WIT Press

Roundabouts And Traffic Emissions At Crossroads


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WIT Press


G. Latini, G. Passerini & S. Tascini


Although largely diffused in Europe, modern roundabouts are scarcely diffused in Italy. Nevertheless, it is well known that the introduction of such roundabouts to replace congested crossings often leads to quite smooth-flowing traffic. On the other hand, the impact of roundabout on traffic emissions is scarcely presumed. In this paper we propose an evaluation, by means of traffic models and vehicle emission models, of the progresses achievable by switching from traffic light-controlled crossings to roundabouts. The improvement, in terms of air quality, is estimated in relation to the characteristics of traffic flow, road capacity and regulation. Significant variations have been quantified with dependence on these parameters. In the first step of our study, national regulations and technical literature about roundabouts have been analyzed to point out fundamental characteristics and the limits of their application. A simple model has been developed in order to automatically compute the parameters modified by varying the input set. The model consists of two sub-models: the first one models a road network and the related traffic flows. The second model estimates traffic emissions. As a case study, the model has been applied to a sample road network consisting of four roundabouts and ten branches. By means of the model, the idling time at every single intersection has been quantified with respect to the different values of traffic flow. This calculation has been executed for both the traffic-light scenario and the roundabout scenario. For the estimation of total emissions, we selected proper traffic emission factors. We have mostly used European Copert factors while EPA instructions were useful to evaluate emissions of vehicles queuing at stops. Data for the traffic disaggregation have been provided by ACI, the Italian Automobile Club. The investigated pollutants, for this study, are Nitrogen Oxides (NOx), Carbon Monoxide (CO), Volatile Organic Compounds (VOC) and Particulate Matter (PM). As expected, waiting times at crossings significantly decreased, in roundabout scenarios, leading to significant pollution abatement. Peak reductions were found for CO with abatements above 50% in rush hours.