How parameters of microscopic traffic flow models relate to traffic dynamics in simulation: implications for model calibration

While several methodological issues in setting up a calibration process for traffic micro-simulation models are still unresolved, the influence of individual parameters of microscopic models on simulated traffic dynamics is also far from clear. In order to address these issues the paper sets up a methodology based on the sensitivity analysis of traffic flow models (the one here used is AIMSUN). Sensitivity analysis was performed by means of a series of 30 analyses of variance (ANOVA). These were designed to evaluate the effect of parameters on the variance of the simulated outputs, and to draw a general inference about i) the proper interval for the aggregation of measurements, ii) the proper measure of performance (e.g. traffic counts vs. speeds), iii) the proper traffic measurement locations iv) the sub-set of parameters to calibrate. The analysis allowed us to quantify the effect of the single parameters on different traffic phases. For example, it was possible to quantify the extent of the influence of the parameter “reaction time” on simulated outputs in locations where free flow, rather than congested conditions, occurs. The great differences among parameters in affecting the different traffic phases suggested that parameters are likely to be calibrated independently i.e. using data from different locations. The first evidence of the possibility of breaking the calibration problem into two sub-problems is given. This entails great benefits in terms of computational time, given the exponential computational complexity of the calibration problem.