Having experience with a P.D. in Colo. The party backing up would be cited for unsafe backing. However if the rider or driver of the car was seriously injured or killed the Officer would call in a Traffic Accident Technician/Investigator (Unit) to do more than just a simple diagram and measurements. Skid marks for instance may establish a minimum speed using the coefficient of friction based on the vehicle weight, in this case the motorcycle, along with the drag factor of the road. In the absence of skid marks, gouge marks in the pavement can be used to help determine speed as can debris distance from the point of impact. The investigation may not change who is cited but may well supply the car drivers insurance co. with additional information to mitigate the car drivers (insurance co's.) financial responsibility. That, along with the reports from the insurance co's highly trained investigators and reconstruction experts may well be used to exonerate the driver of the car from all responsibility, both civil and criminal.
Of course if only minor or no injuries are sustained there would be much less investigation. And if the driver of the MC admitted how fast he was going he's twice a fool.
Disclaimer; There is more to it than I've patience to type out, and although I have 32 years of experience I was only in traffic for a few years a long time ago. But I have played a cop on TV and I've stayed at a Holiday Inn express at least a half dozen times.