Is a construction worker in the street from a job site (not road work) to manage traffic vested with the legal authority to do so?

Is a private construction worker vested with any legal authority to stop and control traffic? For instance, a construction worker in jeans and a T-shirt steps into the street from a private construction site (not road construction) and stops traffic to allow a truck to maneuver onto the roadway. Is the construction worker vested with any legal authority to control the traffic? Is the traffic legally obliged to obey him? BTW the construction guy is not a flagman. Suppose the truck messes up repeatedly while maneuvering and the traffic becomes impatient, ignores the construction worker and carefully moves past him. Have they broken any law?

Similarly, my son does the same so I can maneuver my boat onto the roadway. Is he vested with any authority to control traffic? Is the traffic legally obliged to obey him or is it just a courtesy?

I asked this some while ago but did not get a good answer. Of course the "rules of courtesy" apply, but I am not asking about courtesy. I am asking about the LAW.