The way I do it is to use a real city map. You can also take out sections or neighborhoods from different cities and paste them together to create unified, singular maps.
Once you have that map you can detail certain specific locations and leave the rest to "neighborhood description."
That is to say you don't have to detail every location the players are likely to want to visit, but rather identify neighborhoods or geographic areas where the players are most likely to find dives and bars and stripper joints and chop shops and gang nests and warehouses and smuggling operations and meth labs (usually in the county), etc.
If you want libraries, or sports arenas or museums or government buildings or law offices or airports, etc, then you are more likely to find those in certain neighborhoods, and not find them in others.
Thats' the way most cities operate as well. Not always, depends on how they developed, but over time most cities tend to segregate "functions" into certain neighborhoods or sections.
Makes it much easier to localize urban functions (unless it is a utility or transportation network, etc) and to segregate adventure types or scenarios. I'd also include vadding opportunities, such as burned out neighborhoods, abandoned mills and factories, underground areas, derelict buildings and blocks, squatting nests, deserted subway systems, etc.
Instead of looking at it as "I need this building for this," look at it as "neighborhood types, precincts, patrol areas, etc." and after you have those basic locales fixed, then establish specific places within that environmental context. Use your map(s) to brief on neighborhoods or beats and you won't have to fix on specific locales until you're ready to specialize. Yet you'll still have a very good idea of what lies where.
Of course you can always have specialized and floater locales. Businesses, agencies, operations, buildings, individual residences, etc which are "out of bounds" for the area they inhabit. Exceptions to the rule. Either operating openly because they have an established foothold, or operating covertly and disguised as something else. You can put fronts anywhere, and sometimes the more outrageous the font the more interesting the adventure.
You can also have your characters establish "safe houses" within certain areas of the city. Places they can retreat too or hide in or operate from secretly if necessary. I used to do this when I worked undercover. It can give your players a defensible position, an escape route, or a concealed point to conduct surveillance. Or all of those things. It also allows you to reconnoiter a neighborhood in relative safety. I recommend this approach for obvious security reasons, though once your cover is blown, best to scrub your fix and never return to that place again. But two or maybe three well placed safe houses can work wonders on a case.
Hope that helped.
