The adoption of zero emission vehicles (ZEVs) is limited by a variety of barriers. Some are region-specific (e.g. availability of charging infrastructure) while others are global in nature (e.g. battery prices) where improvements spill over between regions. This study explores regional spillover effects and GHG impacts of strong ZEVfocused policy, specifically the ZEV mandate in place in ten U.S. states ("ZEV States") which requires automakers to sell a minimum amount of ZEVs each year. We use a dynamic technology adoption model to simulate passenger vehicle sectors in North America, focusing on the case of one small region (British Columbia, covering 0.7% of the market) as potentially free-riding off of ZEV States' policy (covering 23% of the market). Results indicate that free-ridership is not effective; even with the ZEV mandate driving very high sales in ZEV States, British Columbia cannot achieve significant ZEV adoption without also implementing its own ZEV mandate. Further, for British Columbia to meet its 2050 GHG targets, it may need a ZEV mandate in addition to complementary climate policies pushing ZEVs to account for 40-93% of new vehicle sales in 2050. In short, regions seeking low-carbon transportation likely need to implement their own stringent policies.