Abstract: A large road vehicle has a cabin subject to accelerations in normal driving conditions in excess of accelerations experienced by the vehicle as a whole and which are sufficient to lock the acceleration responsive retractor of the driver's seat belt system unnecessarily. To prevent such locking except under genuine emergency conditions, a solenoid in the retractor holds inoperative a locking mechanism responsive to retractor acceleration, until an inertia switch located on the vehicle chassis (and thus less exposed to acceleration than the cabin mounted retractor) senses a predetermined vehicle acceleration, when the switch de-energizes the solenoid to permit the retractor acceleration sensing mechanism to operate. The seat belt system is passive and a switch operated by the handle of the door adjacent the seat is open when the door is closed but closed when the door is opened so as to energize the solenoid and thereby prevent inadvertent locking during door opening movement.