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Subjects › Transportation › Roundabouts
Updated 07/2012

Roundabouts

Contents

Introduction

Modern roundabouts are alternatives to signalized intersections that provide increased safety and operational efficiencies as well as lower O & M costs. Many of the publications referenced below are careful to distinguish between modern roundabouts and traffic circles that have some of their characteristics. In June, 2000, the Federal Highway Administration (FHWA) distilled several years of research and empirical design into Roundabouts: an Informational Guide  which contains the following definition:

Roundabout - a circular intersection with yield control of all entering traffic, channelized approaches, counter-clockwise circulation, and appropriate geometric curvature to ensure that travel speeds on the circulatory roadway are typically less than 50 km/h (30 mph).

Roundabouts as an intersection control are in common use throughout the world, but only in the last dozen years have they gained acceptance and begun to be used without reservation in the United States. This has been mostly due to a lack of objective nationwide guidelines on planning, performance, and design of roundabouts prior to the development of this FWHA guide, as is the case with signalized intersections. That roundabouts are still somewhat of a novelty in the United States is evidenced by the (perceived) need for 'How to' guides on some websites of agencies that have installed roundabouts.

Design

[Excerpted from Chapter 1 (Adobe Acrobat Document1 MB) of Roundabouts: An Informational Guide, FHWA]

A roundabout is a type of circular intersection, but not all circular intersections can be classified as roundabouts. In fact, there are at least three distinct types of circular intersections:

  • Rotaries are old-style circular intersections common to the United States prior to the 1960's. Rotaries are characterized by a large diameter, often in excess of 100 m (300 ft). This large diameter typically results in travel speeds within the circulatory roadway that exceed 50 km/h (30 mph). They typically provide little or no horizontal deflection of the paths of through traffic and may even operate according to the traditional  yield-to-the-right  rule, i.e., circulating traffic yields to entering traffic.
  • Neighborhood traffic circles are typically built at the intersections of local streets for reasons of traffic calming and/or aesthetics. The intersection approaches may be uncontrolled or stop-controlled. They do not typically include raised channelization to guide the approaching driver onto the circulatory roadway. At some traffic circles, left-turning movements are allowed to occur to the left of (clockwise around) the central island, potentially conflicting with other circulating traffic.
  • Roundabouts are circular intersections with specific design and traffic control features. These features include yield control of all entering traffic, channelized approaches, and appropriate geometric curvature to ensure that travel speeds on the circulatory roadway are typically less than 50 km/h (30 mph).

For the purposes of the Guide, roundabouts are categorized according to size and environment to facilitate discussion of specific performance or design issues. There are six basic categories based on environment, number of lanes, and size:

  • Mini-roundabouts
  • Urban compact roundabouts
  • Urban single-lane roundabouts
  • Urban double-lane roundabouts
  • Rural single-lane roundabouts
  • Rural double-lane roundabouts

Multi-lane roundabouts with more than two approach lanes are possible, but they are not covered explicitly in the Guide.

Safety

[Excerpted and adapted from Chapter 5 (Adobe Acrobat Document) of Roundabouts: An Informational Guide, FHWA] Roundabouts may improve the safety of intersections

  • by eliminating or altering conflict types
  • by reducing speed differentials at intersections
  • by forcing drivers to decrease speeds as they proceed into and through the intersection.

Reasons for the increased safety level at roundabouts are:

  • Roundabouts have fewer conflict points in comparison to conventional intersections. The potential for hazardous conflicts, such as right angle and left turn head-on crashes is eliminated with roundabout use.
  • Single-lane approach roundabouts produce greater safety benefits than multi-lane approaches because of fewer potential conflicts between road users, and because pedestrian crossing distances are short.
  • Low absolute speeds associated with roundabouts allow drivers more time to react to potential conflicts, also helping to improve the safety performance of roundabouts.
  • Since most road users travel at similar speeds through roundabouts, i.e., have low relative speeds, crash severity can be reduced compared to some traditionally controlled intersections.
  • Pedestrians need only cross one direction of traffic at a time at each approach as they traverse roundabouts, as compared with unsignalized intersections. The conflict locations between vehicles and pedestrians are generally not affected by the presence of a roundabout, although conflicting vehicles come from a more defined path at roundabouts (and thus pedestrians have fewer places to check for conflicting vehicles). In addition, the speeds of motorists entering and exiting a roundabout are reduced with good design.
  • As with other crossings requiring acceptance of gaps, roundabouts still present visually impaired pedestrians.

Links

Design and General Information

Safety

Roundabouts in Washington State

The Kittelson and Assoc. Modern Roundabouts Website maintains a nationwide database of roundabouts. WSDOT has a State-wide map showing roundabout locations. Some of the roundabout projects in Washington are listed below as well as public information about roundabouts:

Projects

Instructional Information

Articles from ITE Journal

Available Online for ITE members or via MRSC Library Loan

  • Articles from ITE Journal February 2009
    • Establishing Right-of-Way Standards for Roundabouts in the City of Calgary, Canada , by Stephen Sargeant, P.Eng., PTOE and Ryan Vanderputten, P.Eng.
    • Mini-Roundabouts for the United States, by Clive Sawers, MA, MICE, C.Eng
    • Roundabouts and Signals: Harmony Even with Increasing Traffic Volumes, by Hillary N. Isebrands, P.E.
    • Trucks in Roundabouts: Pitfalls in Design and Operations, by Edmund Waddell, Michael A. Gingrich Sr. and Mark Lenters, P.E.
  • Article from ITE Journal April 2009
    • A Simple Exercise in Ingenuity: The San Antonio, TX, USA, Peanut-About, by Edward N. Mery, P.E.

  • A Comparative Evaluation of the Safety Performance of Roundabouts and Traditional Intersection Controls, by 
    Nambisan, Shashi, S.; Parimi, Venu, ITE Journal, March 2007.

Need more information?

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