A jackknife accident is when a tractor-trailer's trailer swings out sideways and folds toward the cab, so the rig forms a V or L shape like a folding knife. It usually starts with wheels locking or losing grip during hard braking or a turn, especially at speed, on wet roads or with a light trailer. The trailer can block every lane, so jackknifes often cause multi-vehicle crashes.
How a jackknife happens
A tractor-trailer bends at the fifth-wheel coupling. When the tractor's drive wheels lock or lose traction, the trailer's momentum pushes the back of the tractor around, and the rig folds. When the trailer's own wheels lock, the trailer can swing out on its own ("trailer swing"). Either way the driver has very little time to recover once it starts.
Common causes
| Cause | Why it leads to a jackknife |
|---|---|
| Hard or sudden braking | Locked wheels lose steering and traction, often after following too closely |
| Speed for conditions | Too fast for a curve, ramp or slippery road |
| Wet, icy or oily roads | Less grip; the first rain after a dry spell is especially slick |
| Empty or lightly loaded trailers | Less weight on the trailer axles means less traction |
| Improperly loaded cargo | Unbalanced or shifting loads change how the trailer behaves under braking |
| Brake defects or imbalance | Out-of-adjustment brakes can lock some wheels before others |
| Fatigue or distraction | Late reactions lead to sudden, hard braking |
How drivers and carriers prevent jackknifes
- Slowing down before curves and ramps and in bad weather, and keeping a long following distance.
- Avoiding hard braking while turning.
- Anti-lock brakes (required on trucks and trailers built since the late 1990s) and stability control systems.
- Regular brake inspections and adjustment, documented on DVIRs and maintenance records.
- Proper cargo securement and weight distribution.
Who is at fault in a jackknife accident?
Because a jackknife is usually caused by how the truck was driven, loaded or maintained, the truck driver and carrier are often the focus. But fault depends on the facts. Investigators look at:
- engine control module (black box) data on speed and braking (truck black box data);
- the driver's hours of service and ELD records;
- brake inspection and maintenance history;
- bills of lading and how the load was secured, which can bring in the shipper or loader;
- whether another vehicle cut in front of the truck.
Texas divides fault by percentage. See who can be liable in a truck accident and comparative negligence in Texas.