Vertical Clearance Assessment of Overpassing Structures on Sag Vertical Curves

Abstract

The paper investigates areas on sag vertical alignments where overpassing structures impose restrictions on truck drivers’ sight distance. The assessment is performed for the least desired case where the driver’s sightline is equivalent to the defined stopping sight distance (SSD). The overall objective is to define the boundaries where such overpassing structures limit the available sight distances for truck drivers below the required SSD and thus to provide valuable guidance to practitioners. Initially, the authors addressed the SSD calculation on variable grades during the braking process through an earlier approach based on the point mass model and the laws of mechanics. As a second step, truck deceleration rates were determined from the literature. The effect of various design speeds was examined, corresponding to specific ranges of grade values and sag vertical curvature rates. The methodology was based on calculating the difference between the truck driver’s line of sight and the road alignment at fixed locations and associating the defined value with the desired vertical clearance rate (VC). The analysis revealed that critical cases arise for design speed values between 90 km/h and 110 km/h. The examined cases for Vdesign = 100 km/h and Vdesign = 110 km/h revealed more critical concerns since the assessment showed that even a VC value of 4.90 m is not adequate to address SSD provision. Such cases require further attention, especially when the entering downgrade to the sag vertical curve is long and steep.