Okay, I know street lights have to blow out sometime, after all they’re lightbulbs, but I’ve never seen it happen. For somebody to change the bulb you’d expect it to hold up traffic. Even if the technition could get there quickly I’d expect it’d still cause some accidents. Can anyone shed some light on this?
maybe they replace them BEFORE they blow? When nobody else is around? Hmmmm…
Most street lights have more then when facing in each direction. (Usually two or four). When one goes out you still have the others to look at. And like happyheathen said, maybe they replace them before they blow out. Maybe each bulb is replaced once a year even if it is still working (especially at main intersections)
Street lights or traffic signals? Sounds like we’re talking two different things here.
And, in either case, I have seen the bulbs being replaced. Usually it is done with a boom truck that parks on or near the sidewalk (to minimize traffic interruptions) and raises the worker up in an insulated basket.
I second SavageNarce’s post. I have seen both burned out lights in the city, and seen them being changed.
In our city, they drive up a large ‘cherrypicker’ and hoist up. The lights have a drop down front (the glass bit) and they change the bulb.
Come to think of it, I don’t recall seeing them do this in the daylight…I think it may be a nightime procedure.
They do it day or night. If they have a bucket truck or “cherry picker” they use that. I have even seen 'em put a ladder in the back of a pickup to change bulbs. And they are only replaced when they burn out.
I don’t know for sure, but I’ll throw out a few theories:
I would expect that they have a backup bulb in there, which the system will switch to automatically when the first burns out.
Knowing that these are critical use lamps, I am sure that the municipality in charge of them has the lamps (light bulbs) on a carefully scheduled group relamping schedule, with replacement lamps being placed in the fixtures at a safe margin of time before the lamp’s expected burnout time.
(FYI: all types of lamps have come with a rated life expectancy. If your lamp says it has a 1000 hour life, that means that in testing in the lab for that particular type of lamp, 50% of the test lamps were still burning after 1000 hours of use, while 50% had burned out. This isn’t a straight 1000 hours, turn-on-once-and-leave-it-on; there is a stated protocol for how long they turn the lamps on, and then turn them off, and re-start them. )
Well, I do know for sure what they do here. I worked for the City for 4 years. There are no “backup bulbs” in traffic signal heads. The guy in charge of signals replaces them as they go out, not on a schedule. Might be different in a larger city, but not here. 
So how does the “guy in charge” know when a bulb has gone out? Is there an automatic alert system, or does he drive all day making sure all the traffic lights are working? When one bulb goes out, do the other lights at the intersection switch to flashing red? (I think that’s how I’d design it, but IANAEngineer.)
Actually, over the next decade many cities will replace incandescent lighting with LED’s, which are brighter, far more efficient, produce far less waste heat and last ten times longer. If you look around, you might see some traffic signal lights that are no longer one large red/yellow/green light, but are composed of several dozen tiny lights. When streetlights go this route, they may have as many as 400 individual white diodes (at least this model does). You could lose ten, twenty or even 50 of these diodes and the streelight will still function. By then, the problem will have been detected and a team dispatched to replace the entire assembly.
LEDs are actually pretty exciting in a techno-geek kind of way. I look forward to them (and flourescents) replacing inefficient incandescents entirely. The electricity saving in a typical city would be huge.