If a blinker is an expected passing signal, you WOULD be looking for it. My blinkers are very bright, and are amber, not red, so they would never be confused for brake lights. Some at BIR use blinkers as a passing signal since most clubs at BIR do run windows up. It works just fine, I think it's far easier to see a blinker than a hand signal. If you dont have blinkers you're probably in a race car where a hand signal is the only option, and thats fine too. Why not use both?