I bought a super bright Serfas light (250 lumens?) for illuminating my way at night. I bought it more for some dark roads, but I find it's bright enough - definitely much more so than a typical bike light - that it does seem to make a difference for oncoming traffic. Mind you, it does cost 6X the regular light...

On the other hand, it was in broad daylight on a clear, calm afternoon with little traffic, I was wearing a bright yellow reflective jacket, had the right of way on a residential road, and a car still couldn't see me and nearly cut across the road in front of me from a side street, and only stopped when the driver saw me about 8 metres away. Go figure.