I remember reading an old magazine article a long time ago (certainly before 1982) about a guy who'd invented a lot of things for cars. One of his ideas was a system to automatically dim the headlights when another car was approaching. His problem was that it would also dim when going up a hill if the light from a streetlight entered his sensor. He then remembered that a distinguishing characteristic between AC and DC powered lights was that AC powered lights flicker. He modified his sensor to ignore flickering lights and the auto-dimmer worked properly. This was one of the inputs that gave rise to my idea back then.

It wouldn't surprise me if the lights didn't go all the way to dark in synch with the cycles. Some lights flicker more than others, with florescent lights being pretty bad. Like I said, the idea should be pretty easy to prove or disprove. I'd point a sensitive photocell at the background light pollution and see if any 60 Hz component can be detected with an oscilloscope.