I've done my own study. When I'm driving a car I notice bicyclists. I find I notice them easier when they're wearing high visitility yellow than anything else. At night I notice people with lots of reflective material and flashing red taillights. I notice people with reflectors on their pedals, because they seem to flash as people pedal.

I see people with flashing red taillights during day rides - especially centuries. I don't think it helps much at all.

Those are the results of the BigBlueToe Visibility Study.