Why have multiple climate policies for light-duty vehicles? Policy mix rationales, interactions and research gaps

Globally, there are a wide variety of policies in place that could help contribute to deep greenhouse gas (GHG) emissions reductions in the light-duty vehicle sector. Most regions are impacted by a mix of such policies. However, the transportation literature has devoted little attention to policy mixes, especially in the light-duty vehicles sector, so here we review and draw insights from the broader, mostly non-transport literature. We identify several rationales for pursuing mixes of policies: (i) the "three legs" approach to transport decarbonization, namely that different policies should address different GHG reduction areas (low-carbon fuels, vehicle efficiency and reduced travel demand), (ii) the "market failure" perspective that a different policy is needed to correct each market failure, (iii) the "political process" perspective that considers the real-world need for a policy mix to be perceived as political acceptability, and (iv) the "systems" perspective that policy needs to send signals to channel technological innovation and break the lock-in of incumbent practices. Based on this review, we develop a simple framework for examining policy interactions across multiple criteria, namely GHG mitigation, cost-effectiveness, political acceptability, and transformative signal. We demonstrate this framework by setting hypotheses for interactions across six light-duty vehicle policies in the case of British Columbia, Canada - including a carbon tax, electric vehicle purchase incentives, infrastructure deployment, and three regulations. We conclude with a summary of important research gaps and implications for policy design, as well as quantitative modeling.