I've ridden on an unlit road with a tiny 5-LED lamp powered by four AAA cells (with about 8-10 hours at full brightness, then another 30-40 hours at progressively lower brightness). Could do 25 mph without worry, but that was on a good road. The ground was lit up well enough to allow me to see potholes and large debris in time at that speed. A bit more light would've been nice, though.

You don't necessarily need something that rivals a car headlight. More light isn't always better either.