Another facet of all this that has not be addressed from a regulatory perspective (as far as I know) is with respect to the changes is headlamp technology, specifically as bulbs have changed over time from traditional incandescent, to halogen, to xenon. Most noticeable was with stark color of xenon bulbs. This is actually based in the science of the "color" of the light. Xenon bulbs were not necessarily brighter than others, but the different colors of the xenon spectrum is what drivers perceived as better lighting, but oncoming drivers perceived as glare.
At least LEDs can be "tuned" to specific colors from cool to warm, which is a big benefit for different applications, but it is likely that regulatory guidelines are far behind the advances in headlamp technology.