KSCE Journal of Civil Engineering

, Volume 23, Issue 2, pp 800–809 | Cite as

Modeling of Unsymmetrical Single-lane Roundabouts based on Stopping Sight Distance

  • Said M. Easa
Transportation Engineering


Evaluation of the adequacy of sight distance at roundabout is usually performed graphically as no quantitative guidelines are available for the lateral clearance needs. This paper presents analytical lateral clearance models to satisfy Stopping Sight Distance (SSD) at unsymmetrical single-lane roundabouts (with different entry and exit radii). The models are developed for three critical locations: approach sight distance, sight distance to exist crosswalk, and circulatory roadway sight distance. Two cases of approach sight distance are presented: sight distance to the crosswalk and sight distance to the yield line. The required lateral clearance at any specified point along the curve can also be determined. Design aids for the required maximum lateral clearance are established for the three locations of sight distance based on accurate SSD formula that accounts for effect of the radial deceleration component of the vehicle. For the circulatory roadway sight distance, infeasible combinations of the circulatory roadway radius and speed are identified based on acceptable driver’s field of peripheral vision. The models and results of this paper, which are applicable to both symmetrical and unsymmetrical roundabouts, should be of interest to highway engineers and designers.


unsymmetrical roundabouts stopping sight distance lateral clearance pedestrians crosswalk 


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Copyright information

© Korean Society of Civil Engineers and Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Dept. of Civil EngineeringRyerson UniversityTorontoCanada

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