Emission factors related to vehicle modal activity

Polluting emissions depend on vehicle characteristics and on traffi c conditions expected after the construction of a road project. Nevertheless, in the assessment of new projects, the task of road designer becomes uncertain when the reduction in polluting emissions has to be evaluated. Moreover, vehicle emissions are highly linked to modal vehicle activity, but current emission rate models do not properly predict on-road vehicle emissions produced by modal traffi c events, as those occurring at intersections. Modal emission models require the analysis of modal activity at a microscale level in order to evaluate emission factors by a single mode (idle, acceleration, deceleration, and cruise). This evaluation can be standardised with reference to the type of road, volume-to-capacity ratio and fl eet composition. On the contrary, the mesoscopic level for vehicle modal activities, as it is usual in traffi c analysis, will result appropriate to reach correct emission estimates. In order to explain factors affecting polluting emissions, a research programme targeted to the development of a methodology to be applied to traffi c studies has been undertaken. This paper reports the results of an exploratory analysis, based on examples of driving patterns, with the specifi c purpose to measure and to interpret vehicular polluting emissions in road situations different for geometric and traffi c conditions. Results referred to in this paper show that, for a specifi c traffi c condition, estimates of vehicle polluting emissions can be obtained from emission factors proper to each elementary modal activity and from proportion of time spent by vehicles in each modal activity as defi ned at mesoscopic level