Perhaps it is common knowledge in your State or area. but new to me.

ROAD PRECAUTION TO FLASHING LIGHTS FROM UNMARKED POLICE CAR

If you see an unmarked car and are not sure whether to stop.

With your cell phone you should call *77, this will put you through to a police radio dispatcher who can tell you if there are any unmarked cars in the area. A young college girl did this and was suddenly surrounded by police vehicles. They also surrounded vehicle following her. After wrestling subject to the ground, she found out that this person was a convicted rapist.

South Western Bell confirmed that Highway Patrolmen are aware of your right to proceed to a safe area.

Thats from a pretty reliable teacher where I work!