Abstract
Uncertainty is a major issue in many phenomena. The operation of highways is no exception. Yet one of the most common methodologies for assessing the performance of facilities such as intersections and freeways lacks the ability to reflect uncertainty in the inputs or the outputs. Methods that allow explicit examination of the variations that arise in the performance of given facilities or systems are necessary. The issue of uncertainty is examined for two freeways. Ideas are presented about how uncertainty might be treated for such facilities. One focuses on uncertainty during the peak hour. The other considers the entire year and asks whether such an approach gives a more complete picture of performance.