Investigating the impact of urban grade-separation on pedestrian PM2.5 exposure

For urban intersections, grade separations have been deemed a way of mitigating traffic congestion. However, the environmental impact of various grade separations, especially on pedestrian exposure to traffic-related air pollution, is unknown. Particulate matter of 2.5 µm or less (PM_2.5) near roadsides comes mainly from vehicle emissions and has negative respiratory health effects. Pedestrian PM_2.5 exposure at ground level at different types of intersection and are studied and compared here. Based on realistic survey data at a current at-grade intersection, and reasonable assumptions about traffic speed and volume changes, MOVES modeling software used the highest resolution approach of operating mode distribution to calculate PM_2.5 emission inventory for three scenarios: at-grade intersection, overpass grade separation, and underpass grade separation. Pedestrian PM_2.5 exposure concentrations were estimated using the AERMOD model. The results of the case study show that both underpass and overpass intersections largely decrease pedestrian PM_2.5 exposure, at almost the same amplitude. From the viewpoint of regional environmental impacts, however, the underpass-type intersection is recommended for replacing the at-grade intersection, in order to both reduce traffic congestion and better contain pollution. The findings of this research indicate that, as three-dimensional urban transportation design is becoming a more popular measure for relieving traffic congestion, environmental analysis will become an increasingly critical part of intersection design.