A jackknife prevention apparatus configured to be mounted intermediate the trailer and cab of a tractor-trailer rig extending bilaterally wing-like from the tractor's frame rails to which it is hinged horizontally. The jackknife prevention apparatus can swing up or down on its hinge between lowest and highest positions by action of a motor which responds to tractor conditions so as to promote safety, maneuverability and convenience. Specifically, when the tractor is parked, the apparatus tilts down near the ground and constitutes a stairway from ground up to the frame-rail tops. When the tractor is unparked, the apparatus swings up to near horizontal, where it is inactive. If tractor speeds exceed perhaps 20 or 30 mph, the apparatus rises to its highest position where it blocks further nearing of a trailer's front corner to the tractor's cab at a tractor-trailer angle of perhaps 15 to 20 degrees, thereby preventing a jackknife accident.