But it's also brightness: legal limits for brightness above the horizon were set in an age when light designers weren't able to do very detailed control of ligh

But it's also brightness: legal limits for brightness above the horizon were set in an age when light designers weren't able to do very detailed control of light distribution and the dimmed area was always a mix of some bright spots and on a much darker average. The legal brightness limits were selected under the assumption that the highest allowed was an exceptional peak that would only occasionally hit an observer for a short time before returning to the lower background. Now headlight engineers have perfect control over the brightness distribution and they blanket the entire cone with what was intended as an exceptional maximum. And it's a full on arms race, because for the driver behind the wheel it's certainly awesome, one of the very few ways cars can differentiate from others (olders!) in terms of the quality of actual day to day driving.