EMISSION FACTORS, VEHICLE MODAL ACTIVITY AND ROAD OPERATIONS. A MESOSCOPIC APPROACH

Current studies underline that vehicle emissions are highly linked to modal vehicle activity and a modal approach to traffic-related air quality modeling is suitable to estimate mobile source contribution to air quality. Pollutant emissions depend on characteristics of vehicles in traffic flow and on road operating conditions predictable after the implementation of the road project; then, the usual emission rate models do not predict properly the on-road vehicles emissions of modal traffic events, such as those ones occurring at intersections. Moreover modal activity analysis carried out at micro scale level, as required by modal emission models, will be still necessary to value reliable emission factors by single mode (idle, acceleration, deceleration, cruise). Nevertheless, this kind of evaluation as regards each pollutant to be considered can be standardized depending on the type of road, volume-to-capacity ratio and fleet composition; so, mesoscopic level for vehicle modal activities, as it is usual in traffic analysis, will result appropriate to reach accurate emission estimates. Starting from the above considerations, the specific purpose of the research presented in this paper is to model and to measure vehicle pollutant emissions in different road situations (for geometry and traffic). The paper reports an exploratory analysis carried out within a more general research program targeted to develop a methodology suitable to be adopted in current practices of traffic studies accounting for effects on risk joined to pollutant emissions. Results referred in the paper suggest that it is possible, for a specific traffic condition, to reach estimates of vehicle polluting emissions from emission factors proper of each mode - such as they can be preventively obtained through modal emission models based on effective speed/acceleration frequency distributions - and from proportion of time spent by vehicles in each modal activity defined at mesoscopic level.