To ask, what map are you on? And a fun thing to try is to build the basic, cheapest version of each type (or the type that emergencies wont spawn) as something in the code is somehow linked to vehicles. It works well as a tutorial or delay aid for new players, but when sandbox or going big at the start, it can hurt.

You should only need them untill the first emergency comes. After you get each type, just remove the cheap stuff and go as large and expensive as you want.