Your family road trip is going well. You've been making good time, and you'll stop at the next exit to get lunch. Then, suddenly, you see brake lights ahead. Traffic comes to a stop. You turn on the radio to find out what's going on. The reporter says that a jackknifed trailer has blocked the road. You sigh. It's going to be a while before you get your meal.
An Acute Problem
Normally, when tractor-trailers travel, the front tractor unit and the cargo trailer form a straight angle. When the semi-truck turns, the front and back ends form and obtuse angle. When a truck jackknifes, its tractor and trailer form an acute angle—an action that resembles the folding of an actual jackknife. The tractor can no longer straighten out the trailer. The truck becomes stuck, blocking the road for oncoming drivers. Severe jackknife incidents can result in death. Trucks jackknife when they lose traction due to bad braking, icy roads, or poorly distributed weight. The back of the trailer swings around toward the front of the truck. The trailer can hit cars in the process or the entire rig can be tipped over. Truckers are trained to avoid these sorts of accidents, but sometimes a situation quickly spirals out of control.
Some technologies have reduced the risk of jackknifing. Anti-lock brakes keep the truck from skidding on wet or icy roads. Engineers are also developing devices to make jackknifing impossible. It's a tricky problem to solve because trucks still need to be able to make turns.
See for yourself: http://www.youtube.com/watch?v=dwUwl7ke1iY
Check out the following links to learn about more technologies that have been developed to prevent jackknifing.