If it is on the roadway it's a vehicle, horse, bicycle, lawnmower. Most states it applies to passengers too. Now wether or not a police officer will write a ticket for it is another story. If you make him or her mad and argue, then you are most likely going to get a ticket for it.
I know this becuase some jerk wanted to argue with my duty investigator and the accident fit the profile for the boss to be called (me) said jerk lied to me, wasn't wearing a seatbelt, rolled a vehicle in a place he shouldn't have been. So I wrote a ticket and the prosecutor processed the case. If said jerk would have said he was being stupid, I wouldn't have gotten a phone call and my investigator might have allowed him to call some friends to upright the vehicle or he would have to have paid for a wrecker service and that would have been it.
Most police officers are reasonable to reasonable people. If you are a jerk and you get a police officer that has been around for awhile most likely the police officer has lots of experience in being a real jerk.