A jackknife occurs when a semi-truck trailer slides sideways and swings forward, folding against the cab like a pocket knife. This happens when the trailer wheels lock up and lose traction, causing the trailer to push the tractor from behind. Once the angle between the cab and trailer passes a certain point, the driver loses all control of the vehicle.
Jackknifing is usually triggered by sudden braking on wet, icy, or snow-covered highways. It can also happen if the driver enters a turn too fast or uses the trailer brakes incorrectly. To prevent this, drivers should avoid slamming on the brakes, use light pressure, and steer gently into skids. Modern anti-lock brakes and stability control systems have reduced the occurrence of these crashes.