Objectives: Operating speeds on roads are critical indicators for evaluating traffic safety. Currently available research on the operating speed's prediction focuses on open roads and highways. Insufficient attention has been paid, so far, to tunnels, which form bottlenecks on expressways. The present research aims to establish an operating speed prediction model for tunnels and analyze the influence of their geometric parameters on the operating speeds of vehicles.
Methods: We consider the speed of vehicles collected through field measurements in the portals and lay-bys of six superlong tunnels (length greater than 3000 ). Using linear regression, a prediction model for the speed in an expressway superlong tunnel is obtained considering tunnel's geometric parameters. The influence of various parameters on the operating speed are analyzed through comparisons with existing research findings.
Results: We establish the first operating speed prediction model for tunnels considering geometric parameters and find that the vehicle type is the most important parameter affecting the operating speed. Other important parameters include the preceding curve length up to speed observation point (), preceding tangent length () and preceding tangent length up to speed observation point ().
Conclusions: The influence of geometric parameters on vehicle operating speed in super long tunnels differs from that observed in non-tunnel roadways. The effects of the preceding or subsequent curve radius ( or ) of the tangent section, curvature (), and curve degree () are not important in this case. Furthermore, we find that the influence of the posted speed limit () is closely related to the driving scene and safety awareness of drivers. These findings can improve the design and joint evaluation of tunnel geometric parameters and traffic safety.
Keywords: Operating speed; driving scene; expressway superlong tunnel; geometric parameters; prediction model; traffic safety.