Mind wandering is a common experience but its prevalence and consequences during routine activities such as driving are unclear. This thesis comprises five studies investigating how often drivers’ minds wander, and the relationship between mind wandering and crash risk. The first study was a questionnaire completed by 502 drivers, to explore their experiences of mind wandering including its variation in different driving contexts. All drivers reported mind wandering at least some of the time. Tendency to report mind wandering during driving was positively correlated with trait tendency towards cognitive failure, and negatively correlated with tendency towards mindful attention and awareness. Drivers reported most mind wandering driving their own car on familiar roads. The second study built on the finding that drivers are most likely to report mind wandering on a familiar trip in their own car through an on-road study of drivers’ thoughts. Eleven drivers traveling in their own cars between home and work were periodically asked what they were thinking about, across 110 trips in total, to establish how often drivers report mind wandering on real streets. Drivers reported mind wandering around two thirds of the time, demonstrating that it is a frequent experience on familiar urban roads. Drivers’ thoughts shifted frequently, triggered by what they saw in the environment and by internal concerns unrelated to driving. The first two studies found relatively high likelihood of mind wandering on familiar roads, so the third study investigated links between mind wandering and crash risk by exploring variation in crash patterns on roads close to home and further away. Analysis of crash distance from home, accounting for travel exposure, confirmed a ‘Close to Home Effect’ for road crashes. New Zealand drivers face increased crash risk on familiar roads within 10km of home, suggesting that higher rates of mind wandering close to home may influence crash risk. The Close to Home Effect was explored in more depth in the fourth study, which was based on analysis of police Traffic Crash Reports. Crash frequencies at different distances from home were compared in relation with posted speed limit, crash location (intersection or midblock), and errors (intentional violations or unintentional lapses of attention). Compared with crashes on roads further away, crashes close to home were more commonly reported on urban than on rural roads; more often at midblocks (stretches between intersections) than at intersections; and were more likely to involve a lapse of attention than an intentional violation such as excessive speeding. The fifth and final study combined an on-road study of drivers’ thoughts on a prescribed urban road route with analysis of crash data from the same route. The purpose of the fifth study was to determine how patterns of mind wandering and crashes vary with respect to road environment factors. Results showed that mind wandering is not random, but varies systematically in relation to task demand and crash risk on familiar urban roads. Overall, results from the five studies showed that mind wandering is ubiquitous during everyday driving. Drivers focus on aspects of the driving task frequently, but typically briefly, in response to momentary actions of other road users, or to situations that are usually demanding, such as roundabouts. Although there is an inverse association between patterns of mind wandering and the places where most crashes happen, there is no evidence that mind wandering causes crashes. Findings from this research support theories of mind wandering that acknowledge its variation in different contexts. However, while theories imply that mind wandering is an infrequent departure from a norm of task focus, this research found that driving task focus is typically a temporary, intermittent departure from mind wandering. Results are generally consistent with the tandem model of driver behaviour, which suggests that drivers do not pay continual attention to driving but rely on an unconscious monitoring process that governs their behaviour most of the time. The main implication of this research for road safety practitioners is that roads ought to be designed to account for drivers’ unconscious, routine behaviours because their minds are often wandering. Drivers’ attention can be captured and directed appropriately where focus on driving is warranted. Overall however, mind wandering is not intrinsically dangerous but is a normal characteristic of safe everyday driving.