Driving safety at spiral tunnel portals under radius–slope combined alignment: A case study based on the improved safety field theory

At tunnel entrances and exits, the abrupt transition in cross-sectional and lighting conditions poses substantial safety risks, which are further exacerbated in spiral tunnels due to their coupled radius–slope design. This study proposes an improved artificial potential field-based risk modeling framework that integrates geometric alignment (radius and slope), lighting conditions, driver characteristics, and vehicle dynamics, enabling more precise quantification of driving risk under the combined influence of multiple factors. Using three alignment design schemes from a real-world engineering project as prototypes for the spiral tunnel, driving simulation experiments were conducted to systematically quantify and compare driving risks at the spiral tunnel portals across different radius-slope combinations. Results show that radius and longitudinal slope near tunnel portals significantly affect driving risk. A small radius coupled with a steep slope markedly increases entrance risk. Compared with open-road sections, internal tunnel zones exhibit a higher risk, and the 100 m before the exit generally presents a greater risk than the 100 m after the entrance. This study clarifies the mechanisms underlying risks at spiral tunnel portals under complex alignment and environmental conditions, and provides theoretical and practical guidance for improving alignment design and mitigating portal risks.