Methodology for Transportation Projects under California Environmental Quality Act Transportation Impact Analysis Process in LA County

Section 3.2.4. - Methodology

Los Angeles County Traffic Study Guidelines section 3.2.4.1. - Project Impact Determination

The County employs the SCAG RTP/SCS Travel Demand Forecasting Model, detailed in the Los Angeles County Senate Bill (SB) 743 Implementation and CEQA Updates Report, to assess changes in Vehicle Miles Traveled (VMT) resulting from proposed roadway projects within their land use and transportation context. This model is specifically designed to quantify VMT changes induced by transportation projects.

To evaluate project impacts directly, adjustments are made to the SCAG RTP/SCS model's base year network to reflect enhancements in vehicle capacity anticipated from the proposed transportation project. The model is then run both with and without the project, keeping the land use inputs unchanged, to isolate the potential change in network VMT attributable to the project compared to the baseline. This assessment covers the entire area expected to experience changes in driving patterns, supported by justification for area selection.

While the SCAG RTP/SCS model adjusts trip lengths, mode shares, and route choices in response to network changes, it does not incorporate modifications in land use or capture latent demand increases. Therefore, induced travel is estimated using an elasticity factor sourced from relevant academic literature.

Consequently, the VMT impact of a transportation project is calculated as follows:

1. Run the SCAG RTP/SCS model with and without the transportation project to isolate the direct change in network VMT attributable to adjustments in trip length, mode share, and route choice.
2. Utilize the SCAG RTP/SCS model to determine the total modeled lane-miles across the project area, capturing changes in travel behavior resulting from the project.
3. Calculate the percentage change in total lane-miles anticipated from the project.
4. Determine the total existing VMT over the same area using the SCAG RTP/SCS model.
5. Multiply the percentage increase in lane-miles by the existing VMT, and then apply the elasticity factor derived from recent induced travel literature to calculate induced VMT.
6. Add the induced VMT to the modeled change in network VMT due to adjustments in trip length, mode share, and route choice.

This comprehensive approach ensures that transportation project impacts on VMT are accurately assessed and mitigated as necessary to align with CEQA guidelines and regional planning objectives.

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