Here in Wisconsin and neighboring Minnesota, people are encouraged to call 911 if an unmarked car tries to pull them over (I know...I know...hard to do on a bike!). The dispatcher will then inform the driver if there are any unmarked cars in the area and if so, contact the unit. The driver will then be encouraged to find a public place to pull over.

My sister was in a car coming back from Minneapolis one night and had an unmarked flash the lights. They did as described above, and were told that there weren't any unmarkeds in the area. They told them to keep driving while "REAL" unmarked cars intercepted the other car. It has become quite the issue in this area where people are actually being robbed or worse by people pretending to be unmarked police.