alysis of the influence of the clothoids on curve negotiation on two-lane rural highways

Most geometric design standards recommend the use of clothoids when connecting tangents and circular curves to produce a smooth, pleasing alignment. Despite the advantages of using clothoids, there are also possible adverse effects and literature results do not yet provide definitive results on clothoids effectiveness. Since the effect of clothoids is still an open issue, this paper provides a contribution in the evaluation of the effect of clothoids on curve negotiation on two-lane rural highways by a driving simulator experiment. The study is carried out evaluating the combined effect of different curve radii, deflection angles, and curve directions. Sixty-eight participants were involved in the study. The experiment was conducted in the VERA (Virtual Environment for Road sAfety) dynamic-driving simulator, operating at the Road Safety Laboratory of University of Naples Federico II in Italy. The variables are: clothoid (2 levels: with and without), curve radius (four radii: 125, 200, 400 and 800 meters), deflection angle (three angles: 20, 40 and 60 degrees), and curve direction (left direction in the outer lane and right direction in the inner lane). Trajectory dispersion and lateral position were used as measures of driving behaviour. Study results show that the clothoids produce significant advantages, reducing the lateral position variability, the maximum and minimum lateral positions, and the number of departures in the opposite lane and in the shoulder. The greatest effectiveness is shown in the curve approach. Clothoids are effective on all curves tested but the effect on the curves with radius equal to 800 m is quite smaller than on curves with radii of 125, 200, and 400 m.