The goal of the present research programme was to find out how common driver fatigue is among New Zealand truck drivers and the degree to which they suffer from fatigue-related effects on their driving performance. To that end, 606 truck drivers were tested at a variety of sites throughout the North Island of New Zealand. The results showed that a considerable number of drivers are operating in excess of the hours of service regulations. The three fatigue measures in our survey indicated that there are significant levels of fatigue in the New Zealand transport industry. One out of four of the drivers' self-ratings of fatigue were in the "tired" range, even though many of them were surveyed at the beginning of their shift. A psychomotor test also indicated a very high level of fatigue in the sample. Overall, 24% of the sample failed the psychomotor performance criteria. Amount of rest and sleep, shift length, and number of driving days per week were all significantly related to psychomotor performance. The results of the daytime sleepiness inventory showed that the drivers in our sample had somewhat higher levels of daytime sleepiness than do heavy goods vehicle drivers in Great Britain. There was significant correspondence between drivers' self-ratings, psychomotor performance, and daytime sleepiness fatigue measures.
Brown (1994) has offered the following definition of driver fatigue: "... the subjective experience of fatigue involves conflict between the desire to rest and the inclination (or perceived commercial pressure) to continue driving to their planned destination ... The main effect of fatigue is a progressive withdrawal of attention from road and traffic demands ... the withdrawal of attention will be involuntary and difficult, if not impossible to resist ... Individuals so affected have been described as `driving without attention' (DWA) because they are apparently oblivious to impending collisions.. (pp. 311-312)." The present study adopts the above use of the term driver fatigue, treating the phenomenon as a generalized subjective state resulting from a combination of task demands, environmental factors, arrangement of duty and rest cycles, and factors such as drivers' consumption of alcohol and medications. Of particular importance to the present study are the performance decrements in driving that arise from the psychological state of fatigue.
While it is difficult to quantify the contribution of driver fatigue to crash rates, a number of overseas studies have produced estimates. The state roading authority in Victoria Australia, has estimated that it is a factor in approximately 25% of all truck-related crashes (Vic Roads, 1995). In the United States, it is estimated that fatigue-related crashes in transportation claim over 15,000 lives and cost more than 12 billion dollars a year in lost productivity and property damage (Caldwell, 2000; Rau, 1996). Other estimates place the incidence of fatigue in commercial driver crashes somewhere between 1% and 56% depending on whether the estimates are from safety researchers, transport regulatory agencies, or coroner's findings (Mitler, Miller, Lipsitz, Walsh & Wylie, 1997). In New Zealand, The 1996 House of Representatives Report of the Transport Committee on the Inquiry into Truck Crashes found that: "fatigue is likely to be a significant contributing factor in all types of crashes, not just truck crashes. Despite its importance, however, it is largely unrecognised as a problem in New Zealand." Estimates of the incidence of fatigue-related crashes vary widely, primarily because fatigue leaves no direct physical evidence at the scene of a crash and thus must be inferred from the circumstances of the crash and potentially unreliable reports from individuals involved (Summala & Mikkola, 1994). Nonetheless, it is generally acknowledged that fatigue is significantly underreported in official crash statistics, and is a …