Effects of the driver perception of combined curves on speed and lateral placement

Several researches have pointed out that optical illusions may occur when the horizontal curve is combined with a vertical curve. In particular the literature reports the hypothesis that horizontal curves appear sharper or flatter when overlapping with crest o sag vertical curves, respectively. This hypothesis was validated from studies on the visual perception of the road through the use of not interactive techniques that do not allow the analysis of the driver reactions at the visual perception. Interactive driving simulation systems, on the contrary, have high potentials for the analysis of the effects of the combined alignment. An experimental study in the driving simulator of Inter- University Research Center on Road Safety was carried out in order to assess the driver behavior on crest and sag combinations, compared to the behavior on horizontal curve with the same radius but on flat grade. Thirtyfive drivers carried out drivings in the simulator during which the local speeds and the local lateral placement were recorded. On the crest combinations the result of statistical analysis of the speeds was entirely consistent with the hypothesis of perception on crest combinations. Also the influence of the crest vertical curve on the path was clear. On the sag combinations the speeds were not significantly higher (at 5% significance level) than those on the correspondent reference curves. Such a result does not seem to strengthen the hypothesis of perception on sag combinations. No significant effect of the sag vertical curve was observed on the path.