How to combine different microsimulation tools to assess the environmental impacts of road traffic? Lessons and directions

In the last decades, traffic microsimulation platforms have a growing complexity allowing a detailed description of vehicle traffic dynamics in a second-by-second basis. However, to project spatially their outputs, some precautions must be followed. Therefore, we analyze some variables used in the microscopic traffic models which have a high impact on further applications, especially when a spatial projection is required. To assess these objectives, a microsimulation framework which includes traffic and emission models was defined to characterize traffic flows and to evaluate vehicular emissions. This general methodology was then applied in a European medium sized city using two scenarios: (i) considering a Lagrangian approach and (ii) using an Eulerian approach of the simulation road traffic platform. The Lagrangian approach shows that if we have long links (some hundred meters, e.g. >500 m), we lose the spatial detail on emissions. On the other hand, using the Eulerian approach to define very small links (some few meters, e.g. <30 m), a significant statistic representation of traffic dynamics, in that link, was not obtained, particularly in areas with low traffic flow. The latter situation can occur because the vehicle speed can be high enough that did not allow recording any information in that link, even considering a high time resolution analysis (second-by-second). Thus, a non-linear trend of the error is identified when such data are analyzed geographically. Accordingly, depending on the use of those microsimulation tools, we identify some best practices related with the traffic model design that must be followed to minimize those errors.