Stopping Sight Distance (SSD) is a key control element which directly affects the 3 suggested values of crucial road design parameters. Although there is a significant 4 difference in SSD values between upgrades and downgrades, many Design Policies ignore 5 the grade effect during vehicle braking on variable grades. Such a case is experienced 6 during the determination of crest vertical curvature rates where the relevant SSD values are 7 extracted assuming leveled road geometry. The paper investigates possible deficiency of 8 this approach, regarding cases where the length of the vertical curve exceeds the control 9 SSD values. 10 The authors addressed the SSD calculation on variable grades during the braking 11 process through a recently developed process that relates the point mass model and the 12 laws of mechanics. 13 For a wide range of design speed values, charts illustrating the required SSDs were 14 drawn as a function of negative ending grade values related to control crest vertical curve 15 rates, as adopted by AASHTO. The process revealed numerous SSD shortage areas, where 16 the authors provided revised crest vertical curvature rates, in order to grant SSD adequacy 17 throughout the vehicles’ breaking process. 18 Furthermore, the authors aiming to provide the designers with ready-to-use vertical 19 design tool associated the amended vertical curvature rates to AASHTO’s road functional 20 classification, as a function of the crest vertical curve’s exit grade value.
|Tags||driver behaviour, road design, road infrastructure, statistical modelling, telematics|