Tag Archives: roundabouts

Reducing Driver Errors at Two-Lane Roundabouts

Researchers studied driving behavior at four multilane roundabouts to better understand the relationship between traffic control designs and driver errors. Data collected showed that certain traffic control changes decreased turn violations but failed to eliminate yield violations. Researchers were unable to identify long-term solutions for improving roundabout design and signage, and recommended further research to improve the overall safety and mobility of multilane roundabouts.

“Even though the study did not provide a silver bullet on how to prevent crashes at multilane roundabouts, it did create an efficient tool to analyze and quantify driving behavior data,” said Joe Gustafson, Traffic Engineer, Washington County Public Works.

“This study has advanced our knowledge in multilane roundabout safety and is one step closer to providing much needed improvements to roundabout design guidance,” said John Hourdos, Director, Minnesota Traffic Observatory, University of Minnesota.

What Was the Need?

Roundabouts have been shown to improve overall in-tersection safety compared to traditional traffic signals. However, noninjury crashes are sometimes more frequent on multilane roundabouts than on single-lane roundabouts due in part to driver confusion. Common driver errors such as failing to yield and turning violations on multilane roundabouts have contributed to an increase in noninjury crashes.

Given the benefits of improved mobility, traffic throughput and injury reduction of multilane roundabouts, reducing the noninjury crash rate at multilane roundabouts is important to facilitating their use by Minnesota cities and counties. Identifying solutions to reduce common driving violations would be more sustainable than the current practice of converting multilane roundabouts back to single-lane roundabouts.

In a previous study on a two-lane roundabout in Richfield, Minnesota, researchers demonstrated that traffic control  changes could reduce some of these driver errors. However, more data was needed to support study results. Understanding driver behavior and improving traffic control devices are key factors in designing safer multilane roundabouts.

What Was Our Goal?

With limited research on modern multilane roundabouts, the Minnesota Traffic Observatory sought to collect more data to evaluate the correlation between traffic control design features and collisions. Instead of conducting manual observations, researchers used an innovative video analysis tool to collect and process recorded videos of driving behaviors at test sites. Based on the analysis, they attempted to identify driver behaviors and error rates to help reduce noninjury crashes at multilane roundabouts.

What Did We Do?

The research team selected four multilane roundabouts in Minnesota — two in Mankato, one in Lakeville and one in St. Cloud — to observe undesirable driving maneuvers. At each roundabout site, researchers mounted video cameras at key locations to record one to two weeks of driving behavior. Only one roundabout could be observed at a time because only one set of specialized video equipment was available.

The raw videos were processed to produce a data set for analysis. Researchers used TrafficIntelligence, an open-source computer vision program, to automate extraction of vehicle trajectories from the raw footages. They used the same software to correct any errors to improve data reliability. The resulting clean data from the recorded videos were supplemented with historical crash frequency data reports obtained from the Minnesota Department of Public Safety. Collectively, data from both sources allowed researchers to thoroughly investigate the frequency and crash types from the four roundabouts. A statistical analysis of the data revealed that turn violations and yield violations were among the most common driving errors.

Researchers also looked at how violation rates vary with the roundabout’s location and relevant design features. Based on these findings, researchers proposed signage and striping changes to reduce driver errors at the two Mankato test sites. After the changes were implemented, they collected additional video data.

What Did We Learn?

This study provided one of the most comprehensive analyses to date of driving behavior at multilane roundabouts. Researchers were successful in finding solutions for reducing turn violations, but they were unable to identify solutions for yield violations despite the vast amount of data.

Minor differences in the design at each roundabout presented specific challenges. The analysis focused on how each varying design feature impacted driving behavior. Proposed traffic control changes such as extending solid lines between entrance lanes, adjusting the position of yield signs and adding one-way signs successfully decreased turn violations. However, data from before and after traffic control changes showed an insignificant impact on decreasing yield violations. Importantly, the study produced a list of ineffective traffic control methods that can be eliminated from future studies, saving engineers time and money.

The TrafficIntelligence tool was crucial in efficiently processing and cleaning large amounts of raw video. With improvements made to the software program, the tool should be an asset to future research on roundabouts and to other studies requiring observations of driving behavior.

What’s Next?

The traffic control changes that were successful at reducing crashes at two-lane roundabouts should be implemented by traffic engineers. In particular, large overhead directional signs or extended solid lines between entrance lanes should be installed to help reduce turning violations. The analysis method used in this study could also be used for a robust before-and-after evaluation of future modifications to traffic control devices.

Additional research could further scrutinize the data already collected, and researchers recommend that more data be collected to examine additional traffic control methods and other intersection design elements to improve the overall safety and mobility of two-lane roundabouts. This research could also serve as an impetus for the study of numerous roundabouts in a pooled fund effort involving several states.


