52-vehicle Pile-up On A Highway A31, Emsland Autobahn, Germany

A multi-vehicle collision (colloquially known as a pile-up) is a road traffic accident involving many vehicles. Generally occurring on high capacity and high speed routes such as freeways, they are one of the deadliest forms of traffic accidents. The most disastrous pile-ups have involved more than a hundred vehicles.
Causes
Pile-ups generally occur in low-visibility conditions as drivers on freeways are sometimes caught out by driving too close to the vehicle in front and not adjusting to the road conditions.