The answer to the above question is “very.” A jackknifing truck poses one of the most dangerous road situations with which you can come into contact. Why? Because no matter how experienced its driver, (s)he cannot control the truck once it begins jackknifing.
Evan Transportation explains that, unfortunately, it does not take bad weather or slick roads to cause a jackknife. Instead, almost anything can cause a trailer to begin behaving erratically and stop following the cab to which it is attached. And once the jackknife starts, no one, including the truck driver, can predict where the trailer, or for that matter, the rig itself, will end up.
Thinking of a jackknifing truck as though it were a closing pocket knife makes the mechanics of a jackknife easier to understand. The truck’s cab acts the same way as your knife; its trailer acts the same way as your knife’s blade.
When you start closing your knife, the angle between its blade and the knife itself becomes more and more acute. The same thing happens when a truck jackknifes. The trailer starts moving toward the cab at an ever-decreasing angle. Your knife, of course, has a slot into which its blade goes. A truck’s cab, however, has no such slot, so ultimately the careening trailer has nowhere to go except to slam into the cab.
This could cause the whole rig to turn over or, alternatively, to wind up crosswise in the middle of the road or on its shoulder. Either way, you have very little time to avoid a collision once the truck in front of you begins jackknifing.