Traffic Forecasting as if Intersection Control Matters: The Sequel

For the second time within 5 years, the traffic forecasting model application developed by the planning staff for the Cedar Rapids, Iowa, area metropolitan planning organization (MPO) has achieved not only a base-year accuracy that exceeds what other, larger MPOs have accomplished, but also a travel-volume and travel-time accuracy for the relatively high-volume streets that, in most cases, is not significantly different from the sampling error of field observations. Comparable success has also been achieved in modeling peak-hour traffic patterns, where applications have also been developed to model peak-hour factoring and design hour volumes for transportation projects. The standard of accuracy that has been achieved for this metro area has been made possible by two major differences from typical MPO practice. First, travel estimates (trip generation rates) are based on field-measured vehicle-trip rates rather than the common practice of a stated-response travel survey of the public (i.e., on what people do rather than what they say they do). Second, and far more important, the modeling is based on the dynamic integration of the Highway Capacity Manual (HCM) methods of intersection traffic control analysis with traditional traffic forecasting processes. Although there are many planning agencies that claim to be using HCM procedures, the process described is shown to be the one that much more faithfully replicates the flexible approach capacities and volume/delay relationships that are the direct result of HCM-level analysis.