I'm not so sure about that. About 5 years ago, I was working on a building rehab job a few blocks from the WTC. The building was mid 1800s and plans were long gone. We found a sub-sub-basement that the owner did not even know existed. It was a half-height door behind piles of junk. Not opened for at least 50 years.
I will remember that smell forever. When I shined a light in, the floor looked like it was moving frantically. It was wall-to-wall rats as far as you could see, their eyes reflecting off the light. Many were bigger than cats and made low-frequency growling noises.
An exterminator was called and said we were lucky to be alive opening such a door full of cornered rats. A few more minutes and they would have turned us into food.
The NYC rats are very elusive and will almost never appear within sight of humans. If you’re on one of the few dead-quiet blocks and quietly look out your window in the middle of the night, you’ll see swarms of huge rats foraging. They scatter at the slightest noise. There are many, many underground passages and chambers around NYC for them to hide.
In 1949 the NYC rat population was estimated at one per 36 people. Average rat weighs around a pound. You do the math.
http://www.snopes.com/critters/wild/rats.asp
Let's hope that's where they go when the cities start to burn....and not flee to non urban areas.