Texas A&M Transportation Institute

Transportation Policy Research

Roundabouts

System Modification

Introduction

Intersections are common locations of frequent congestion and crashes. Intersections in the United States are traditionally controlled by traffic signals or stop signs. These traffic control devices are natural sources of delay because they stop one direction of traffic to allow another to proceed.

The modern roundabout design lessens these sources of delay. It is an intersection design that has been long used in other countries and is being used more frequently in the United States. A roundabout is a type of circular intersection that uses:

  • Yield signs to control entering traffic.
  • Splitter islands on the approaches.
  • Appropriate roadway curvature to reduce vehicle speeds on each approach.1

All vehicles enter at lower speeds and move through the roundabout. It is not necessary for any vehicle to stop unless waiting on a gap in traffic to enter the roadway.

Executive Summary

Target Market

Modern roundabouts are often used on residential area minor and major streets. They can also be used in rural and urban areas where right of way is available and the design is appropriate for the approach speeds.

Roundabout designs are typically considered for intersections that have been identified as needing major safety or operational improvements. This includes intersections with freeway interchange ramp terminals where signal coordination and regular traffic lines can be problematic.2

How Will This Help?

  • Reduces delays, shortens travel times, and lowers speeds compared to traditional signalized intersections. The design helps to reduce the amount of time vehicles are stopped at the intersection. It decreases delay and travel time since vehicles only have to yield as they approach. The roundabout design also encourages drivers to slow down as they go through the intersection.
  • Improves safety by preventing crashes and reducing crash severity at the intersection. A roundabout eliminates some of the conflicting traffic—such as left turns—that often causes crashes at traditional intersections. Because roundabout traffic enters or exits through right turns only, severe crashes are greatly reduced.3

Implementation Examples

Modern roundabouts have been added in many locations across the country, particularly in Colorado, Florida, Kansas, Maryland, New York, and Washington. There are also successful installations in Texas.

Golden, Colorado
A roundabout corridor in Golden, Colorado, replaced a series of signalized intersections with four roundabouts on a 0.5-mile roadway segment.4 A comparison of before-and-after conditions indicated that speeds were lowered between major intersections in the corridor. Travel times were also lowered (reduced from 78 to 68 seconds). In addition, the average measured delay before the roundabouts was 28 seconds, with a maximum of 118 seconds. After installation, average delay was reduced to 13 seconds, with a maximum of 40 seconds.

Traffic volumes increased from 11,500 to 15,500 vehicles per day, but the number of annual crashes dropped from 123 to 19. Calculated crash rates declined by 88 percent, from 5.9 to 0.4 crashes per million vehicle miles. Injury crashes were reduced from 31 in the three years prior to installation to only one in the 4.5 years after—a 93 percent decline in the injury crash rate.

The study also reported sales tax revenues along the corridor increased 60 percent, and 75,000 square feet of retail/office space was built after installation.

Application Principles and Techniques

The purpose of the roundabout design is to create an intersection that allows vehicles to move through the intersection with minimal stopping. Instead, vehicles simply yield to oncoming cars currently in the roundabout. This design improves the traffic flow through the intersection and decreases congestion and delay.

Determine if a roundabout is appropriate for the location based on traffic volumes and available right of way.Planning is key when deciding whether to add a roundabout. Practitioners should:

  • Identify the adequate size.
  • Evaluate the possible impacts of the design.
  • Provide education and outreach to help transition drivers into using the new design.5

Practitioners should also consider using different designs for roundabouts. Examples include:

  • Considering the angle of approach so that it provides slow entry speeds and more consistent speeds throughout the roundabout.
  • Determining the appropriate number of lanes for adequate capacity.
  • Evaluating lane volume balance.
  • Assessing lane continuity.
  • Providing smooth flow that is intuitive to drivers.
  • Designing for proper vehicles.
  • Designing for pedestrians.
  • Ensuring adequate sight distance and visibility.5

Issues

Three primary issues affect the successful implementation of modern roundabouts:

  • Sufficient right of way to install the circulatory roadway and approaches.
  • Design that ensures reduced speed for entering and circulating vehicles and that provides access for all road users.
  • Education of drivers and other road users that may not be familiar with modern roundabouts and how to navigate them.

Right of Way
In some developed areas, right-of-way purchase may be a major portion of the project’s cost and timeline. However, in undeveloped areas and at locations where large intersections are being converted, there may already be sufficient right of way to add all of the necessary elements for proper operation.

Design
Proper site selection and lane flow for motorists, bicyclists, and pedestrians are essential. In particular, it is important to ensure safe space for bicyclists at higher-speed roundabouts and for pedestrians with visual or mental impairments. A peer review of the design, such as that provided by the Federal Highway Administration (FHWA), 6 is also a recommended step in the design process.

