Bike lights with beam cut-off to avoid dazzling drivers?
I have actually dramatically improved the cutoff on my 1200 lumen dual LED (just for reference) with a sort of brim made out of aluminium plate.  On road I run it on minimum brightness (guessing about 1/4 -- 1/3 power). On pitch dark bike paths I do change the angle a little as well as turning up the brightness.  

Super-speedy sketch (go inkscape!):



The light body is about 60mm long, and the reflector is in approximately the right proportions.  I mocked it up with cardboard first, before using some 1mm plate - it's tucked away under the bars, under a front bag mount and all edge and corners are nicely rounded off.  

The mounting underneath the handlebar is another issue -- overly reflective silver-grey brake and gear cables right in front of the light if it sat on top of the bars.

It's not quite as good as it was before I tweaked the handlebar angle. On (slight) riser bars using the riser as intended I had it tipped up on the left to illuminate roadside signs (UK).  I've twisted the bars so the rise points forwards at about 45° and the aim isn't quite what I'd like.

I promised pictures:


Side view





Front view





Illumination pattern (high ambient light levels)