Huh, saw the same thing yesterday on 4th between Brannan and Bryant.
That IS 4th between Brannan and Bryant.
Somebody is clearly doing something VERY important.
It's a hydrant diffuser for reducing water force out of the hydrant when flushing sediment, which has to be done once every few years on old mains.
YOU GET TO DRINK FROM THE FIRE HOSE!
And they say USENET isn't relevant any more.