The development of a typology of commercial vehicle driving environments, estimate of the percentage of drivers falling into each type of driving environment, and driving environment effects on driver fatigue are described. A model of commercial motor vehicle driver fatigue, based on literature sources and focus groups of industry professionals, is proposed. Three driving-environment factors (regularity of time, trip control, and quality of rest), comprising 25 indicators, are included in the model. Data were collected via a nationwide survey of 502 randomly selected over-the-road commercial truck drivers. Data analysis revealed 12 driving-environment indicators to be good predictors of fatigue and crash outcomes. Various 2x2x2 driving-environment typologies were created by using different high-low combinations of these 12 indicators. A typology based on the single best predictors of fatigue and crash outcomes from each driving-environment factor was selected for examination (i.e., favorable and unfavorable combinations of driving the same hours, waiting longer than expected for loads, and starting the work-week tired). The percentage of drivers working in each type of driving environment ranged from 5.2% to 20.1%. Additionally, the typology was significantly related to frequency of close calls and perceptions of fatigue. The 12 driving-environment indicators collectively accounted for 5% and 23% of the variability in close calls and fatigue perceptions, respectively (p</=.001), and 2% of the variability in crash involvement (p</=.07). Implications for fatigue management are also discussed.