Most of this discussion has been about unmarked cars and plain clothed officers. Unfortunately, there individuals out there who use fake cruisers and uniforms. Case in point.
In Arkansas a number of years reports were coming in about a marked uniformed deputy stopping women on secluded roads and rapeing them. The perp was caught when he passed a Deputy who did not recognize him and the jig was up.
Investigation revealed that the individual had purchased a used police Crown Vic equiped it with lights, siren and appropriate decals. He had purchased his uniform and insignia through a mail order house.
I don't remember how many victums there were but the number 18 stands out.
Prior to him being caught, The law enforcement community recommended that women not stop until they were in a safe place.
The good part of the problem was that LEO Management excised good leadership and instructed their officers not to attempt a stop unless the contact could be made in a lighted occupied location. Or with mutiple units. These instructions saved a lot of hate and discontent.
I think in this day and age all LEO's should make stops only in that manner. This will not only protect the public from impersonators but will protect the officers as well.