To my knowledge, the cop trumps the red traffic light.
He sure does, in most jurisdictions. Also, you'd be surprised how often the average driver will let just about anyone tell them what to do if the person is directing traffic as if they know what they're doing.
About 10 years ago I was driving down a narrow street when there was a serious accident a few cars in front of me. I pulled over into a driveway and parked, then got out to see what I could do to help the person who was knocked down. Someone else with better first aid training was already there, and another was calling for an ambulance. However, the traffic was backing up in the street in both directions with more wanting in. So I walked back about 100 feet to the last intersection, it was controlled by traffic lights, I stood in the intersection and directed all traffic wanting to enter the street to down the cross street, not one single driver objected while I kept the street clear of traffic. Thus when the ambulance arrived he had a clear run up to the site, and then a clear street to drive down once they got the person on-board.