How do freeway tunnel portal locations and the corresponding horizontal alignment affect traffic safety? Insights from driving simulation experiment and reliability analysis

Previous research has indicated that freeway tunnel portals are prone to traffic accidents, with alignment at tunnel portal being a significant factor influencing crash occurrence. However, how to ensure safe design of freeway tunnel portal alignment remains unclear. This study developed a freeway model with five tunnels, whose portals are located at different positions along a tangent-curve section, to comprehensively investigate the impact of portal locations on driving behavior and traffic safety. Microscopic driving parameters were obtained through experiments and further analyzed using reliability analysis. Based on real-world crash data, two failure modes were considered: insufficient stopping sight distance and excessive lane departure. The probability of failure (PoF) was calculated using the Monte-Carlo sampling algorithm as an effective indicator of driving risk. To further explore various features affect tunnel portal traffic safety, sensitivity analysis was conducted on four key indicators, include curve radius, spiral length, pavement friction coefficient, and driving speed. The results show that the design locations of the tunnel portal significantly affect drivers’ speed and lane departure behavior. When the portal is located on a tangent section, the distribution of driver speed and lane departure behavior are the most concentrated. In contrast, when the portal is situated on a circular curve or spiral section, the distribution becomes more dispersed. The failure modes and PoF are related to the portal location. Besides, the PoF based on insufficient stopping sight distance increases continuously with the curvature at the portal, while the PoF based on excessive lane departure increases with the deviation of curvature. The synthetic PoF indicates that when the portal is located 3/4 of the spiral section, the PoF is the highest, reaching up to 35.66% at the entrance and 25.31% at the exit. The curve radius, spiral length, pavement friction coefficient, and driving speed all influence the PoF at the tunnel portal. Among these factors, increasing the curve radius and ensuring a sufficient pavement friction coefficient have the most significant impact on reducing the PoF. This study proposes recommendations for the alignment design of freeway tunnel portals and traffic safety management, providing valuable references for road designers and freeway administrators to enhance the traffic safety of freeway tunnels.