CORSIM, a microsimulator for vehicular traffic, is being studied with respect to its ability to successfully model and predict behavior of traffic in a 36-block section of Chicago. Inputs to the simulator include information about street configuration, driver behavior, traffic light timing, turning probabilities at each intersection, and distributions of traffic ingress into the system. Data are available concerning the turning proportions in the actual neighborhood, as well as counts of vehicular ingress into the neighborhood and internal system counts, during a day in May 2000. Some of the data are accurate (video recordings), but some are quite inaccurate (observer counts of vehicles). Previous use of the full dataset involved “tuning” the parameters of CORSIM—in an ad hoc fashion—until CORSIM output was reasonably close to the actual data. This common approach, of simply tuning a complex computer model to real data, can result in poor parameter choices and completely ignores the often considerable unc...