Stopping Sight Distance Requirements for Basic Turbo Roundabouts

Abstract

Turbo roundabouts, originating from the Netherlands, have been proven to have safety and operations benefits compared to traditional multilane roundabouts. Turbo roundabouts are gradually being adopted by North American transportation and road authorities. However, no design guidelines have been developed yet per North American practices. Sight distance requirements were not even specifically addressed in the Dutch mature design guidelines for turbo roundabouts. This paper focuses on basic turbo roundabouts, examines stopping sight distance (SSD) characteristics, recommends SSD requirements for three critical locations—circulatory roadway, entry approach, and crosswalk on the exit approach—based on AASHTO and Transportation Association of Canada’s (TAC’s) SSD guidelines, presents analytical models, and develops design guidelines for the required lateral clearance and intersection clear sight triangles to meet the required SSD. The values of the required SSD at the circulatory roadway and to the crosswalk on the exit approach equal the values for the speeds. The required approach SSD is the same as for any roadway section with the same design speed. The minimum crosswalk setback of a one-vehicle gap of 6.0 m between the queued vehicle and the crosswalk is sufficient to meet the required SSD. The required lateral clearance and intersection clear sight triangles are recommended. The recommended guidelines would help develop future design guides per North American practices and provide a guide to property needs for planning basic turbo roundabouts. Turbo roundabouts initially built in the Netherlands have gradually attracted North American transportation and road authorities’ attention, given their safety and operations benefits. After more than a decade’s effort in advancing and promoting turbo roundabouts, the first turbo roundabout was built in the US in 2022. However, design guidelines per North American practices are lacking, such as the required stopping sight distance (SSD). The required SSD at the circulatory roadway and entry approach and SSD to the crosswalk on the exit approach are three critical locations. The values of the required SSD at the circulatory roadway and to the crosswalk on the exit approach equal the values of the travel speeds. The required approach SSD is no different from any roadway section with the same design speed. The minimum crosswalk setback of a one-vehicle gap of 6.0 m between the queued vehicle and the crosswalk is sufficient to meet the required SSD. Sight obstructions should meet the required lateral clearance at the roundabout approach and circulatory roadway due to the curve alignment. Clear sight triangles at the roundabout quadrants should be provided to meet the required SSD to the crosswalk on the exit approach.