Simplified 4-Step Transportation Planning Process For Any Sized Area

Mann, William W.; Dawoud, Mazen · 1999 · ROSA P / Virginia. Department of Transportation

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Summary

This paper introduces a streamlined, simplified four-step travel demand forecasting model designed to replicate the results of complex regional models while remaining accessible for smaller jurisdictions and subarea studies. The development was motivated by the increasing complexity and cost of Metropolitan Planning Organization (MPO) models following the Clean Air Act Amendments of 1990 and the Intermodal Surface Transportation Efficiency Act of 1991. While these comprehensive models address policy-sensitive variables, they are too resource-intensive and time-consuming for local agencies requiring quick turnaround times. The authors argue that every MPO needs both a complex regional model and a simpler subarea model that can be executed on personal computers with minimal setup. The study presents the Virginia Department of Transportation (VDOT) model, a streamlined version of the Washington D.C. region’s MPO model, executed via new software called TP/4in1. This DOS-based software runs the entire four-step process—trip generation, distribution, mode split, and assignment—in a single execution on a PC, taking approximately two hours for the Washington region. A key modification in the trip generation step is the calibration of suburban/rural trip rates to 10.0 vehicle trips per detached household, which aligns better with smaller communities than traditional MPO rates. The model applies these rates at the zone level based on area density rather than district level. Trip distribution uses a gravity model calibrated to Census data and ground counts, while mode split factors are derived from regional data ("freeze-dried") rather than explicitly modeled. The assignment process utilizes four iterations of incremental capacity restraint loading and a modified Bureau of Public Roads (BPR) speed-flow equation to relate congestion to speed changes. The model was calibrated and validated using data from the Washington D.C. region, including Fauquier County, Virginia. Calibration involved adjusting friction factors to match ground counts and ensuring that simulated vehicle miles traveled (VMT) matched ground count VMT across various screenlines and jurisdictions without bias. The modified BPR curve, which uses a flatter function for volume-to-capacity ratios exceeding 2.0, successfully eliminated the need for typical speed feedback procedures by ensuring input and output speeds remained consistent. The software allows for detailed outputs, including trip ends by purpose, VMT summaries, and link analysis. The significance of this work lies in providing a practical, cost-effective tool for transportation planning in areas ranging from rural counties to large metropolitan regions. By reducing the computational burden and complexity, the VDOT model and TP/4in1 software enable local jurisdictions and novice practitioners to perform rigorous travel forecasting without the resources required for full-scale MPO models. The authors conclude that this approach fills a critical gap in transportation planning, allowing for efficient subarea studies that replicate regional model accuracy while being adaptable to local conditions or usable as a default model for any sized area.

Key finding

The streamlined VDOT model produces traffic assignments that match ground counts and gravity model input speeds without requiring iterative speed feedback procedures.

Methodology

modeling

Provenance

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Summary generated by qwen3.6-27b-prismaquant on 2026-06-10; verification: verified.

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