They are street cleaners, flushing the obvious dirt and trash off of the street.  Because quality and being clean only really matters from appearance from a distance.

Happy Birthday is just their warning call to tell you to get out of the way unless you want a bath as well.

The first time I heard the truck playing that little tune, I thought maybe the ice cream truck was coming around the neighborhood.  What a disappointment

Yeah when you hear the happy music you know to RUNNNNNNNNNNNNNNNNNN!!!!!!!

Yes, it happened to many of us, that equated the Happy Birthday song to an ice cream vendor truck.  Here, I have seen them doing what it seems like a dual function.  One, they are used to clean the streets using water.  One truck easily will do three or four lanes at a time. 

And second to water the plants on sidewalks and middle ground.  And there pedestrians better watch out, they do shut the stream all the way to the top of each tree too, not only at ground level.

They do that almost everywhere. You better stand back next time you wouldn't want to get wet. They do it in the big cities since they have to do that to clean the streets and clean the pollution a bit. It's a service of the city what the tax dollars pay for. Just like garbage collecting etc., 

They do it in this city too, and it's not a big city. The ones here don't play a song, and they don't spray as high or as far, so as long as you're not standing on the curb you won't get very wet.