This post pertains to the LRRB-produced Report 2017-30, “Evaluation of Safety and Mobility of Two-Lane Roundabouts,” published July 2017. A webinar recording of the report is also available.

Choosing Effective Speed Reduction Strategies for Roundabouts

Using survey results and prior research, this project developed a new resource to enable Minnesota local road engineers to select appropriate speed reduction measures for roundabouts. Further research is needed to determine the relative effectiveness of different measures alone and in combination.

“Although roundabouts are becoming common, single-vehicle crashes from drowsy, inattentive or unfamiliar drivers are still a concern, particularly in rural areas,” said Joe Gustafson, Traffic Engineer for Washington County. “This project provides an overview of existing speed reduction treatments that have been used in both roundabout and nonroundabout contexts, and a framework to properly evaluate the effectiveness of new treatments.”

“Rather than try to identify the right combination of treatments, the research was designed to give engineers a variety of options to consider for a given location,” said Susan Chrysler, Senior Research Scientist, Texas A&M Transportation Institute.

Gustafson served as the technical liaison for the study, and Chrysler was the principal investigator.

What Was the Need?

2017-14-p1-image

Roundabouts can provide a safer alternative to traditional intersection control devices like traffic signals and stop signs. Roundabouts have been proven to reduce crash severity by requiring drivers to decrease speed during the approach to the intersection. But failure to slow down sufficiently could result in a crash.

Signs and markings are key treatments used to communicate to drivers that they must slow down as they approach the roundabout. When navigated appropriately, roundabouts can eliminate or reduce the severity of crashes, reduce delays and reduce fuel consumption.

What Was Our Goal?

This project had two goals: to analyze existing research and conduct a survey of roundabout design and installation practitioners to determine best practices; and to develop a resource that engineers can use to identify appropriate speed reduction treatments for high-speed approaches to roundabouts.

What Did We Do?

Investigators surveyed transportation engineers from Minnesota and other states, along with technical consultants, to learn their experiences managing roundabouts with high-speed approaches. The survey addressed geometric design parameters and traffic control methods, changes in maintenance practices, crash history and speed reduction measures that were considered or eventually enacted.

Previous research on the subject was studied, including the Federal Highway Administration report Roundabouts: An Informational Guide and National Cooperative Highway Research Program Report 672: Roundabouts: An Informational Guide, Second Edition. Design manuals from four states were reviewed to provide a sample of the material avail-able to practitioners seeking guidance on design of high-speed roundabout approaches.

Based on their research, investigators provided information on the effectiveness of various treatments and on their installation and maintenance costs. They also developed a methodology for conducting a speed study to assist engineers in determining the most effective treatment for a given intersection. Treatments for alerting drivers that a round-about is ahead include traditional signs, pavement markings, illumination and other indicators, plus advanced devices like speed-activated, LED-enhanced warning signs.

What Did We Learn?

Each roundabout presents unique challenges. Local road engineers need to evaluate the characteristics of the intersection being considered (such as geometric design and adjoining land use) and the costs of installation and maintenance before recommending a specific treatment or combination of treatments.

Other findings include the following:

  • Speed reduction techniques found effective for horizontal curves, urban-rural transition zones and isolated rural intersections should be effective for rural roundabouts with high-speed approaches.
  • In rural locations, speed reduction treatments that have been used at railroad crossings, T-intersections and work zones may also be applicable to roundabouts.
  • Some unique treatments used internationally hold promise, but further study is needed before these treatments can be recommended for use in the United States.

What’s Next?

This study was the first phase of research. The findings provide the methodology to select, install and evaluate treatments at different locations. Further research is needed to accomplish the following:

  • Analyze the effectiveness of speed reduction treatments at different locations
  • Determine the impact of different combinations of treatments
  • Establish the comparative benefits of two or more treatments that fall within the same general cost and maintenance grouping
  • Analyze the impact of roundabout infrastructure (such as gateway treatments and illumination), various pavement markings and the long-term effects of specific signing treatments.

This Technical Summary pertains to the LRRB-produced Report 2017-14, “Strategies for Effective Roundabout Approach Speed Reduction,” published May 2017. 

Reducing confusion at two-lane roundabouts

Minnesotans have grown accustomed to roundabouts as they’ve proliferated throughout the state, but many motorists are still confused by the less common two-lane roundabout.

While roundabouts have been shown to reduce vehicle delay and severe crashes, the few Minnesota cities with this type of multi-lane roundabout have had a prevalence of driver mistakes.

In Woodbury, two such roundabouts were converted into smaller, one-circulating-lane designs due to driver confusion.

