These generally occur in low-visibility conditions, like rain or fog. In such conditions drivers on highways often drive closer together than they should. If one car develops a problem, those behind it cannot stop in time, hitting it. As cars are forced into other lanes and oncoming traffic more vehicles become involved. The most disastrous pile-ups have involved more than a hundred vehicles.

They are particularly deadly as the solid mass of crumpled vehicles makes escape difficult. A fire in one part of the accident can quickly spread to spilled fuel and cover the entire crash area. Vehicles in a pile up are often hit multiple times, increasing risk of injury to the passenger. Moreover cars are often spun during an accident and are subsequently hit from the side, increasing risk of injury or death. Some vehicle occupants choose to get out of their vehicles during a pile up, making them vulnerable to oncoming vehicles. Pile-ups can also overburden local firefighting, ambulance, and police services making quick rescues more difficult.

The large scale of these accidents can close important routes for several days. The destruction and intense heat of fires can also damage roadways, particularly by melting and burning the asphalt. A pileup inside a tunnel is by far the worst, as there is little means for escape in older ones, and the unvented heat may even cause the concrete lining to come apart.

Determining the cause of such accidents is also difficult for investigators and it is often impossible to tell if negligence caused the crash.