D. Wylie Associates


Fatigue causes impairments that affect vigilance, attention, perception, and decision making – processes that are crucial to safe driving. These objective, measurable fatigue effects can occur without any marked degree of prior physical exertion, and the driver may not be aware of these impairments. Another serious fatigue threat to safe driving is drowsiness – the tendency to fall asleep at the wheel. This is also a condition that drivers tend to underestimate (or to be totally unaware of) while it is actually happening to them. Circadian rhythms interact powerfully with fatigue and sleep debt, and have a major impact on human error.

Driver fatigue can also have subjective effects that the driver is aware of, characterized by lack of motivation, feelings of exhaustion, boredom, discomfort, and a disinclination to continue driving. These effects of fatigue can impair driving safety by impairing sustained attention and safe decision making.

The causes of driver fatigue, fatigue-induced cognitive impairment, driver drowsiness, and the subjective experience of fatigue include:

  • excessive hours of work
  • inadequate quantity or quality of sleep
  • night time driving
  • irregular work-rest schedules

In 1995 the U.S. Department of Transportation sponsored a National Truck and Bus Safety Summit. The goal was to bring together representatives of the many organizations involved in motor carrier safety to prioritize the safety issues facing the industry. Groups of experts represented drivers, law enforcement, shippers and carriers, researchers (Dennis Wylie was one of the scientific representatives), highway safety, business and vocational associations, safety management systems, government organizations, and manufacturers/suppliers. This comprehensive assembly came to the consensus that driver fatigue was the number one safety issue of the motor carrier community. National Transportation Safety Board chairwoman Deborah Hersman said in an interview with the American Trucking Associations’ newspaper Transport Topics (July 12, 2010) “Fatigue is just not as simple as revising hours of service regulations. It’s about compliance. It’s about enforcement. It’s about fatigue risk-management programs that encompass education and training.” Driver fatigue remains a leading safety issue today.

The Commercial Motor Vehicle Driver Fatigue and Alertness Study

The importance of driver fatigue led the U.S. Department of Transportation and Transport Canada to commission the largest, most comprehensive over-the-road study of commercial driver fatigue and alertness ever conducted. Dennis Wylie was the Principal Investigator, and he and his associates designed, executed, and documented a study involving 80 U.S. and Canadian tractor-trailer drivers in an operational setting of real-life, revenue-generating trips totaling more than 200,000 miles and 4,000 hours of driving. The scientists monitored the drivers and trucks continuously by electronic instrumentation. The study focused on several work-related factors, including:

  • Hours of driving during a work period
  • Number of consecutive days of driving
  • Time of day when driving took place
  • Schedule regularity

One of the most important findings of the study was that drivers under the then-current hours of service rules were not getting enough sleep to prevent dangerous fatigue and loss of alertness. This study provided the scientific data most frequently cited by the DOT’s Federal Motor Carrier Safety Administration to justify the first major overhaul in 50 years of the federal hours of service regulations, which, among many changes, increased by 25% the daily rest period required of commercial drivers.

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