I know usually for side streets or yard lights, the neighbors to the light report if it's not working. But how do crews know if a light on the highway is not working or needs a new lamp? Since you can't tell during the day and no one is going to report it. And I assume the DOT maintains the fixtures. But at least 1/2 of the LED fixtures installed here about 2 years ago don't work, so most of the road is dark at night. Yet, the parts with HPS still work but maybe two or three lights, and I saw one was greened out.

I think with the LEDs its a wiring or switching problem, since so many are off.

Tradditionally they expected certain failure rate as a function of their age and so the just do the maintenance on scheduled dates. Because the reliability performance was pretty well known, the result were just acceptable number of nonworking lights.
The LEDs bribg two new things there: First the reliability performance of the new technology, so the corresponding schedule is still unknown, as a head start many operators choose to blindly trust the makers (with tradditional technologies that worked pretty well, mainly because the makers had a lot of reliable data; with LEDs are no userul dataavailable and so the industry is heavily contaminated with marketing BS, the undermaintained systems are the result).
Other problem is, the new fixtures were put on the old poles and wiring, an attempt to save cost. And after the time the wiring served there everything becomes brittle and ready to break, but usually continues working when kept untouched. So such messing with it caused many defects, so the group problems. Such massive overhauls never happened in such large scale before, so it was quite "easy" to underestimate the consequences (before the fixtures were just spot replaced when some failed, or mass replaced when there was wiring overhaul)

Here in Kiryat Ata, they doing a periodically a monthly maintenance, when they dayburning groups of lanterns, so they can see if some lanterns don't works, and relampping them and/or turning on the breaker if it was tripped.

Where I live, I believe they do group relamping after a certain number of hours.  Also, most state maintained street lights are on group photocells, so a stretch of lights can be turned on during the day for inspections.