Yes, they’re other colors as well, but why ARE they usually yellow?
I think it’s probably because yellow is the most visible colour. That’s probably why most taxis are yellow, too (in the States, anyway).
It’s better than the alternative of using all-white school buses in the winter time and all-black ones at night.
You have a very large object making frequent stops on the various roads and highways, at rush hour, with several months of pre-sunrise darkness and fog, picking up small people that are easily damaged.
I’d go for visibility. (Especially back in the days when flashers weren’t very bright and nobody had even considered putting a strobe light on a vehicle.) Now, of course, it’s Tradition.
First of all, that ain’t just yellow, it’s called “school bus chrome.”
Second, while I would agree that it was probably chosen for visibility way back when (and also probably for purposeful unique garishness so as never to be confused with other types of vehicles) I would not be surprised if the color has since been legislated into the books.
Here in NYS every single school bus has certain features (color, the words “school bus” in black letters facing back, etc.) that leads me to believe that they are required by law statewide, if not nationwide.
When I was in college many moons ago I had a psychology professor who cited a study (sorry I don’t have the cite any more) that showed that statistically yellow cars are the least likely to have accidents compared to other colored cars because they are the most visible. Sounded reasonable to me and I know that he drove a yellow car for that very reason. Assuming that people who drive yellow cars are on average no better drivers than anyone else I can’t explain why yellow cars would have less accidents UNLESS it had something to do with the color… but I’ve never seen stats on it one way or another since.
My guess is that school buses were painted yellow long before anybody did a study on accidents versus colors and that it just became a de facto standard after so many years… and yes, in California their color is regulated to some degree too.
So the kids who DON’T ride can tease the one who do by saying they ride “The Big Cheese!”
Cecil covered the taxi cab yellow conundrum and I am too lazy ( my isp is run by dead gerbils) but the gist of it is visiblity.
What I want to know is: Why are school busses roof tops white? The newer busses in our area are all white and I am thinking it is because of reflecting the heat of the sun,but that is a stab in the barrel of monkeys.
Shirley,
You’ve hit the nail on the head (leave the monkey alone!). Since most school boards are to strapped for cash to afford air-conditioning for the buses, the cheapest thing is a roof that’s painted white.
If they got up off another few bucks, I think they’d find a silver (colored) reflective paint is probably on the market that’s a bit cooler than white.
Sschool buses in the United States and Canada are made to extremely strict standards - specifically, CAN/CSA D250, or its UL equivalent, which I don’t have the name of handy. Those standards are invariably referenced in law. No deviation is permitted. So the answer to the question is that school buses are that color because they have to be. 
There has actually been some suggestion at the D250 technical committee level to change the color of school buses (and a lot of other things) to neon green. (Or something that looks neon green, I’m sure it has some other name.) Tests apparently show that this shade of green is the most noticeable color there is. However, there is some concern that even if school-bus-yellow isn’t technically as noticeable, it’s very recognizable; you see a big orange/yellow vehicle and you instantly think “School bus, kids around.” Changing colors might cause people to not take as much care around them as they did before.
Now I am just waiting for someone to ask why school buses don’t have seat belts. (Although, IIRC, Cecil answered it already.)