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The Impacts of Mixed-Use Development on Vehicle Miles Travelled and GHG Emissions:

A National Investigation and Policy Implications

Principal Investigator(s): Simon C. Brewer PhD; Reid Ewing PhD; Shima Hamidi PhD

Project Partners: University of Utah, California Air Resources Board (CARB)

Research Project Funding: $161,449

Project Start and End Date: Oct 1st, 2023 – September 30st, 2024

Project Description: To help reach goals of improving air quality and reducing greenhouse gas emissions, recent legislation in California has required that the impacts of new developments are measured in vehicle miles traveled (VMT). Existing planning tools are focused on pre-existing targets, principally level of service. The proposed research will address this mismatch between requirements and existing methods by establishing a predictive model of VMT for mixed-use developments (MXDs). The analysis will be based on an expanded dataset of MXDs from 38 metropolitan regions. Model parameters will be used to create tools for use by planners and engineers, facilitating the assessment of development impacts, and providing a base to accelerate the adoption of these standard in other regions.

US DOT Priorities: This project fits well within the US DOT strategic goals of Climate and Sustainability and Transformation, aiming to improve sustainable communities, including the reduction of VMT and associated pollutants. The model results and spreadsheet tool will help in the mission, by providing end-users with an innovative method to optimize developments that reduce VMT.

Outputs: The main product of the proposed work is a spreadsheet tool to be used directly by planners in assessing the VMT impacts of MXD development. This proposed research fits well within the goals of the Sustainable Transportation and Communities Division of CARB, that aims to improve sustainable communities, including the reduction of VMT and associated pollutants. Our model results and spreadsheet tool will help in the mission, by providing end-users with a method to optimize developments that reduce VMT.

A secondary product will the development of an R package, which we anticipate having a greater interest among academics as it will permit automated testing of multiple scenarios. The R package has the potential to provide a basis for future research that can integrate a) machine learning methods into the estimation of VMT, likely improving predictive skill; b) optimization routines to find MXD development scenarios that maximize VMT reduction given certain constraints.

Outcomes/Impacts: The proposed research will benefit from two areas of existing knowledge. First, the existence and use of travel time-based tools indicates that the planning community has experience of working with these tools, which will facilitate adoption. Second, co-PIs Ewing and Hamadi have previously studied the link between VMT and development, and these studies will provide a basis and reference for the development of the new tool, which will be designed to closely match existing spreadsheet tools used to assess different development impacts (trip generation).

There exist numerous studies that demonstrate that mixed-use development can impact VMT. However, given current and historical emphasis on using level of service as a metric to assess development, the results of these studies have not been integrated into an off-the-shelf tool that planners can use. In contrast, several tools exist, and are commonly used to assess impacts on travel times. The change in emphasis to using VMT as an impact metric that is currently spearheaded by California is not currently supported by these tools, and the current proposal aims to address these gaps.

We expect this tool to achieve two key goals/impacts. First, it will provide CARB with a method to track how mixed-use developments help or hinder progress toward the goal of VMT reduction under SB 743. Second, it will provide a standard tool to help planners quickly examine different development scenarios, providing a measure that can help with compliance with goals of VMT reduction. Although we expect the adoption to be slower outside of California in the short term, the availability of an easy-to-use tool for impact assessment may help accelerate the use of VMT as a standard development impact metric in other states.