A full jackknife occurs when
- the driver of a combination vehicle separates the tractor from the trailer.
- the road is wet.
- the tractor and the trailer form an angle of 90 degrees or less. ✓
- the driver loses control of a combination vehicle.
Answer: C. the tractor and the trailer form an angle of 90 degrees or less.
A full jackknife occurs when the tractor and trailer form an angle of 90 degrees or less relative to one another.
Why this is the answer
The commercial driving handbook defines a full jackknife by the angle between the tractor and trailer: 90 degrees or less. It also warns that this can happen even at very low speeds and says jackknifing is often caused by poor braking techniques, including improper use of retarders. That definition points directly to the choice describing the tractor and trailer forming that angle, not to uncoupling, wet roads, or a general loss of control.
Why the other answers are wrong
- A. the driver of a combination vehicle separates the tractor from the trailer. — Separating the tractor from the trailer describes uncoupling, not the angle between them that defines a full jackknife.
- B. the road is wet. — A wet road is not the definition; the handbook ties jackknifing to poor braking techniques, including improper use of retarders.
- D. the driver loses control of a combination vehicle. — Losing control is a general description, not the specific angle between tractor and trailer that the handbook gives for a full jackknife.
Remember it
Picture the rig folded to a right angle or tighter — that is the full jackknife the handbook describes.
Official handbook: ICBC Driving Commercial Vehicles, 2 Heavy vehicle braking – Braking, p. 26