A Bayesian approach to planning support on two-lane rural roads for automated vehicles: The role of geometric design in disengagement events

This paper presents an on-road study examining the interaction between Level 2 automated vehicles (AVs) and the geometric features of two-lane rural roads. These roads constitute the largest percentage of the road network in most countries. Their complex geometric features and lack of design consistency present a significant challenge for the operation of automated vehicles (AVs). Although AV systems aim to reduce driver workload and improve road safety, their performance remains limited on these roads, often resulting in automated system disengagements that require driver intervention. Such disengagements highlight emerging challenges for road administrations regarding road readiness and the safe integration of AVs into existing infrastructure. Based on an on-road study using two instrumented AVs, this study investigates how geometric features of Homogeneous Road Segments (HRS), including Curvature Change Rate (CCR HRS ), average radius, tangent length, and average parameter of vertical curves, affect driving system engagement rates. A Bayesian approach was used to quantify the relationship between road geometry indices and AV disengagements. The findings reveal that the primary factor affecting engagement status is CCR HRS , showing that higher values of CCR HRS (>400 gon/km) are associated with AV engagement dropping below 25%, highlighting a direct impact of geometric complexity on AV reliability. The other geometric indices have a moderate positive effect. The findings underscore that road geometry significantly affects AV reliability. These findings have significant implications for infrastructure planning and policy. By identifying geometric thresholds that compromise AV functionality, this research supports the definition of Operational Road Sections (ORS) aligned with Operational Design Domains (ODD). The proposed model offers a practical tool for road administrations to assess road suitability for AV deployment, inform upgrading priorities, and establish design guidelines that facilitate the safe integration of AV technologies into existing rural networks. As automation advances, aligning infrastructure design with AV capabilities becomes essential for equitable and effective transport policy.