By the way, I don't feel that a helmet light alone is the best solution, particularly in locales where cyclists are less common. Drivers aren't expecting to see vehicle lights any higher than headlight-level, and unless you're riding a recumbent, your head is a lot higher than that.
I often think that the only reason I see people react to my helmet light is because they've seen enough of us riding around and they've come to expect lights at higher "altitudes".