at 20 mph, much farther than people think. I really love my 1600 lumen Serfas E-lume

however hands down my 800 lumen Light & Motion has the best reflector and the best throw even on medium setting. On my road bike, after a certain brightness I really don't care how much brighter the Elume is. I really only care about being able to see what is coming with enough reaction time to compensate.
which makes the L & M my main rural light.
I actually use both. L& M for the rural 20 mph stretches, and then the E lume in town for the brightness and wider throw. and for the sake of being seen, Basically set apart from all of the other lights people are seeing.

Edit:
I really wish the US would pass laws require lights to have a height cut off on lights used for street use! No need to see the tops of trees, we only need to see the street. Shining anything above the dash of on coming cars just serves to blind them.