Before the mercury vapor lights were installed I remember as a kid in Chicago during the late 1970’s all street lights were “white” but every so often there was a light pole with a yellow bulb? What was the purpose of that yellow bulb or street light? I think maybe it was a indicator of a police call box or fire alarm. Please tell me I’m not imagining this.
The yellow ones were older, and just hanging in there, due for replacement. Chicago used to have a more yellow-toned bulb in all the lights, but over the years they switched to something closer/purer to white as the older ones burned out.
They weren’t yellow because they were old, they were yellow because that is what was used in prior years – a bulb that just wasn’t as pure white as newer bulbs that replaced the older ones.
Yellow is sometimes used to cut down on light pollution. Street lights in my city of San Jose, CA are yellow for the sake of the Lick Observatory on Mt. Hamilton in the hills nearby. A certain filter can be used to effectively cut out all the light from the city below.
This is not, however, a reason why some lights are yellow and some white.
High-pressure sodium lights are yellow. Don’t know how common they were back in the 1970s, though. I know in my area they started getting a push in the mid 1980s.
High pressure sodium (HPS) lights got popular in most areas in the 70’s, replacing the old mercury vapor lights (the push was lower cost/pollution).
I might have it reversed. Newer HPS lights at the time were probably getting mixed in with other lights that were more white.
Still, that is the gist of the answer: That cities used different bulbs at different times, and different types were/are more yellow or white or blue (more common now). Most cities didn’t run around replacing all bulbs when HPS got popular, but opted to replaced non-functioning ones as maintenance.
In some cities, blue is here and white is there… except the white is really more yellow if it were compared to an older white bulb. 
They were likely low pressure sodium lights rather than either high pressure sodium or mercury vapor. LPS lights tend to look yellowish (or sometimes almost salmon colored). HPS tends to look whiter, particularly with newer generation HPS lights.
Low pressure sodium have very high efficacy (meaning more light output per Watt of power input), but their color rendition is generally pretty poor. It’s mainly a cosmetic defect — they’re perfectly fine for lighting streetways, but building owners frequently don’t like them in their parking lots because although you see things just fine, you frequently can’t tell what color things are. (What color sweater was that assailant wearing? Where’s my rental car? — it was a green one, but I can’t see anything that looks like that color…)
Heh, I had read that factoid, garygnu, about the yellow street lights here in SJ. When we went to the big island of Hawaii recently, we noticed that street lights there were yellow, too, and it took us about one second to realize “Hey, it must be because of the Keck Observatory on Mauna Kea!”
My WAG is that the city bought too many yellow lights used on traffic bridges and other places they might blind drivers and just started installing them on regular light posts.
No, that guess is wrong. Those yellow hazard lights, and the yellow in traffic signals, are just white lights with a yellow cover.
I only drove in that area a couple times when I lived there, but it was awful. The lights were all about the same color as the yellow on stop lights. It was very confusing. I found myself stopping at a green light before I realized it was green, it was some other light that was yellow. I guess you’d get used to it eventually.
I grew up in Chicago and remember this.
My kid sister once said “It means a monster is coming”. 
Factual answer: marks location of either a fire hydrant or police box or something. There was actually a meaning to it along those lines, but time has faded the exact one from my memory.
Thanks Stan at least one person remembers those. At least I have some kind of proof they did exsist. No one I talked to remembers these lights or bulbs but I distinctly remember them as a child in the 70’s. I think they were used to mark police boxes or fire hydrants. These bulbs/lights were a bright yellow and they stood out clearly over the white bulbs/lights
And i am pretty sure they had significance to emergency personnel or something. Perhaps someone who knows will chime in, assuming this thread is not declared a zombie.