The effects of radius and longitudinal slope of extra-long freeway spiral tunnels on driving behavior: A practical engineering design case

The combined effect of the radius and longitudinal slope in an extra-long spiral tunnel can impose substantial demands on driving tasks. In this study, we conducted driving simulations using three alignment design schemes, with the Midicun Spiral Tunnel in China as the prototype design. We constructed a comprehensive indicator framework that considers four driving tasks, i.e., speed control, lane-keeping, distance-keeping, and collision avoidance, to investigate driving behavior characteristics within the spatial context of spiral tunnels. We used a matter-element model to compare the effects of different radius and longitudinal slope combinations for the three alignment design schemes. The results reveal that spiral tunnels constrain the driver's choice of speed more than ordinary straight tunnels, prompting more conservative speeds, especially when executing right turns and uphill maneuvers within the tunnel. The combined effect of a short radius (<= 530 m) and a steep longitudinal (>= 2.6 %) slope adversely impacts the driver's choice of speed during right turns and uphill maneuvers inside a spiral tunnel, leading to many instances of driving below the minimum speed limit. Compared to ordinary straight tunnels, the exit zone of an extra-long spiral tunnel induces greater driving risks, characterized by a severe decline in speed control and lane-keeping performance. Notably, we identified the unusual finding that, in the continuous circular curved sections of a spiral tunnel, drivers tend to drive close to the tunnel wall on the inside of the curve. The comprehensive evaluation results indicate that the spiral tunnel's Scheme A (R = 550 m/530 m, S = -2.5 %/2.6 %) is not recommended. The study results provide valuable reference for understanding drivers' driving behavior within spiral tunnels and optimizing spiral tunnel alignment design.