Traffic offences present danger to the offender, and to others. This study examines differences in decision making and personality between traffic offenders and non-offenders. Fifty-one traffic offenders participating in penalty courses were compared to a control group of 36 drivers who were not penalized for traffic offences in the 5 years prior to the study. All participants performed the Iowa Gambling Task (IGT), a popular decision task employed for assessing cognitive impulsivity, and completed the "big five" personality questionnaire. The results showed that traffic offenders made fewer advantageous choices on the IGT; and an analysis with a formal cognitive model, the Expectancy Valance model, suggests that this results from offenders' high weighting of gains compared to losses. An examination of personality factors reveals that traffic offenders were more extraverted. The predictive power of IGT performance was comparable to that of the personality factor. These results demonstrate that the IGT can be useful for studying individual differences in risk taking in a real-world task, and combined with the EV model, identify the sources of these differences.