The geometric design principles traditionally hypothesize that trajectories of vehicles can be assumed corresponding to the road longitudinal axle of the road or, more correctly, to the median line of each allowed lane. In real conditions, instead, vehicles always travel along trajectories each other different; the variability is due to dynamic actions affecting the motion and, in a great measure, because the control of vehicle trajectories, performed by users, is not perfect. In order to consider if theoretical models can be effective for safety and comfort verifications in design process, it is important to evaluate how a reference trajectory can statistically represent the whole population of road users. In fact, the difference between a real trajectory of a generic vehicle and the theoretical one can emphasize the safety problems related to geometric characteristics of roads. To deal with these problems, it appears interesting to analyse the dispersion of trajectories in various road sections; in this way, in fact, the "reference trajectory" along a road alignment can be recognized by means of a statistical approach. Starting from surveys on real road elements, the paper presents a method aimed to obtain trajectories that have formal geometric expression and that can correctly represent the scattering of vehicles' position, because reference lines are defined after a statistical analysis of collected data.
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