Letting off steam

This is one of the things that surprised me about the city.
It's still one of the things that freaks me out.
Actually, the thingy itself doesn't freak me out, nor does the steam cloud or the traffic jockeying around the obstruction (ok, that does a little).

What freaks me out is the idea that the city has so much pressure and heat and steam building up from all the stuff that happens underground, that if they don't let it out, the ground beneath our feet (and tires) will explode.

To prevent this explosive sauna, the city puts up these thingies (anyone know the official name?) at seemingly random locations for seemingly random lengths of time. Who decides where one will be put and how long it will stay? Is there some poor under ground sap who has a pressure/steam/heat meter and walks around until it flags that area as needing release? Does it have something to do with Teenage Mutant Ninja Turtles? When it comes to what goes on under NYC, I guess it's anybody's guess (unless of course you're the poor sap with the meters in the dank tunnels- then I suppose you know everything and are planning to write a book when you retire).