How to Design a Roundabout for Improved Traffic Flow and Safety

With the right design, roundabouts can significantly improve the traffic flow and safety of an intersection. Here are some key design principles that help guarantee success.

A single-lane roundabout in Dublin, Ireland. Photo by Lucas Miguel on Unsplash.

A single-lane roundabout in Dublin, Ireland. (Photo by Lucas Miguel on Unsplash.)

Roundabouts are becoming increasingly popular across the country—and for good reason. Their advantages are well-documented. Communities concerned about traffic safety or congestion can often make roads safer and quicker to navigate by implementing a roundabout.

Poorly designed roundabouts, however, can create just as many problems as they solve.

Here’s how civil engineers typically approach roundabout design and cover the best practices to ensure that they provide a safe and efficient upgrade to an area’s roads.

The Basic Principles of Roundabout Design

Engineers must take a variety of local factors into account when designing a roundabout. Major considerations are the area’s speed limit, average traffic flow, desired speed limit and the number of streets that will connect to it.

Designers may also need to consider the needs or wants of various stakeholders. For example, safety may be the top priority for one developer or community, while improving traffic flow may be essential for another.

A few basic design principles are important for all roundabouts, regardless of their location or predicted traffic flow. These can help guide designers during the planning process.

1. Roundabout Curvature

The geometric curvature of a roundabout is likely one of its most important characteristics. It must provide a good angle of entry for drivers, smoothly channel cars in and out of the roundabout, and offer a good line of sight.

2. Channelization, Delineators and Signage

The roundabout design must clarify where drivers, cyclists and pedestrians should be as they approach the junction and travel through it. Effective road markings, signage and delineators—like reflectors—can accomplish this.

Yield signs tell drivers that approaching traffic must yield before entering the roundabout. These signs are how most designers accomplish entry control.

Various channelization strategies exist, but they all ensure the smooth flow of traffic into and out of the roundabout.

3. Deflection

Vehicles entering a roundabout are directed to the right at an entry deflection angle that is determined by the design and size of the roundabout.

This entry deflection angle can have a major impact on driver behavior. Typically, entry speed is determined less by signs or markings and more by the angle those entering the roundabout are forced to take.

Certain deflection design strategies, like S-curves, can reduce a driver’s or cyclist’s speed as they enter the roundabout.

Roundabout Design Process

In most cases, roundabouts are designed and constructed in a multiphase process that will roughly follow these steps, beginning with an analysis of the intersection the roundabout will replace.

Land surveying can be a critical step in roundabout design. Photo by Scott Blake on Unsplash.

Land surveying can be a critical step in roundabout design. (Photo by Scott Blake on Unsplash.)

1.    Site Analysis, Safety Assessment and Operational Analysis

The roundabout designer will first gather information and conduct background research on the existing intersection. They’ll conduct an assessment that determines the safety level of the current intersection using information like the historical frequency of accidents, projected rate of future crashes and collision cost analysis.

During this process, designers may need to gather or update information about the land where the intersection is located. Typically, surveyors will use total stations to collect site surveying data. Other tools like 3D scanners, GPS, tape measures and theodolites may also be necessary.

Survey data will help the design team by providing valuable information that digital building solutions contractors may use when constructing the roundabout.

Next, designers will measure the current effectiveness of the intersection.

The operational analysis for an intersection should produce two different estimates—the capacity of the junction and its level of performance, which are typically measured in terms of delay or vehicle queuing.

Comparing this analysis with the estimated operational effectiveness of the roundabout will help planners determine if one is necessary or desirable in the study area.

These volumes will also help designers ensure that the new roundabout will increase area traffic flow.

2. Warrant Analysis

This analysis examines the proposed site where the roundabout will be built and determines if the new junction is warranted.

At this stage, planners may also need to consider alternatives to the roundabout, like expanding the existing intersection and adding traffic signals or road markings. In some cases, other options, like a diverging diamond interchange, may also be important to consider.

The primary measures of effectiveness (MOEs) for roundabouts are cost, safety and capacity. Other MOEs are typically environmental and aim to evaluate the ecological benefits and impact of the roundabout.

In some cases, roundabouts may also improve walkability or area aesthetics—as in the case of the town square roundabout redesign in Newark, Ohio.

If a roundabout can cost-effectively improve one of the primary MOEs, it will typically be justified on its own merits. Additional measurable benefits may make a new one even more likely.

The U.S. Department of Transportation, in its guide on roundabout design and construction, offers some recommendations for what information should be included in a roundabout study report:

  • Identification of the selection category that demonstrates why the roundabout is a logical choice to replace the existing intersection
  • Identification of current or potential traffic safety issues at the intersection
  • Configuration proposal for the new roundabout
  • Demonstration of the feasibility of the proposed roundabout configuration
  • Potential complicating factors and mitigating efforts that may be needed

3. Conceptual Design

Once the project is determined to be feasible and warranted, planners will typically begin determining the roundabout’s geometry. This is an iterative process that typically involves multiple design passes.

Designers will model the structure of the roundabout using data captured during the site and operational analysis. They’ll then use this model to create a concept sketch of a possible design.

This sketch will be analyzed in terms of appropriate design vehicle, fastest path, overlap and potential project impact.

4. Detailed Design and Construction

After the designers have created a roundabout design that appears feasible, they can move on to more specific details. Typically, they’ll begin by considering factors like horizontal curve, roundabout alignments, and drainage. The designers will create cross-sections of the construction.

Next, they’ll begin to design road markings, signage, crosswalks and landscaping to keep the roundabout easy to navigate and aesthetically pleasing.

Informing a Community About a New Roundabout

Designers and planners may also want to consider informational campaigns that explain the roundabout’s design. While roundabouts are becoming increasingly common in the United States, they are still unfamiliar to some drivers.

It’s not unusual for local governments, municipalities or planners to produce brochures and informational documents that explain how the roundabout works, its expected benefits, and predictions about how the new junction will impact traffic or navigation.

Key Considerations for Engineers Designing Roundabouts

The safety, congestion and environmental benefits of roundabouts often make them a valuable alternative to more conventional intersections. However, those built without good design practices can be inefficient or even dangerous.

Civil engineers need to apply a few roundabout design principles to ensure that the concepts they develop will help make an intersection safer while improving traffic flow. The design of roundabouts will increase in importance as these road junctions become more common.