flashing strobe like street light, failure mode?

High-pressure sodium lights exhibit a "cycling" on-and-off behavior when they reach end of life. However, when I've seen this it's been a fairly slow cycle, and didn't exhibit the sort of bright strobe-like flash you describe.

I suppose that you could be seeing a mercury-vapor lamp with a failing bulb and/or ballast, where the ballast sends a strong pulse to "strike" the lamp and get the arc flowing (hence the flash) and a fault in the bulb causes an over-current or under-current condition which then shuts down the arc almost immediately.

Pragmatically, you might want to write down the streetlamp pole number (if it has one) and contact the city's public-works department and report the problem.

red light cam?

I'll have to drive past there on the other road. My impression was street light, not near the traffic signal.

It's a part of town not conducive to hanging around watching for it to recur.

It seems thats were "they" like to put the red light cams.

Dave Platt wrote: "High-pressure sodium lights exhibit a "cycling" on-and-off behavior when they reach end of life. However, when I've seen this it's been a fairly slow cycle, and didn't exhibit the sort of bright strobe-like flash you describe. "

There's a street light across from our house that cycles on and off over a period of 30-40 sec., but does it on random nights(!). We might see it do it repeatedly for one night, then do it again a week or a month later, no pattern.

I got a better look yesterday. It's a street light, no camera. Besides, I see the flash when I'm stopped at a red light, so the cross traffic would have a green.

Good thinking though.

Yeah, that sounds like normal failure mode. I've not seen one flash and stay lit though.

It is a worn-out lamp. When they get old, they start to run hot and the ballast cycles power every few minutes. The repeated starting, maybe 10 times an hour, increases the wear on the lamp. When they get really bad, the ignitor can't strike the arc, and so the ignitor keeps trying. The strobe effect, maybe a couple flashes a second, is the ignitor trying to get the lamp to restart.

Jon

You could be right. I need to get a better look at this one.

It doesn't sound like that to me, because a lamp that old shouldn't stay normally lit most of the time. It should be burned out trying to start most of the time. But I haven't heard a better explanation either.

I might call the city engineer and just ask.