Unmarked vehicles being illegal to make traffic stops is one of the few good things about Ohio traffic laws. Having an entire state "police" force whose primary function is writing traffic tickets is bad enough.
My wife has instructions to never stop for anyone in an unmarked vehicle and to keep moving until she finds a marked vehicle or a police station. Fortunately (or unfortunately..depending on your point of view) one rarely has to drive very far on Ohio's highways to find one of our uniformed professional ticket writers.