Much of California’s sustainability policy is premised on the idea of reducing Vehicle Miles Traveled (VMT). Under current state law, development projects cannot be approved without first estimating how much VMT they are likely to create, and then (often) taking steps to mitigate that VMT. The obstacle California faces, for this regulatory regime to be successful, is one of accounting. It remains hard to determine if a given policy reduces VMT, and if so, by how much. More accurate predictions of VMT impacts would reduce the likelihood that resources are being wasted on ineffective policies and offer assurance to stakeholders that state policies are achieving their intended pollution and greenhouse gas goals. This project focuses on three areas of improvement: 1) A more accurate and complete measurement of VMT, particularly using phone and odometer data; 2) A more accurate causal understanding of how development or transportation projects with different characteristics affect VMT; 3) A more accurate and readily accessible way to forecast VMT impacts of projects in comparison to well-defined counterfactual scenarios. This new analysis will focus on the VMT impacts of land development projects and of transportation projects believed to mitigate or reduce VMT. Results will be translated into one or more demonstration tools showing how to better estimate project-level VMT or VMT-mitigation, drawing in part from consultations with practitioners who will constitute an advisory group for the project.