[chrisweekly]

head taps?

[ggreer]

People on motorcycles signal "police ahead" to riders in the opposite direction by reaching up with their left hand and tapping their head/helmet.

[datax2]

on a motorcycle when you pass a cop you tap your helmet to warn other riders.

[chrisweekly]

thanks. and yikes! I've been a motorcycle rider for over a decade, many thousands of miles, now on my 3rd bike -- and somehow I'm just now learning this.