I use a Light Brain twin controller for my bicycle headlights. Though it has an emergency function for flashing the lights, I don't use the function because I think it is distracting and confusing to drivers. I also don't think it complies with federal regulations regarding modulating headlight brightness on motorcycles.

Edit: Oh. You want a momentarily flashing headlight to attract attention. The Light brain is for a continuous flashing mode. Never mind...