Evaluation of air pollution impacts of a signal control to roundabout conversion using microsimulation

Traditionally, the comparison between alternative types of road intersection control has focused on efficiency and safety. In recent years, the increasing importance of air pollution produced by vehicular traffic has suggested that environmental considerations should be added to the above aspects as a criterion for intersection design. Situations in which a conversion of traffic control type (e.g. signal to roundabout) is applied to an existing intersection are frequent in real networks, and it is interesting to evaluate how pollutant emissions may change as a result. The increasing availability of microsimulation software and emission models makes it now possible to conduct such detailed analyses at an operational level. The research described in this paper focused on a comparison of a roundabout and a signal-controlled intersection in terms of environmental performance. A real road intersection where a roundabout has replaced a fixed-time traffic signal has been analyzed. S-Paramics® software was used to extract detailed vehicle data required as inputs for instantaneous emission models. Microsimulation model parameters were calibrated for obtaining reliable outputs in terms of traffic pollutant emissions. Emissions of NOx, PM10 and total carbon were considered. It was found that the roundabout, generally, outperformed the fixed-time traffic signal in terms of vehicle emissions, although the difference between the two types of control was smaller in terms of environmental impacts than in terms of operational traffic performance.