"A well-designed roundabout has the potential to improve the conditions of an existing intersection or can be a highly effective component of a new transportation system. A modern roundabout offers benefits such as increased safety, increased capacity, reduced delay, and calmer traffic. In addition, air and noise pollution can be reduced and the aesthetics of the area enhanced.”

"Generally both capacity and delay improve with the use of a roundabout when compared to alternate intersection control methods. The capacity at a roundabout will likely be higher than a signalized intersection because there are no losses of time due to yellow and red signal intervals. In addition, vehicles are able to enter the roundabout from each leg simultaneously, which is not possible with other forms of intersection control. Thus, the higher capacity of roundabouts results in shorter delays.”

If you think that quotes above come from a radical roundabout advocate you are mistaken. They are direct quotes from PennDOT’s Guide to Roundabouts (Publication Number 414, May 2001). All too often we find that people with the least knowledge are the ones with the strongest opinions. This appears to be the case with roundabouts as many people who have criticized their potential usage have failed to grasp the very fundamental concept that roundabouts are NOT traffic circles any more than a house cat is a tiger just because they are both felines.

When asked why Maryland had decided to consider utilizing roundabouts, Maryland Department of Transportation engineer, Michael Niederhauser responded without hesitation, “because we owe it to our residents.” We believe that all public officials are so obligated. We are not suggesting that roundabouts are the solution to every traffic problem, but we would suggest that for public officials and the media to dismiss roundabouts out-of-hand, without bothering to learn how and when they can work to the public benefit is an absolute abdication of responsibility.

The information on roundabouts provided below is intended to help clarify what a roundabout is, their potential advantages and when they may be appropriate for consideration. Included is an overview of the issue as well as information from a wide variety of unbiased sources. Additional information on Roundabouts can be found in the Transportation Issues Section of our Environmental Resource Center.

Before and after shots of Maryland’s largest roundabout. The old intersection was backed up for hours during peak periods and the city center was deteriorating because the traffic made the area impossible to get to.

Today, over 47,000 vehicles per day use the roundabout with over 3,900 vehicles per hour using it in the PM Peak Period. Delays have been reduced by 70% and the roundabout is credited with spurring a $35 million, privately funded revitalization effort.

*Photos and statistics courtesy of the Maryland Department of Transportation.

For more information please see the Roundabout Overview

Links

One of the biggest problem with roundabouts is the negative, initial public perception. However, that often changes after the roundabout is installed, a fact that is referenced in many of the documents below and reinforced in this editorial from a Maryland newspaper:

Roundabout Reversal

This 82 page report from the Transportation Research Board - National Research Council is must reading for anyone seriously studying roundabouts. It provides an in-depth look at roundabouts from a wide variety of perspectives and includes significant “before and after” data on such topics as safety, congestion relief, capacity, public acceptance and more:

National Cooperative Highway Research Program Roundabout Study (1.75 MB)

An in-depth look by Maryland’s Department of Transportation into the effectiveness of roundabouts in that state. Included is a variety of study data regarding safety, congestion relief and a very interesting cost/benefit analysis:

Maryland’s Roundabout Accident Experience and Economic Evaluation (1.3 MB)

This listing of Maryland’s roundabouts not only provides locations for those who wish to go see a roundabout in action, but also provides information on the size and the number of vehicles that use each roundabout. A glance at the chart will show that Maryland has used roundabouts on a wide range of roads from low volume rural intersections (to reduce right-angle accidents) and to settings where the traffic counts exceed those at the infamous Five Points. (202/309/463 intersection in Montgomery County, PA):

Maryland Roundabouts (.15 MB)

These studies and articles by Insurance Institute for Highway Safety (the same folks who have won our trust with their excellent offset car crash testing program) focus on the effectiveness of roundabouts in reducing crash related losses:

Crash Reduction Following the Installation of Roundabouts (.5 MB)

IIHS Roundabout Status Report-1 (.4 MB)

IIHS Roundabout Status Report-2 (.25 MB)

This 6 page guide is an overview to their extremely comprehensive and very technical Roundabouts: An Informational Guide. In this report, the FHWA not only condones but encourages the use of roundabouts in certain situations and provides engineering criteria for their construction. Because of the size of the file, we recommend that you access the full guide via the following link to the FHWA site - http://www.tfhrc.gov/safety/00068.htm . If the link disappears, let us know as we do have the full version saved on our server:

Federal Highway Administration Roundabout Informational Guide (Brief) (.35 MB)

The FHWA also has available a slightly dated overview of roundabouts that focuses primarily on European experience that makes interesting reading for those who are really interested in the subject:

FHWA Overview

Even though Pennsylvania does not have any roundabouts as of yet, and roundabouts have been thoroughly dismissed as an option by District Six (Southeastern PA.), the more intelligent minds that occupy PennDOT’s central administration have recognized the important role that roundabouts can play in Pennsylvania’s future. This guide provides both an overview as well as specific engineering information for their construction:

PennDOT Guide to Roundabouts (2.2 MB)

Although much of the focus in the discussion on roundabouts centers on reduced accident rates for motor vehicles, this 50 page study which was prepared for the Southeastern Transportation Center, looks at roundabouts from the perspective of pedestrian safety:

The Effects of Roundabouts on Pedestrian Safety (1.5 MB)

This is an excellent site that has a wealth of information and photographs of roundabouts. Make sure to click on “Articles” to see how other communities are approaching roundabouts:

Roundabouts USA

Another great roundabout site with history, photos and discussion of roundabout design:

Alaska Roundabouts

Diagrams and information on Clearwater Florida’s large roundabout. This roundabout clear illustrates the need for good design. When originally constructed, it had serious design flaws that caused it to function improperly and accidents went up dramatically. Corrections to the design were made and the accident rate plummeted to near zero:

Clearwater Florida's Roundabout

Roundabout Responses

Roundabout Photo Gallery
Under Construction