Understanding Driver Perception of Sharpness, Risk, and Speed on Horizontal Curves: An Instrumented Vehicle and Questionnaire Study of Two-Lane Highway

Abstract

Inappropriate speed choice is one of the major causes of single-vehicle run-off-road crashes on horizontal curves. Safe speed negotiation depends on drivers’ perception of visual cues, such as curve sharpness, speed and available sight distance, which in turn shape their risk perception. Thus, drivers’ perceptions of sharpness, risk, and speed play a critical role in speed selection on horizontal curves. However, existing research has largely focused on geometric factors for enhancing curve safety. This study first examines the effects of road geometry and driver characteristics on drivers’ perceptions of speed, sharpness, and risk, and second, how these perceptions affect objective curve speed. A naturalistic driving experiment was conducted on a 22 km stretch of a two-lane rural highway containing 28 horizontal curves. Thirty-four drivers drove an instrumented sedan equipped with a GPS data logger to collect speed and perception data. The objective speed data were collected using a GPS data logger, while a surveyor seated in the vehicle recorded perception data via a questionnaire, taking responses within 15 seconds to capture drivers’ short-term perceptions. Perceived speed was negatively correlated with perceived sharpness and perceived risk. Deflection angle, curve length, curve gradient, gradient of the preceding tangent, and education emerged as significant predictors of perceived speed. Deflection angle, curve length, and transition curve length significantly influenced perceived sharpness, whereas age significantly influenced perceived risk. Among perceived speed, sharpness, and risk, perceived speed significantly influenced the mean observed curve speed. These findings highlight the importance of integrating driver perceptions into horizontal curve design and safety interventions. Targeted measures, such as appropriate signage and visual cues, can encourage safer speeds, while education and training programs for younger and novice drivers may reduce misperceptions and enhance curve safety. Future research across diverse road sites and driver groups can provide more insights into these findings. © © 2025. Published by Elsevier B.V.