I can't say this is a hard set fast rule.
It depends on the person. We try to discourage anyone from riding in with us. However, we also base our decisions on allowing it, on the situation and person. Most of the time, it is usually an older spouse, who has a ride home, or a parent. Even then, they sit in the front seat, divider door is closed, and the ambulance doesn't go in gear until the seatbelt is buckled.
Is someone shows any hint of emotional distress, or it is a distressing call, no ride if I am on the truck, driving or EMT.