Geometric Threshold Effects on Motorcyclists’ Risky Behaviours at Roundabouts Using CHAID and Regression

This study examines the influence of roundabout geometric design on motorcyclists’ risky behaviours in mixed-traffic environments, where motorcycles form a dominant traffic component. While conventional safety analyses often emphasise the performance of four-wheeled vehicles, limited attention has been given to how geometric design shapes riders’ behavioural responses and risk perception. To address this gap, this study integrates multiple linear regression and Chi-squared automatic interaction detection (CHAID) to capture both linear effects and nonlinear threshold behaviours. Data were collected from multi-lane roundabouts using a drone and ground-level video observations. Regression results indicate that larger radii and wider geometries are associated with increased risky behaviours, including unsafe stopping and signalling non-compliance. In contrast, CHAID identifies exit radius as the most influential factor, with specific ranges (≤12.43 m and 30.2–32.57 m) associated with more consistent behavioural patterns, while larger radii (>32.57 m) are linked to increased risk. These findings highlight the importance of context-sensitive, motorcycle-oriented geometric design in improving safety outcomes in mixed-traffic environments.