Most of the cities are based on an existing city in the 'States. For me Metropolis was always in New York, Gotham was either in Jersey or NY as well as Bludhaven. Midway is supposed to be something like Detroit. Central City was always in Kansas, and for some time it's sister city Keystone was across the bridge in Missouri. St. Roch was meant to be something like New Orleans. Star City is of course something like Seattle. The only ones I don't really have much reference for would be Ivy Town and Opal City. 

~KL~