On the 15th of September, 2011, near Sao Paulo in Brazil one of the largest ever traffic accidents in history took place. In heavy fog, several vehicles collided, beginning a chain reaction that ultimately consumed around 300 cars and trucks. The incident left one person dead, and many more injured. While notable for its sheer scale, this pileup is just one of many such multi-vehicle collisions… many of which have, at their root, the same basic cause.