Citation

Lenters M. Transp. Res. Circular 2005; 2005(E-C083): 20p.

Copyright

(Copyright © 2005, U.S. National Academy of Sciences Transportation Research Board)

DOI

unavailable

PMID

unavailable

Abstract

Over the last ten years, roundabouts have spread to North America with good results reported. Except for grade separated interchanges no other type of traffic control exhibits such good safety performance, particularly for high traffic flows at high-speed intersections. Therefore, roundabouts as an engineering initiative can make a significant contribution to a road authority’s safety vision. Intersection safety reviews and safety audits, aimed at improving the physical and operational characteristics of an intersection, are a proven means of facilitating a reduction in crash frequency and crash severity. The process of an in-service safety review requires the assessor to determine what an unfamiliar user would experience as they drive, ride or walk into and out of a roundabout intersection. Although lists of issues affecting safety are identified herein, this document is not a prescription or checklist, but instead points to design principles as the fundamental basis for roundabout safety. Adherence to the principles outlined herein still does not ensure good design; this remains the responsibility of the designer. Although this piece focuses only on safety considerations of roundabout design, the principle that roundabout geometry, signs and markings act holistically must be kept in perspective if roundabouts are to solve both crash prone and highly congested intersections. If not well understood or properly managed, the cumulative effects of poorly managed geometry-safety interactions linked with roundabouts can amount to a significant congestion and/or collision risk. As discussed in this report, the British empirical research into these relationships demonstrates that the separate elements are dependent on the others for their effectiveness. The role of quantifying the potential crash reduction through an examination of the relationship between crashes, traffic flows and geometry represents a significant improvement opportunity in the science of road safety audits of roundabouts not articulated in previous studies. The in-service safety auditor must be aware that even a poorly designed roundabout can operate reasonably well at low traffic flows. A good roundabout design is not proven until the design year flows are experienced because near capacity operations bring out all the design flaws. One hypothesis of this article is that equal consideration should be given to both visual and crash model assessment methods in roundabout safety reviews. In particular, there is considerable potential for optimizing safety at the design stage through an understanding of the interactions of geometry and the ability to manipulate these relationships. Through recent study presented herein, knowledge of safety-geometry interactions can be used just as effectively in assessments of in-service roundabouts as in pre-construction design audits. Experience with other types of intersections indicates that there are four basic demands of a safe intersection design. In this report, the design checks required to attain predicted safety performance and safety enhancements for in-service roundabouts that incorporate established guidelines are presented with examples of design pitfalls and a list of checks to be carried out. This paper provides safety assessment guidelines for all sizes of roundabouts, except mini-roundabouts with fully traversable central islands.