Do Voluntary and Mandatory Clean Truck Programs Improve Air Quality Around Ports?

ABSTRACT States and port authorities have implemented “Clean Truck” programs (CTPs) to encourage trucking companies to transition to newer, cleaner trucks to reduce the emissions around ports. We examine the comparative effectiveness of mandatory and voluntary CTPs implemented across major United States ports. Leveraging insights from the awareness‐motivation‐capability framework, we theorize how environmental regulatory programs shape carrier behavior under different regulatory structures in the trucking industry. Using a difference‐in‐differences design and a database containing detailed truck and trailer information and air quality measures, we find that mandatory programs significantly accelerate truck model‐year upgrades, narrowing pre‐existing disparities in fleet upgrades across carriers, whereas voluntary programs show no measurable effects. Mandatory CTPs are associated with reductions in nitrogen dioxide concentrations, while effects on particulate matter are less consistent. Air quality improvements are most pronounced in inner harbors and decrease with distance from port activity. Our findings contribute to supply chain and regulatory policy literature by highlighting the effectiveness of mandatory environmental programs in driving industry‐wide change and the limited impact of voluntary initiatives.