Traffic capacity under air quality constraints

The air pollution in urban areas of large cities represents a significant problem concerned by the public, government, and researchers. The Ambient Air Quality Standards set limits to protect public health and welfare. Once pollutant concentrations are found to exceed the limits, it is already difficult to adjust traffic flows to comply with the air quality standards. This paper defines the concept of traffic capacity under air quality constraints, and develops a method to estimate it in advance to regulate the traffic to remain within the capacity. Traffic capacity under air quality constraints is defined as the maximum hourly flow rate that produces pollutant concentrations less than the limits under the worst wind condition. Equations and an algorithm are developed to estimate the capacity. A case study of an arterial street in Nanjing is presented. Vehicle registration and mileage accumulation were collected to estimate emission factors. Traffic data, pollutant concentrations, meteorological data and topography were measured at the street to validate air dispersion model CALINE4 for predicting carbon monoxide (CO) concentrations. The algorithm is implemented to estimate the traffic capacity under CO constraint at the street. The results show that traffic capacity under CO constraint is much less than the highway capacity. The lowest wind speed and traffic composition has significant influences on the traffic capacity under air quality constraints. The capacity increases when the lowest wind speed increases. Traffic capacity under air quality constraints should be considered for air pollution control in addition to highway capacity under roadway and traffic conditions.