Exploring the impact zone at spiral tunnel entrances and exits: Analyzing the vehicle control process from invalidation to recovery under environmental perturbations

Abstract

At tunnel entrances and exits, rapid transition in cross-sections and abrupt lighting changes create a brief “environmental perturbation period,” characterized by unstable vehicle dynamics until drivers regain control required by the new driving situation. During this period, the transition of vehicle control from invalidation to recovery is a critical zone for tunnel traffic safety. Different alignment radii may introduce unexpected perturbation to vehicle control in spiral tunnels with small-radius curves. This study adopts the Midicun Spiral Tunnel as a prototype, with three alignment design schemes for driving simulation experiments. A sliding window-based sample entropy method is applied to reveal the adaptation process of vehicle control from invalidation to recovery, and two indicators, model invalidation position (P_MI) and model recovery position (P_MR), are introduced for analyzing the spatial distribution characteristics of the perturbations. By considering both lateral steering and longitudinal speed control, the impact zone length of environmental perturbations at spiral tunnel entrances and exits is quantified using the 15 % and 85 % quartiles of P_MI and P_MR, respectively. Results reveal that vehicle control often invalidates before the tunnel entrance/exit and recovers within a certain distance afterward. While spiral tunnels do not significantly increase the overall impact zone compared to straight tunnels, the spatial distribution of these zones differs. At tunnel entrances, lateral steering control is the key to determining the impact zone length, which is longer than that influenced by speed control. Smaller alignment radii in spiral tunnels reduce steering impact zones before entrances but extend them after, with longer zones observed as the radius decreases. At the tunnel exit, steering control remains the key to determining the impact zone length. Before the exit, spiral tunnels exhibit larger steering impact zones than straight tunnels, especially with a 530 m radius. Conversely, straight tunnels show longer pre-exit impact zones due to earlier speed control invalidation. These findings, using straight tunnels as a reference, provide valuable insights into the safety management zones at spiral tunnel entrances and exits, offering theoretical support for optimizing alignment design and safety management. Furthermore, this study provides a methodological insight for investigating the impact of perturbations on the recovery mechanisms of vehicle control.