A Hybrid Approach for Determining Traffic Demand in Large Development Areas

Trip distribution is an important element of traffic impact analysis and is typically performed using expert judgment. Impacts are very sensitive to this step, and for large, mixed-use developments with many parcels, trip distribution can be very time consuming and potentially error prone. Regional trip distribution, on the other hand, is generally computed using a gravity model as part of a three or four step modeling process, which is efficient but that does not allow for the introduction of expert opinion, and outputs must generally be smoothed prior to use for traffic or geometric design. We present a hybrid approach that first obtains trip distribution patterns from a regional (TransCAD) model. Origindestination (OD) pairs and graphical highway network are then imported into an Excel spreadsheet using a comprehensive set of VBA macros. The paper discusses additional macros developed that (1) implement a driveway-spread method to assign peak hour trips, (2) allow for quick, easy, and intuitive revision of flow patterns, and (3) export network and intersection flows in a traffic program (Synchro) for operational analysis and simulation. We provide a real-world case study where the tool was applied within a rapidly growing community (Johnston Iowa, 2,600 -acre development area with 40 sub-zones). The hybrid approach is expected to greatly reduce time and errors associated with manual trip distribution and site impact analysis.