Does traffic-related calibration of car-following models provide accurate estimations of vehicle emissions?
Résumé
A convenient way to assess the traffic fuel consumption or pollutant emissions is to couple a microscopic traffic flow model with an instantaneous emission model. Traffic models are usually calibrated using goodness of fit indicators that are related to the traffic behavior. Thus, this paper aims to investigate how such a calibration influences the accuracy of fuel consumption and NOx and PM estimations. Two traffic models are investigated: Newell's and Gipps' models. It appears that Gipps' model provides the closest simulated trajectories when compared to real ones. Interestingly, a reverse ranking is observed for fuel consumption, NOx and PM emissions. For both models, the vehicle emissions of a single vehicle are very sensitive to the calibration. This is confirmed by a global sensitivity analysis of the Gipps model that shows that non-optimal parameters significantly increase the variance of the outputs. Fortunately, this is no longer the case when emissions are calculated for a group of many vehicles. Indeed, the mean errors for platoons are close to 10% for the Gipps model and always lower than 4% for the Newell's model. This latter model then clearly outperforms the first one for emission calculations. Another interesting property is that optimal parameters for each vehicle can be replaced by the mean values with no discrepancy for the Newell model and low discrepancies for the Gipps model when calculating the different emission outputs. Finally, this study presents preliminary results that show that multi-objective calibration methods are certainly the best direction for future works on the Gipps model. Indeed, the accuracy of vehicle emissions can be highly improved with negligible counterparts on the traffic model accuracy.
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