The Phystester, an ignition interlock system based on a predriving performance test, is evaluated. Results and observations from programs conducted by General Motors and other independent organizations are analyzed. The sensitivity of the psychomotor task offered by the Phystester to blood alcohol concentration, performance asymptoticity through distributed training, age, intelligence, drinking habits, sex, and digital dexterity is quantified. Finally, parametric optimization of the task variables and of the test criterion and correlation of Phystester with driving performance at various levels of blood alcohol concentration are discussed.