The answer in Ohio is: “maybe.”
the rule in Ohio is that in order for a police officer to be competent to testify against you in a court of law, if he was primarily on duty for the purpose of enforcing traffic laws, then he must be in the proper uniform of the day and he must be in a marked police car. This issue of markings and uniforms comes up a lot in the context of off-duty officers who observe traffic violations.
From the outset, it is important to note that it has always been the case that an off-duty officer, with an unmarked car and no uniform can report his observations of you to an on-duty officer. It is only natural that this is the law, as a civilian could do the same thing. No police action is involved on the part of the off-duty officer in that case.
For instance, in the case of State v. Leonard,[1] an off-duty officer and his wife (also an off-duty officer) were driving in their private vehicle when they observed the suspect driving erratically. They phoned the Ohio Highway Patrol with a description and a state trooper observed the suspect driving erratically. The suspect was pulled over and arrested for DUI. The Court of Appeals upheld the decision of the trial court allowing the off-duty officers to testify because the off-duty officers merely relayed information to the highway patrol and did not actually participate in the stop and arrest.
But sometimes the off-duty officers get more involved than the ones in the Leonard case. In the case of State v. Shahan,[2] an off-duty police officer observed Mr. Shahan driving erratically. When Mr. Shahan pulled into a private driveway, the officer identified himself as a police officer and called an on-duty officer to the scene. The court ruled that since the officer was off-duty, not on traffic patrol, and because he had waited until Mr. Shahan stopped his car on his own, then the off-duty officer was competent to testify at the trial against Mr. Shahan.
As a general rule, it appears from the case law that so long as the officer only relays information to on-duty authorities, or waits for you to stop on your own, then these factors will cut in favor of the officer being competent to testify at your trial. But the officer is on shakier ground if he takes steps to pull you over with his unmarked vehicle.
