumination, that is, require bright sunlight or even brighter reflections of the sun off of chrome wheels. It also might depend on being in peripheral vision rather than central attention. A former boss of mine, a video expert, once told me that he had seen strobing on car wheels, but did not say if they appeared to turn backwards.

Edit - if it's the shadow-caused effect, it might only be visible if you were NOT tracking the motion of the car. If you were following the wheel, the rotation might blur everything, but if not, the strobing flashes would be distributed across your retina. I think the cases I have seen were when I was not tracking the car motion.