Plus, you jumped to a felony-stop scenario (”???”) when metmom simply asked “”How are they going to arrest someone in a moving vehicle?”
I think the question posed was “what probable cause would police have to pull over a vehicle that is moving and proceeding in an orderly fashion in (congested) traffic?”
Anything the police want...one was not observed staying evenly in their lane, not signaling far enough in advance or for too long as a examples.