I am not sure what it is, but I have heard this yelled in my direction several times in the last few days. I don't normally hear anything from anyone, so I can only assume it is happening now that more car windows are being rolled down to enjoy the milder weather.
Last night, fully expecting a typical smart-ass comment from the individual, I turned to look at him and just as his wise-crack was about to come out he stopped in the middle of the first word and said, "Whoa! Check out the light on that dude's helmet!"
So, not only are flashing lights good for safety, they can spare you from jeering as well.