The Circadian Alertness Simulator (CAS) was developed as a practical tool for assessing the risk of diminished alertness at work. Applications of CAS include assessment of operational fatigue risk, work schedule optimization, and fatigue-related accident investigation. Based on the documented work schedules of employees, sleep and alertness patterns are estimated and a cumulative fatigue score is calculated. The risk assessment algorithms are based on physiological sleep/wake principles including homeostatic and circadian processes. The free parameters of the algorithms were optimized using over 10,000 d of sleep and alertness data sets collected from transportation workers performing their regular jobs. The validity and applicability of the CAS fatigue score was then tested using work/rest and accident data from three trucking operations. Heavy truck drivers involved in DOT-recordable or high-cost accidents were found to have significantly higher CAS fatigue risk scores than accident-free drivers. Implementing a risk-informed, performance-based safety program in a 500 power-unit trucking fleet, where dispatchers and managers were held accountable for minimizing driver CAS fatigue risk scores, significantly reduced the frequency and severity of truck accidents. Further examination of CAS risk assessment validity using scenarios provided in a fatigue modeling workshop indicated that the CAS Model also performed well in estimating alertness with a real-world transportation scenario of railroad locomotive engineer work/rest patterns.
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