Effects of combined curves on driver’s speed behavior: driving simulator study

The probability to occur in drive perception errors increases as the complexity of the road alignment increases. In particular perceptions errors could be significantly relevant in the conditions of horizontal curves overlapped with sag vertical curves (sag combinations or combined curves). The highway geometric design guidelines of several Countries provide suggestions for the coordination of the sag combinations in order to avoid combined configurations which can bring to undesirable optical effects and a reduced safety. Such suggestions come from studies based on the drawing of the perspective of the road. This drawing method is heavily limited with regards to the simulation of the perspective view of the highway to the driver during the dynamic task of the driving. Interactive driving simulation methods are deemed to be more efficient for these aims. This paper reports the results of a study carried out using an interactive driving simulator and aimed at evaluating the effects on the driver’s speed behavior of different configurations of sag combinations and non-combined curves on flat grade with the same features as the horizontal curves of the sag combinations (reference curves). The main result was that on suggested sag combinations the driver’s speed behavior did not differ in any statistically significant way from that on the reference curves. Whereas the critical sag combinations (configurations that should be avoided) caused a high reduction in speed along the tangent–curve transition, which pointed the driver’s reaction to the wrong perception of the road alignment This result, therefore, confirmed the effectiveness of the road design guidelines for the coordination of horizontal curves and sag vertical curves.