Often times major map slow-downs are caused by placing dynamic lights of any kind in high-polygon areas. So for starters make sure you don't have any dynamic lights in the street area (flickering or wavering lights, lights that turn off or on, etc). If it's still too slow, try simplifying the squad cars, maybe make a custom texture or 2 for the cars to compensate for the low-poly models. Also try simplifying the bank's architecture. And make the windows opaque from the outside, make them look tinted or something. That will further reduce the polys seen from the outdoor perspective.
Hope those ideas help.
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"I love war and responsibility and excitement.
Peace is going to be hell on me."
Gen. George S. Patton, Jr.