Do it with a textured satellite-style and show the zombies as a grey-black texture carpeting the "walkable" areas of the city, pressing towards the tunnels and the bridges as they search for food now that the city itself is devoid of life. You can throw in some cars, trucks, tanks, treetops, lightposts, etc. for interest. Actually, I sort of started thinking about that as a bit of a joke...but I think it might work. Ever read World War Z?
M