Daydreaming, not paying attention, not keeping a look-out or simply being negligent is a major, major cause of accidents. The Erie Insurance Group studied some 65,000 fatal accidents occurring in 2010-2012 and found that some 40,000 (62%) were caused by driver daydreaming. One in ten of the accidents was attributable to driver distraction such as texting, talking on a cell phone, etc. The study is discussed further in Popular Mechanics: Just How Dangerous Is Daydreaming While Driving?