Determining Cumulative Impacts for Development Projects under the California Environmental Quality Act (CEQA) Transportation Impact Analysis Process in Los Angeles

Los Angeles County Traffic Study GuidelinesSection 3.1.4.3: Cumulative Impacts Determination

Land use projects must consider both short-term and long-term effects on vehicle miles traveled (VMT). Short-term effects are evaluated through detailed project-level VMT analysis, while long-term (cumulative) effects are assessed based on consistency with the Southern California Association of Governments' (SCAG) Regional Transportation Plan/Sustainable Communities Strategy (RTP/SCS).

The RTP/SCS serves as the regional plan for compliance with air quality conformity requirements and greenhouse gas (GHG) reduction targets. Projects consistent with this plan in terms of location, density, and intensity contribute to meeting regional air pollution and GHG goals, and thus would generally have a less than significant cumulative impact on VMT. Conversely, development in locations not specified for growth in the RTP/SCS may indicate a significant transportation impact. However, if a project does not show a significant impact in the project-level analysis, it can be concluded to have a less than significant cumulative impact.

Projects adhering to the efficiency-based impact thresholds outlined in the RTP/SCS are aligned with the long-term VMT and GHG reduction goals. However, projects that: (1) show a project impact after applying an efficiency-based VMT threshold, and (2) are not consistent with the SCAG RTP/SCS, may have a significant cumulative impact on VMT. In such cases, further evaluation is necessary.

This evaluation can be performed using the SCAG RTP/SCS Travel Demand Forecasting Model. This involves comparing a cumulative "no project" scenario, representing the RTP/SCS cumulative year conditions, with a cumulative "plus project" scenario, reflecting the reallocation of population and employment growth based on land use changes from the proposed project. The analysis calculates baseline area VMT, residential VMT per capita, or employment VMT per employee, with any increase above the RTP/SCS forecasted values constituting a significant impact.

For regional-serving retail projects, the cumulative analysis involves additional steps:

1. Determine the traffic analysis zone (TAZ) for the project location.
2. Convert project land uses into appropriate employment categories in the RTP/SCS model.
3. Adjust socioeconomic parameters in the TAZ to reflect changes due to the project.
4. Run the four-step model for both the cumulative "no project" and "plus project" scenarios across four time periods (AM peak, midday, PM peak, nighttime).
5. Calculate total VMT for each scenario and determine the net change in daily VMT by subtracting the "no project" scenario VMT from the "plus project" scenario VMT.

Land use plans that: (1) show a project impact after applying an efficiency-based VMT threshold, and (2) are not consistent with the SCAG RTP/SCS, may also have significant cumulative impacts. This can be evaluated using the SCAG RTP/SCS model to compare the cumulative "no project" scenario with the cumulative "plus project" scenario. The analysis involves calculating total VMT and VMT per service population for both scenarios, with any increase above the RTP/SCS forecast indicating a significant impact

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