I have been night road cycling for a number of years. One and two watt headlights make you visible, but are way too weak to see chuckholes and road debris safely. The minimum I suggest for riding at 15 mph is 10 watts, and at 18-20 mph, 15 watts. My current light is a NightRider Digital Headtrip, with a single beam on the helmet adjustable for 6, 10, or 15 watts. The 15 watt level burns for about 100 minutes. The NiMh battery pack fits in my jersey pocket and the whole system weighs under 600 grams. Okay, it's pricey, but I really enjoy night riding and I definitely got my money's worth out of it. Heck, it's equivalent to the combined cost of an Arc LSH-P and an Arc AAA.
I LOVE having the beam on my helmet for going around corners, reading road signs and, of course, for shining deer. The earlier discussion of the "cam effect" is interesting and important, although I ride a recumbent and am more likely to have arm and leg rash than whiplash. On long night rides, I'll either go slower and use the 160 minute 10-watt setting or take along a flashlight as backup. The LEDs don't have enough throw to get that beam out 15 feet in front of you for safety, but the above comment on their utility for being seen, especially when flashing is so true. Keep in mind most bicycle accidents involving cars are NOT from the rear, but from on-coming cars making left turns and not seeing you!
I'm eager to try a double luxeon headlight to see it's utility, but as beautiful as the beam is, the throw is too short for other than maybe around-the-block casual riding.
I did rig up a neat tool for attaching a small flashlight to a helmet. I attached a heavy duty rubber band(5/8" wide x 4" circumference to a small aluminum plate(1"x1.5"). A small nylon loop is attached to the other side of the aluminum plate. I slip the flashlight into the rubber band and with the nylon loop, pull the plate through the central helmet vent until the plate "clicks" flat, and, presto the light is securely fastened to the helmet. To remove, I reach inside the helmet, pull the nylon loop and simultaneously twist the aluminum plate sideways to allow the whole thing to pass out of the vent. Have used it with an E2e, a UK4AA, and a PT Rage. I only use these if I wandered too far from home and my HeadTrip has drained. To me the E2e has a beam equivalent to the HeadTrip 10W setting.
Happy riding.