I’ve never understood this. The number of parking spaces that can’t be used because of fire hydrants is enormous, most of those hydrants will never be used, and it seems like a car in front of a hydrant would only be a very small delay for firefighters.

Are fire hydrants so close together in some places they make a significant amount of parking unusable?

Every fire hydrant is thirty feet of unparkable space. Some streets "double up" by putting them at intersections (so it's only ten additional no-parking feet), but that's enough to fit three or four cars.

It seems like there's usually about one per block. That's at least a couple percent of street parking spaces.

On the other hand, I'm not sure it's proven that having more parking spaces translates to better outcomes, namely less pollution caused by lowering time spent to find a parking space.