Education
In addition to right-of-way concerns, education of drivers and pedestrians is also critical because they may not be familiar with the new design. Public announcements, brochures and flyers, public meetings, and videos with images of anticipated operations (or of existing intersections at other locations) are useful for educating the public on how to navigate the roundabout. FHWA7 provides examples of educational programs and materials.

Who Is Responsible?

In many cases, cities are the responsible agencies since roundabouts are often installed on major and minor roadways within city limits. The state department of transportation is involved in installations on city streets that are also part of the state highway system and at rural intersections.

Project Time Frame

The timeline for a modern roundabout project is similar to the time needed to construct a traditional intersection (e.g., 3 to 12 months). Depending on existing conditions, additional time may be required for right-of-way acquisition, utility relocation, or extra construction phases to carry existing traffic through the work area.

Cost

If sufficient right of way is available, construction costs may be similar to building a traditional intersection. Additional costs may be required if the amount of paved surface is greater than previous conditions. This amount includes not just the path for vehicles but also pedestrian paths that may be added. However, there are no costs related to traffic signal installation and maintenance, so the cost of a roundabout may be equal to or less than a signalized intersection over its expected life cycle.

Data Needs

When considering the installation of a modern roundabout, practitioners should examine the:

  • Nearby development and access needs.
  • Available right of way.
  • Approach volumes.
  • Turn volumes.
  • Pedestrian and bicycle counts (including crossing counts per hour).
  • Average speed on each intersection approach.
  • Intersection crash history (type and frequency).
  • Intersection layout.
  • Typical delay.
  • Traffic signal data.

Roundabouts Best Practices

  • Type of location: A wide variety of intersections (arterials, collectors, and local streets); frequently used at low- to moderate-speed and/or volume locations, though higher speeds and volumes are also possible.
  • Agency practices: Coordination between planning, design, safety, and operations.
  • Frequency of reanalysis: After substantial land use changes or development, as travel increases or trips change in the area, and at the time of roadway widening or reconstruction.
  • Supporting policies or actions needed: Capability to fund improvements, multi-agency agreements, policies where roadways cross jurisdictional boundaries, and driver education campaigns.
  • Complementary strategies: Intersection improvements, such as pedestrian treatments, and access management.

 For More Information

American Association of State Highway and Transportation Officials. A Policy on Geometric Design of Highways and Streets. Washington, D.C., 2004.

Crawford, J. A., T. B. Carlson, W. L. Eisele, and B. T. Kuhn. A Michigan Toolbox for Mitigating Traffic Congestion. Texas A&M Transportation Institute, College Station, Texas, September 2011.

Texas A&M Transportation Institute. Mobility Improvement Checklist: Adding Capacity, Vol. 3. College Station, Texas, September 2004.

References

  1. Federal Highway Administration. Roundabouts: A Safer Choice. Report FHWA-SA-08-006, U.S. Department of Transportation, Washington, D.C., 2008.
  2. J. Lindley. Guidance Memorandum on Consideration and Implementation of Proven Safety Countermeasures. Federal Highway Administration, July 2008. . Accessed October 28, 2011.
  3. L. Rodegerdts, M. Blogg, E. Wemple, E. Myers, M. Kyte, M. Dixon, G. List, A. Flannery, R. Troutbeck, W. Brilon, N. Wu, B. Persaud, C. Lyon, D. Harkey, and D. Carter. NCHRP Report 572: Roundabouts in the United States. National Cooperative Highway Research Program, Washington, D.C., 2007.
  4. A. J. Ariniello. Are Roundabouts Good for Business? December 1, 2004. http://www.chico.ca.us
    /capital_project_services/documents/roundaboutpaper.pdf    . Accessed June 2011.
  5. L. Rodegerdts, J. Bansen, C. Tiesler, J. Knudsen, E. Myers, M. Johnson, M. Moule, B. Persaud, C. Lyon, S. Hallmark, H. Isebrands, R. B. Crown, B. Guichet, and A. O’Brien. NCHRP Report 672: Roundabouts: An Informational Guide, 2nd ed. National Cooperative Highway Research Program, Washington, D.C., 2010.
  6. Federal Highway Administration. Roundabouts Peer-to-Peer Program. September 2014. http://safety.fhwa.dot.gov/intersection/roundabouts/p2p/. Accessed October 28, 2011.
  7. Federal Highway Administration. Roundabouts. November 2014. http://safety.fhwa.dot.gov/intersection/roundabouts/. Accessed October 28, 2011.