Policymakers have sought to spur consumer adoption of advanced clean vehicles by granting them single-occupancy access to high-occupancy vehicle (HOV) lanes. We offer the first causal evaluation of these policies that accommodates geographic variability in the magnitude of this policy’s treatment effect. Focusing on the outcome of plug-in electric vehicle (PEV) adoption in California, we employ a generalized propensity score matching approach that allows for continuous, rather than binary, treatment effects. We estimate a state-wide dose-response curve to show that access to 6, 20, and 100 miles of nearby HOV lanes leads to 1, 3, and 10 additional PEV registrations in a census tract. We predict that with a 95% confidence interval, roughly one quarter of California PEV registrations during 2010-2013 were a result of the HOV lane policy. We identify geographically-specific marginal policy effects that are smaller in Los Angeles, but relatively larger in San Diego and Sacramento.