Revision of Calculation of Stopping Sight Distance

Stopping Sight Distance (SSD) is a critical parameter for roadway geometric design and safety evaluation. It experienced several revisions in AASHTO Green Books (A Policy on Geometric Design of Highways and Streets) since 1940. This paper firstly lists the revision history of SSD model in the last 70 years in the USA as example, and then points out that the more critical conditions for vehicles driving at curves with superelevation should be used for SSD calculation. Based on that, this paper conducts a revised SSD calculation model which is universal and more reasonable for SSD calculation. Finally, a case study was completed to show the difference between the existing and the revised calculations of SSD. The revised SSD model suggested basically increasing longer SSDs as speed varies from 30 km/h to 110 km/h, and a little decline at speed 120 km/h.