If the traffic stop on a theory of presenting a hazard to others was due to a mistake of law, it was a reasonable mistake of law, given the state's law about changing lanes.
As an element of the lane-changing law is the driver's omission of checking to see if the lane is clear, the police had reasonable cause for the stop in order to determine if the required visual check had been performed.
https://www.ca10.uscourts.gov/opinions/17/17-2008.pdf
Since 2011. An occasional commonplacing of the U.S. Circuit Courts of Appeal.
Neither an authoritative statement of the law nor legal advice. Quick work.