Effects of Grade Designs and Zone Characteristics on Driving Speed in Long Mountainous Freeway Tunnels with Crest Vertical Alignment

Safety issues in freeway tunnels are prominent, and how drivers adjust their speed in response to various grade designs in long mountainous tunnels with crest vertical curves has not been adequately explored. To bridge this gap, this study established a freeway model incorporating 10 tunnels with different vertical alignments to conduct driving simulation experiments, aiming to explore the impact of crest vertical curve design on drivers’ driving speed. Based on the variations in grade, each tunnel was divided into three zones. Speeds within zones and variations between zones were obtained and statistically analyzed using repeated-measures one-way analysis of variance ANOVA and mixed linear regression. The results show that under various vertical alignment conditions, drivers’ speeds generally exhibit an increasing trend in long tunnels with crest vertical curves. Significant differences in driving speed were observed across different tunnel zones and grade design conditions. The entrance grade, exit grade, grade change, zone characteristics, as well as the interaction effects between these variables all have significant influences on the speeds within zones and speed variations between zones. Based on the results, this study proposes design parameters and management strategy recommendations for design and management authorities. The output of this study can help understand the traffic safety issues in long mountainous freeway tunnels from an ergonomic perspective and provide insights for freeway tunnel designers and administrations to enhance freeway tunnel traffic safety.