Traffic control for air quality management and congestion mitigation in complex urban vehicular tunnels

Urban vehicular tunnels, generally connected to the ground road network via on- and off-ramps, are geometrically complex with multiple entrances and exits. They are designed with in-tunnel ventilation systems and multiple pollutant discharge points for air quality control. In addition to traditional mechanical ventilation tools, traffic control has been recognized as a useful approach for air quality management and congestion mitigation in tunnels. This study aims to develop quantitative system analytic models for optimal traffic control considering both traffic and air quality improvement goals for complex urban tunnels. Moving vehicles inside the tunnel are not only the emission source, but also a key factor affecting aerodynamics in the tunnel. For a control cycle, steady-state traffic flow, aerodynamics, and pollutant dispersion models are established by tunnel segment, in which the air flow speed under a given mechanical ventilation scheme is mainly affected by tunnel traffic. An optimal time-of-day ramp traffic metering and mainline inlet traffic control model is proposed using nonlinear programming (NP) techniques to meet multiple air quality/emission as well as traffic throughput requirements. In-tunnel air quality requirements and emission limits at discharge points are modeled as nonlinear constraints for traffic control decisions. For illustration, the model was applied to an urban tunnel in Hangzhou, China for optimal traffic control to (1) assist the tunnel ventilation to meet environmental requirements under varying traffic demand; (2) provide a mechanism for tunnel air pollution control when fan operation alone is not enough during peak traffic period; and (3) maintain traffic efficiency by preventing recurrent congestion in the tunnel. The proposed tunnel traffic control method proves a useful complementary strategy to traditional mechanical tunnel ventilation for the improvement of urban tunnel transportation environment.