I suppose they can . . . but if its unmarked, then by definition it isn't identifiable as a police vehicle.llwatson wrote: . . . So yeah, I think unmarked vehicles can make traffic stops.
A few years back right here around Austin there was a police impersonator in an unmarked vehicle stopping people - usually women - for no good purpose. He'd outfitted his sedan with lights behind the grill (probably from Warshawsky or JC Whitney) and was able to put on a convincing light show.
Certainly a marked car with a light bar on top is almost certainly a real LEO . . . but an unmarked vehicle? I'd probably keep going until one or more clearly marked units showed up. (I expect by that point I'd have to keep my hands up, too, but with the alternative being taken by a fake cop . . . bad no matter how you look at it.)