Continuous three-dimensional Stopping Sight Distance control on crest vertical curves

In current road design practice still a 2D approach is in use both with respect to the road configuration and the vehicle’s motion, despite the fact that a 3D approach regarding these aspects is the only accurate and realistic method to control the provision of critical design parameters such as the adequacy of Stopping Sight Distances (SSD). This paper describes an accurate SSD control method that relates the 3D configuration of a roadway to the dynamics of a vehicle moving along the actual roadway path, based on the difference between the provided and the demanded SSD. The SSDDEMAND is defined from the mass point model adopted by AASHTO enriched by the actual values of grade and friction variation. On the other hand the SSDPROVIDED is described as the 3D driver’s line of sight towards the object height. From this method the conditions of SSD provision could be derived for a variety of 3D alignment combinations and a wide range of design speeds and compared with the design elements’ limiting values as suggested by the AASHTO 2004 design guidelines. The relevant investigation revealed a satisfactory SSD reserve only in cases of horizontal curved alignments overlapped with vertical curves. The analysis showed that, in order to ensure SSD adequacy on road alignments designed according to the AASHTO design guidelines, a slight increase of the AASHTO minimum values for Crest Vertical Curvature Rates is necessary. The SSD control methodology presented in the paper is suitable for SSD evaluation regarding both existing and new road sections.