Switching from LOS-Based to VMT-Based Traffic Impact Analysis: What Should the Threshold of Significance Be? A challenging upcoming question in California environmental review

Traffic impact studies traditionally frame traffic impacts of projects in terms of their effect on delay on automobiles or automobile flow as measured by roadway Level of Service (LOS). While analysis of LOS impacts measures how congestion changes locally around a project, it has been argued that traffic impact analysis based on LOS can work counter to environmental protection. This is problematic in California as traffic impact studies are conducted as part of the environmental review process under the California Environmental Quality Act (CEQA). As a result, the California legislature passed Senate Bill 743 (SB 743) in 2013 that called for the replacement of LOS as the traffic impact metric. Implementation documents for SB 743 identify Vehicle Miles Traveled (VMT) as the replacement metric. In the analysis of any environmental impact in California under CEQA, it must be determined whether a project has a “significant” or “less than significant” impact on the environment. The important dividing line between a significant and a less than significant impact is known as the “threshold of significance”. Implementation documents for SB 743 propose that development projects be evaluated versus the VMT of existing conditions. This paper examines several different interpretations of the VMT of existing conditions and the thresholds that would result from those various options.