I do not stop for unmarked cars. I allow the sirens and beacons to follow me until I'm in a crowded area, such as a shopping center parking lot, supermarket parking lot, etc.
After I explained why I didn't pull over, I was immediately off the hook.
"How many people, in this city, over the past five years, have posed as cops, with a little portabubble blue/white/red light atop their car, and then rape/kill the person who stops for him, believing him to be a REAL police officer?"
"Okay, case dismissed".