Easiest way to kill tons of Z's in a city. Is at a building construction site, of a sky scraper that is not to far along.
When it is still in the 'Pit' stage.
http://blogs.kcrw.com/whichway...SC07001-1024x575.jpg
Gain access to the crane. Put a couple of people in a cranebasket, about eight feet out from the edge. Make a hole in the fence, surrounding the site. Lure the zombies to just walk off the edge, to fall to 40-50 feet below.. When it gets a little full there, just move the crane basket over a little, etc. You could fill the pit to about 15 feet from the top. Or just do a burn every once in awhile.
If the building is further along like this one.
http://www.bing.com/images/sea...&selectedIndex=5
Just create a path/cattle chute that takes them up to at least the 4th or 5th stories up. Using the cranebasket to the same effect, to lure them to just walk off the edge and fall, shattering their skulls. Can be moved in the same way, when things are piling up.
If you don't plan on using the sewers. You can also funnel them to fall down into an open sewer hole in the street. Less bodys piling up and stinking up the place.