I always used to use lumicycle halogens, helmet mounted, pointing quite sharply down, when I commuted in London.
It was brilliant when you spotted someone about to pull out into you from a side road despite all the other lights / reflectives I had on, you could just aim the light at them straight on, and suddenly they slammed on the brakes and stopped. Worked 100% of the time. I was pretty careful not to dazzle oncoming drivers / riders, or point it straight into people’s rear view mirrors or whatever, but it was very very useful for cycling in a busy city.
Nowadays my commute is much more straight country roads, with few side junctions, so I don’t bother with a helmet light for it, but I can see why people do in cities.