Abstract: A tractor-trailer coupling including a fifth wheel on the tractor pivotally receiving a king pin on the trailer. Cooperable structure is provided between the tractor and trailer which is operable during highway driving to prevent the tractor and trailer from pivoting relative to one another to a sufficient extent to cause vehicle jackknifing. This cooperable structure can be rendered inoperative to allow greater pivotal movement between the tractor and trailer in tight maneuvering conditions. In addition, cooperable stops on the tractor and trailer prevent the tractor and trailer from pivoting so far that their sides can engage and cause damage to the vehicle when the jackknife preventing structure is inoperative. The jackknife preventing structure is rendered operative and inoperative from within the tractor cab.