Differentiation of visibility and alcohol as contributors to twilight road fatalities.

Elevated road fatality rates at night involve multiple variables, each of which contributes to an unknown degree. In order to determine the relative contributions of reduced visibility and drivers' consumption of alcohol, we performed two quasi-experiments analyzing data recorded by the U.S. Fatal Accident Reporting System from 1980 through 1990. The results confirmed that both variables play a major role in nighttime road fatalities and revealed new evidence that they contribute differentially to two classes of fatal collisions: Degraded visibility in low illumination is associated primarily with collisions involving pedestrians and pedalcyclists, whereas drivers' consumption of alcohol plays a larger role in other fatal collisions.