Emission-based signal control policy: is there economic justification?

Urban traffic signal control has traditionally focused on reducing delay, often without accounting for the environmental externalities associated with vehicle emissions. This study introduces the Marginal External Cost Ratio (MECR) as an economic screening metric to assess when emission-oriented signal control is justified at urban intersections. Using operational data from three locations in Bangkok's central business district, MECR is computed from Intersection Fundamental Diagram (IFD) variables to compare the marginal external costs of congestion and emissions under different traffic states. The results show that emission-focused control is economically justified only under low traffic accumulation, whereas delay-minimizing control becomes preferable once accumulation reaches moderate or high levels due to the rapid growth of congestion externalities. A sensitivity analysis further demonstrates that lower emission-rate functions—a reasonable proxy for higher electric-vehicle penetration or cleaner fleet composition—reduce the range of traffic conditions under which emission-priority would be warranted, thereby strengthening the predominance of delay-oriented strategies. The MECR framework provides a transparent, data-driven basis for evaluating the external-cost implications of signal-control policies and supports the development of context-sensitive approaches for sustainable urban traffic management.