Abstract: An anti-jack-knifing safety device for use on tractors and trailers operating on the highways and streets. It includes a gear rack mounted on the front end of the trailer, and secured against rotation thereon, and a second gear rack mounted on the rear end of the tractor, and also secured against rotation on the tractor. Electromagnetic solenoid means are provided on the tractor portion of the device, for moving the gear rack from retracted position to extended position and thus into intermeshment with the gear teeth on the front end of the trailer, thus blocking relative rotational movement of the trailer and tractor and thus blocking jacknifing. The solenoid is actuated from the tractor battery and through a foot switch controlled by the driver when jacknifing is imminent.