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Understanding Rules Noncompliance 9 same mode or stage of processing will suffer significantly more than tasks that rely on different cognitive resources (Wickens 1984). Previous research has also found that "cognitive load com- bined with the loss of exogenous cues, which can occur when the driver briefly glances away from the roadway, may be particularly detrimental." (Lee, Lee, and Ng 2007). This diversion can occur willingly, for example, using a cell phone or tuning radio controls, or as a consequence of some environmental stimuli, like passing a billboard or a flashing warning sign (Regan, Lee, and Young 2008). With the increase of technology proliferation into vehicles, driver distraction is becoming more common, leading to an increase of risk exposure. However, even though the effects of driver distraction are well documented, research into fully understanding the sources of distraction, the underlying causal mechanisms, and mitigation techniques are all still undeveloped and lacking (Regan, Lee, and Young 2008). Workload The study of workload has a long history in psychology as well as human factors transporta- tion research. In this project, only mental workload is considered. Gopher and Donchin (1986) define a measure of workload as "the difference between the capacities of the information- processing system that are required for task performance to satisfy expectations and the capacity available at any given time." There is an optimal level of mental workload. Under circumstances where workload is either too high or too low there are performance decrements. Low-workload conditions do not provide enough arousal to sustain vigilance and high-workload conditions cause employees to over-focus resulting in cognitive tunneling, both of which potentially lead to human error (Proctor and Van Zandt 1994). Cox-Fuenzalida (2007) noted that previous research described a general decrement in per- formance following a decrease in task demand. In an experimental study of workload vari- ability, she confirmed that a condition involving a shift from high workload to low workload impaired performance. Additionally, she observed that abrupt increases or decreases in work- load led to a loss of accuracy and slower response time. The high to low condition may be an arti- fact of fatigue. Regardless, measuring the dynamics of workload is important given the potential for performance decrement. Fatigue Research has documented the performance effects of fatigue. The performance effects of inad- equate sleep can affect an individual's ability to work safely and efficiently. Belenky et al. (2003) have shown that performance declines initially with mild to moderate sleep restriction of 7 and 5 hours, respectively, and after a few days it stabilizes at a less than fully rested level. The relevant performance effects include the following (Institute of Medicine 2006): Response time slows Attention to intensive performance is unstable, with increased errors of commission and omission Involuntary microsleeps occur Performance declines in short-term recall of working memory The susceptibility to any of the above becomes an urgent concern when the job carries safety risks for the employee, co-workers, or the public. The job performance of public tran- sit operators getting less than 7 hours of sleep on workdays is likely compromised. Research of Van Dongen, Mullington, and Dinges (2003) has shown that sleep loss-related perfor- mance declines often go unrecognized by the affected individuals making them at increased risk of error.