As far as I know none of the radar guns are designed to be able to measure the speed of a vehicle from another moving vehicle. The Bushnell Radar Guns models are definitely not sophisticated enough to be able to handle the task, but at the same time neither are the Decatur Radar Guns, which are the best units for traffic control around. Decatur Electronics is what is used by law enforcement agencies.
Sorry that I was not able to answer your question.