Carbon emission implications of toll discount policies and network expansions in highway

Highways account for a significant share of transportation-related CO₂ emissions, yet policy interventions in mitigating these emissions remain underexplored. This paper investigates the impact of toll discount policies and network expansions on highway carbon emissions, drawing on long-term Electronic Toll Collection (ETC) data. We propose a bottom-up framework for estimating highway emissions and use a combination of time series decomposition and causal inference methods to isolate policy impacts. Our results reveal that toll discounts exert a significant and positive effect on highway carbon emissions, the shift of freight traffic onto highways reduces overall system-wide emissions: on average, each kilometer of discounted highway travel yields a net reduction of 52 g of CO₂ per day in the transportation network. These findings underscore both the potential of policy-driven highway utilization to lower total carbon footprints and the importance of carefully calibrated policies to manage induced passenger travel.