Abstract: A sobriety testing device subjects the operator of a motor vehicle to a test before permitting the engine of the motor vehicle to be started, and prevents the engine from being started if the test reflects that the operator's reflexes are impaired. The sobriety testing device includes a hand-held unit having a base position button, as well as left and right test buttons. During testing, the left and right test buttons are randomly illuminated, and a determination is made of the reflex time required to move the operator's finger from the base position to the indicated test button and back to the base position. A base norm reflex time is recorded by the testing device representing the operator's reflex time when free of any physical or emotional impairment. The sobriety testing device compares the operator's current reflex time with the previously recorded base norm and disables the engine if the operator's performance is significantly below the base norm.