While driving back from Montreal last night, Brian and I encountered a vehicle that seemed to have no lights on. As we got closer, we realized that it was a dark colored truck towing a trailer that was not hooked up to the truck's electrical system, so there were no taillights visible. On the dark highway through the Adirondacks, I could envision a fast-moving, unobservant driver plowing into the rear of the trailer, not seeing it until it was too late to prevent a collision. The driver was also swerving erratically, not quite staying in a lane, and if s/he was signaling, the lack of taillights made it impossible to tell anyway.
I had Brian call the police on a cell phone, and I stayed behind the vehicle, about six car lengths behind, with my flashers on to prevent others from rear-ending the vehicle. Brian told the operator our mile-markers as we traveled down the highway, and an officer was positioned at exit 18 to pull the truck over. The operator told us we could stop and talk to the officer if we wanted, but we didn't have to. We chose not to, and after seeing the truck getting pulled over, we turned off our hazard lights and continued onward to home and sleep.
I'm sure that the truck's driver thought we were incredible jerks for turning him in, but if I'd just passed him and not done anything, then later heard about an accident because of it, I'd be very ashamed of myself.