Abstract: A safety belt arrangement for protecting passengers in motor driven vehicles comprises a strap roller having a spindle mounted for rotation in a housing, a belt strap wound on the spindle, biasing means for urging the belt strap to wind on the spindle, a locking mechanism to lock the spindle against unwinding rotation, a detector for sensing a reference surface or surfaces on the spindle or on the belt strap and a memory device to register the time interval between signals received from the detector to determine the unwinding velocity of the belt strap and to activate the locking mechanism when a preselected value of the unwinding velocity of the belt strap is exceeded.