The City of Richfield had no such option at the high-volume Portland Avenue and 66th Street, a formerly signalized intersection that carries about 30,000 vehicles per day. (See video)

Crash-prone and congested prior to its reconstruction in 2008, a two-lane roundabout seemed to be the practical solution for this intersection. But although the roundabout reduced overall crashes, the intersection still had more fender benders than designers were comfortable with, according to City Engineer Kristin Asher.

“The crashes were primarily related to improper left-turns from the outside lane and failure to yield at the entry,” she said.

Not only were drivers unsure which lanes they should use to enter or exit the roundabout, they didn’t know how to respond to other cars inside the roundabout. (See news story)

“People don’t understand they have to yield to both lanes inside the roundabout,” explained University of Minnesota researcher John Hourdos.

In a recently completed study funded by the Minnesota Local Road Research Board, researchers from the Minnesota Traffic Observatory examined whether sign and pavement marking changes would improve performance.

The city of Richfield extended the solid lines leading up to the intersection from 50 to 250 feet to encourage drivers to choose the correct lane before entering the roundabout. It also replaced fish-hook-style roundabout signs with traditional lane designation signs and did away with complex striping patterns.

These before and after photos show the original fish-hook style pavement markings, left, which were replaced with a more traditional design. (Photos courtesy of the city of Richfield)
Before and after photos show the original fish-hook style pavement markings, left, that were replaced with a more traditional design.
(Courtesy City of Richfield)

Hourdos examined two years of traffic data to see how motorists responded to the improvements that were made in 2011.

He found 50 percent more drivers entered the correct lane from the get-go, which led to a reduction in improper turns within the roundabout. Lane violations were also reduced by 20 percent.

“One of the main problems was drivers didn’t know they have to choose one of the two lanes,” Hourdos explained. “Then once they were inside the roundabout, they were forced to either deviate from their course or commit a violation.”

The city also increased sign visibility to address yielding problems; however, these changes didn’t seem to make a difference.

With state and federal guidelines lacking much guidance for how to sign two-lane roundabouts, the LRRB is funding a new study for three other multi-lane roundabouts: in St. Cloud, at Highway 169/494 and one planned for the future realignment of County Roads 101 and 61 between Chanhassen and Shakopee.

Report: Effect of Signing and Lane Markings on the Safety of a Two-Lane Roundabout (PDF, 4 MB, 72 pages)

Update (1/30/2014): Watch the LRRB’s new video on how to navigate a multi-lane roundabout.

About those roundabouts

One of my unofficial duties as a MnDOT employee is to respond to a near-constant barrage of opinions from my family and friends regarding the condition of our state’s roadways. (My wife, for example, half-jokingly tries to ascribe personal responsibility to me for the congestion she faces on her morning commute.) Interestingly, one of the issues that gets brought up to me most often in private conversations is roundabouts — the circular intersections that are widely praised by engineers but often vilified by a skeptical public.

From a public interest perspective, the verdict on roundabouts is overwhelmingly positive. With very few exceptions, roundabouts have been shown to dramatically reduce both the frequency and seriousness of traffic accidents when compared to other types of intersections. One oft-cited source, the National Insurance Institute for Highway Safety, reports that U.S. intersections converted to roundabouts have experienced a 35-47 percent decrease in crashes and an 72-80 percent decrease in injury crashes (source here). Moreover, because the don’t have stop signs or traffic lights, roundabouts have been found to reduce traffic delays and pollution.

Perhaps not surprisingly, research on these potential benefits has precipitated a rash of roundabout construction. In Minnesota alone, 115 have already been built, with another 39 either planned or under construction, according to the Pioneer Press. Love them or hate them, roundabouts are becoming a fact of life here.

Of course, not everyone loves them. In spite of their stellar  record, roundabouts remain something of a political lightning rod. This article in the Mankato Free Press and this news segment from KSTP provide typical examples of the kind of skepticism officials face when proposing to put in a roundabout. The problem is persistent enough that many officials see a need to develop a public relations game plan. On June 19, the Transportation Research Board is offering a free webinar entitled “Community Outreach: Successful Outcomes for Roundabout Implementation,” designed to help transportation professionals understand and respond to political opposition to roundabouts. It’s free for employees of TRB sponsor organizations (including MnDOT); a $99 registration fee is required for employees of non-sponsors.

For those who are unfamiliar with roundabouts, there are some good resources designed to help people understand their purpose and benefits. Several years ago, the Local Road Research Board produced the video above (along with an accompanying brochure). MnDOT also has a resource page devoted to explaining the use of roundabouts.

Those with more than a passing interest in the subject might also want to check out these recent MnDOT/LRRB-sponsored studies: