A year long study by the National Transportation Safety Board of 182 heavy truck crashes in which the driver was fatally injured has found that fatigue was the most frequently cited cause or factor of the crashes. Of the 57 drivers (31 percent of the total) cited for fatigue, 19 were also impaired by alcohol and/or other drugs. Nineteen per cent of the professional drivers in the sample, for whom a determination could be made, were found in violation of the hours of service regulations. Drug free drivers appear to have crashes during certain time periods (when there is increased exposure and at times when people tend to be less alert and in accordance with low periods in circadian rhythm) whereas drug-involved drivers appear to have crashes throughout the day. Based on this study, it is clear that fatigue, drugs which are taken to counteract the symptoms of fatigue, and drugs which aggravate fatigue are major factors in causing fatal-to-the-driver heavy truck crashes [NTSB, 1990].