September 2017 Vol. 25, No. 3

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Strategies to make drivers slow down at roundabouts

roundabout

Photo: LRRB

What’s the best way to configure signs, pavement markings, illumination, and other treatments to get drivers to slow down enough to keep roundabouts safe? An LRRB-sponsored project developed a new resource to help Minnesota local road engineers select appropriate speed reduction measures.

“Although roundabouts are becoming common, single-vehicle crashes from drowsy, inattentive, or unfamiliar drivers are still a concern, particularly in rural areas,” says Joe Gustafson, traffic engineer with 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.”

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 also studied, and design manuals from four states were reviewed.

The final report provides information on the effectiveness of various treatments and on their installation and maintenance costs. It also includes 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 roundabout is ahead include traditional signs, pavement markings, illumination and other indicators, plus advanced devices like speed-activated, LED-enhanced warning signs.

According to the investigators, 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.

“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,” says Susan Chrysler, senior research scientist with Texas A&M Transportation Institute and the study’s principal investigator.

Other findings include:

  • 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.

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