Sight distance and horizontal curve aspects in the design of road tunnels vs. highways: Part II (trucks)

Abstract

The design of road tunnels is an essential component in highway geometric design. The study implements reasonable criteria for obtaining the sight distance and horizontal curve requirements of road tunnels vs. open roadways while considering a significant number of trucks in the traffic stream. This document continues a previous study assuming that the design vehicle is a passenger car. The engineering principles for considering trucks in the traffic stream are similar i.e. the use of perception-reaction time and longitudinal friction characteristics for obtaining the sight distance (and developing horizontal curve radii values for highway design) is applicable for trucks as well. However, truck performance characteristics affect the longitudinal friction parameters, side friction parameters, maximum superelevation, and the horizontal sightline offset (HSO) e.g. tunnel pavement status is irrelevant for deriving trucks' sight distance. 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 the equilibrium requirement generated lower horizontal curve radii for the whole range of design speeds. 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 trucks in road tunnels are considerably lower than the open roadways' radii for certain lower range of design speeds (50–80km/h). However, the reduction percentage from open roads can be considered less significant in the higher range of design speeds (90–120km/h). The results are useful to improve traffic safety if the design vehicle is a truck.