A seat belt system for a vehicle has a shoulder-belt extending diagonally across the seat. A belt tensioner applies tension to the belt in response to detection/prediction of a potentially unsafe situation by a safety sensor/control system. A support arm has an upper end mounted to the vehicle structure above an upper outboard portion of the seat. The support arm extends downwardly and a guide member is attached to the lower end of the support arm and engages the belt. The lower end of the support arm is movable relative to the upper end/mounting point such that when the tensioner is activated, the belt urges the guide member from a comfort position relatively far from the neck and/or upper shoulder of a seat occupant to a safety position closer to the seat.