Roundabout operations : a summary of FHWA's - 'Roundabouts: An Informational Guide'

Abstract

Paper presented at the 20th Annual South African Transport Conference 16 - 20 July 2001 "Meeting the transport challenges in Southern Africa", CSIR International Convention Centre, Pretoria, South Africa. ABSTRACT: As in many other countries, roundabouts are receiving renewed interest in South Africa.The Federal Highway Administration (FHWA)publication, 'Roundabouts: An Informational Guide', presents a comprehensive discussion of roundabout planning, performance analysis, and design. No specific local guidelines for roundabouts have been developed and local engineers rely mostly on English and Australian methods. This guide provides another useful tool for the traffic engineering toolbox. One of its features is the definition of six categories of roundabouts, ranging from small, urban mini-roundabouts to relatively large, rural double-lane roundabouts. Distinctions have been made between the various categories in terms of characteristics, planning procedures, operational analysis, and design treatments. This paper presents the various elements of operational analysis included in the guide, including measures of capacity and performance analysis,and provides a discussion of many of the decisions made during the development of the guide. The paper presents capacity curves for urban compact roundabouts, single-lane roundabouts, and double-lane roundabouts, as well as adjustment factors to account for the effect of short lanes (flaring). The paper also presents the methodologies and recommended practices for determining degree of saturation, delay, and queuing.