Sight distance and horizontal curve aspects in the design of road tunnels vs. highways

摘要

The design of road tunnels is an essential component in highway geometric design. The study implements reasonable criteria for obtaining the sight distance requirements of road tunnels vs. open roadways based on research overview of driving behavior, longitudinal friction characteristics, and highway safety considerations in road tunnels. These criteria assist to generate design values of horizontal curve radii for a certain range of design speeds. The assessment of stopping sight distance for road tunnel is performed by assuming: (1) a lower perception reaction time of the driver due to drivers' vigilance along the bounded cross section, and (2) higher values of friction coefficient depending on the tunnel surface conditions: moist or dry tunnel. The stopping sight distance (SSD) design values were also generated for the "end of tunnel" (EOT) zone assuming wet pavement for desirable design. The implementation of unique criteria for stopping sight distance in road tunnels reduced significantly its design values. The analysis of horizontal curve radii in road tunnels is based on two concepts: (1) equilibrium i.e. balancing the centrifugal force by the superelevation and the radial friction factor adjusted to road tunnels; (2) conforming stopping sight distance requirement by analyzing three alternatives of the driver position along the inside lane (centerline of inside line, left hand curve, and right hand curve). It is concluded that the critical concept for safe horizontal curve radii in road tunnels (as in open roadways) is the stopping sight distance. The analysis has shown that only for the lowest design speeds (i.e. 50 and 60 km/h), the equilibrium requirement generated higher horizontal curve radii. The driver position (left hand or right hand curve) has a considerable impact on the design values of horizontal curve radii. The horizontal curve radii analyzed for road tunnels are considerably lower than the open roadways' radii. The difference increases as the design speed increases. The tunnel pavement status (especially dry tunnels) and driver behavior characteristics have therefore a substantial impact on the stopping sight distance and on the horizontal curve radii. The results are useful to provide traffic safety if the design vehicle (passenger car) speed is maintained by appropriate traffic management and enforcement.