Well, since the sewers are full of mutant rats, wouldn't city services do something to keep them under control. For instance, say, roving mecha that destroy all living creatures in a cyclic pattern throughout the sewers, save those with the proper security chip inserted under the skin?
Just an eensy-meensy thought.
*grins evilly, thinking of dismembered corpse thingys floating in the wake of a big, semi-smart, spiked metal ball that rolls endlessly through SD's sewers*
Yet another two shares of Enron, brought to you by