What Causes a Semi-Truck to Jackknife (and Who Is Liable)

A jackknife happens when the trailer of a tractor-trailer rotates outward, eventually folding toward the cab at a sharp angle. At highway speed it can sweep across every lane of traffic in seconds. Here is what causes it and who is on the hook.

The physics

Jackknifing happens when the trailer's wheels lose traction while the cab's wheels keep moving forward — or vice versa. Once the trailer begins to swing, recovery is extremely difficult, especially with a heavy load.

Four root causes

Most jackknife crashes trace back to one of these:

• Brake-system imbalance — trailer brakes locking up before the tractor brakes engage. Brake-related issues are a contributing factor in roughly 29% of large-truck crashes per FMCSA.
• Empty or lightly loaded trailers — less weight on the trailer wheels means less traction.
• Slick roads — rain, ice, oil. Driver training on speed reduction is supposed to mitigate this.
• Driver error — abrupt braking, sharp lane change, downshifting into a turn at speed.

Who is liable

The driver almost always shares some responsibility — but rarely all of it. The motor carrier is usually liable through one or more of: inadequate driver training, negligent hiring, failure to maintain the brake system, dispatching the truck in unsafe weather, or putting an empty trailer on a route at speed.