Simulator evaluation of drivers’ behaviour on horizontal curves of two-lane rural highways

To improve design consistency of existing and new roads, several studies developed operating speed prediction models and investigated drivers’ speed behaviour. Most of the existing models are based on spot speed data assuming constant operating speed throughout the horizontal curves and occurrence of acceleration and deceleration only on tangents. To overcome limitations associated with these hypotheses, this study investigated continuous speed profiles of individual drivers by a driving simulator experiment carried out on a two-lane rural highway using the VERA high-fidelity dynamic-driving simulator, operating at the Road Safety Laboratory of University of Naples Federico II (Italy). The experimental route consisted of the succession of 20 tangents with length equal to 1,000 m and curves with radius equal to 400 m. The tangent-to-curve transition was carried out by spiral curves with parameter equal to 150 m and length equal to 55 m. Study results show that speed is not constant along the curve and deceleration rates are considerably higher than acceleration rates. These findings question the traditional hypotheses used to develop operating speed profiles. The analysis of individual drivers’ behaviour showed that: (a) 85th percentile of the speed reduction experienced by individual drivers is more than twice as the operating speed difference in the tangent-to-curve transition, and (b) deceleration and acceleration rates experienced by the individual drivers are approximately double than deceleration and acceleration rates used to draw the operating speed profiles. Study results emphasize that the analysis of the individual speed profiles adds important information to the analysis of the operating speed profile and has the potential to identify supplementary design inconsistencies and safety issues.