This news story in Popular Science asks whether car headlights are getting brighter and concludes they are not:
So are headlights actually getting brighter?
The short answer is no—although there was a tweak to headlight regulations in 2008—but multiple factors can cause them to seem brighter.
But their reasoning seems to be entirely based on US regulations about headlight brightness:
Headlights are governed by a overwhelming slew of regulations known as FMVSS 108 that dictate how bright they can be, at a maximum and minimum.
One key point pertains to how much light is allowed above the horizontal plane of the low-beam headlights—picture looking at your headlights from the side of the vehicle, and imagine that light shouldn’t be allowed to escape above a line parallel to the ground that comes out of your headlight covers. “Anything above that horizontal level is supposed to be restricted brightness,” says Matthew Brumbelow, a senior research engineer at the Insurance Institute for Highway Safety, or IIHS.
Though they admit that manufacturing misalignment might represent an unintended way round the rules.
However, many headlight after-market retailers specifically advertise that their kits are much brighter than standard headlights (this from XenonHIDs):
LED headlights are the latest innovation for vehicle forward lighting. They are much brighter than regular halogen headlights
Also rules in other countries are very different from the US ones.
Some observers have argued that LED headlights are too bright:
Have those on new cars become too bright? They have, according to the RAC's research which reveals that 65 per cent of motorists have been dazzled by the latest-technology headlights even when they are dipped.
Is there any observational evidence that headlights are getting brighter in the real world (ie observational evidence, not evidence based on assuming everyone follows the rules)?