Driving behavior and safety performance in spiral tunnel groups: A naturalistic data analysis

The characteristics of naturalistic driving along freeway tunnels were examined to better understand driving operations and safety performance along a series of closely-spaced spiral tunnel environment. Thirty-four valid participants completed runs in both travel directions of spiral tunnel groups over 25 km. Driving behavior was quantified using speed and speed consistency, longitudinal acceleration, lateral lane position, time-to-line crossing, steering wheel angle, and steering entropy. These measures were treated as safety performance indicators and were integrated using the rank-sum ratio method to construct a composite safety performance index for comprehensive evaluation and comparison. The results showed that: (1) Clear directional differences in driving operations. In the downhill direction, vehicles operated at higher speeds with poorer speed consistency and a stronger tendency to speed. In the uphill direction, acceleration and deceleration fluctuated more, indicating greater longitudinal control effort. (2) Safety risk was significantly higher in the downhill direction. The composite safety performance index was markedly worse, accompanied by more frequent speeding and larger dispersion of lateral lane position, reflecting reduced control stability. (3) Pronounced section-dependent degradation was observed. Connector tangent sections exhibited the poorest safety performance. The tunnel entrance zone also performed significantly worse than the interior middle zone and the tunnel exit zone. (4) Longitudinal grade dominated horizontal curve direction in explaining safety performance variation. The effect associated with uphill versus downhill direction was substantially larger than that associated with left versus right turning direction, indicating that grade-driven speed regulation and longitudinal load were primary contributors to elevated risk in spiral tunnel groups. These findings pinpoint high-risk directions and critical sections and support data-driven improvements in geometric design and safety management for freeway spiral tunnel groups.