It is actually well known that road safety is crucially related to human factors. The road geometry itself often induces incorrect driving behaviors. This is confirmed by recent on road experiments, using an instrumented vehicle, such as by some occasional studies based on simulation. A validated driving simulation based approach is efficient and user friendly. This paper shows at first the outcomes of a laboratory survey that involved 120 drivers (> 2400 simulations) using a simple PC image generator. One entry straight, a transition curve (spiral) and a circular curve, a symmetrical spiral and a final straight geometrically compose the considered road. The dispersion of the trajectories for different speeds and lengths of the spirals and different radius of the circular curve has been analyzed. The outcomes show that usually assumed approaches in road geometry design can sometimes induce a deficit of safety. For example, when the angular deviation of the circular curve is greater than 40*, the safety standard decreases if the spiral length increases. This result seems to disappoint the well-established know-how. It cannot be explained through physical relationships, because of it refers to human perception and reaction. Secondly the paper presents more advanced results, that have been obtained using an interactive driving simulator. A road, in closed path configuration, is designed for simulation. A complete simulation survey has been carried out, recording the trajectories, the speeds, the accelerations, some bio-medical parameters, and more than 30 other parameters. This experimental survey greatly confirms the effectiveness of the simulation approach to validate the standards of the road geometry, for the safety assessment. Finally the study, sometimes contrary to the usual traditionally adopted technical rules, demonstrates it is not possible to set a geometrical safe standard, unless the impact on the human factors is verified.
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