When is jack-knifing most likely to occur?

Jack-knifing is a term used to describe a situation where a vehicle, particularly a tractor-trailer, skids and causes the trailer to swing out to one side, creating an angle that resembles a jackknife. This dangerous situation is most likely to occur when trying to reduce speed sharply while travelling downhill due to the combination of the vehicle's momentum and the increase in gravity’s pull.

When going downhill, the vehicle naturally gains speed due to gravity, and any sudden attempt to apply the brakes can cause the wheels to lock or skid as the trailer, which has a different center of gravity and momentum, may not immediately respond to braking. This disparity can lead to the trailer sliding out and creating the jack-knife effect. Thus, sharp speed reduction while descending is a critical scenario to be cautious about to prevent such an incident from occurring.

While turning sharply, driving on wet roads, and reversing can also lead to loss of control, they do not usually result in jack-knifing in the same way that rapid deceleration on a decline does, which significantly alters the dynamics between the tractor and the trailer.