Roundabouts

Safe Roundabout Design: a Vermont Context

There have been great challenges to the successful implementation of roundabouts as a strategy to increase the safety of Vermont intersections. Unfortunately, a few example intersections implemented using either a hybrid, or an inadequate design for the context of its environment have held the public limelight and practically redefined mainstream understanding of this traffic device. The “Winooski Circulator” in Winooski, VT serves as a hybrid-design example of an intersection having some characteristics of roundabout. However, the device departs from genuine roundabout design by employing signalized pedestrian crossings which bisect traffic midstream. Furthermore, the circulator’s elongated oval shape encourages higher operating speeds along its eastern and western sections. This design departure has advanced the intersection to the top of the Vermont Agency of Transportation’s High-Crash Location (HCL) listing for 2006-2010 with a reported 191 crashes and 42 injuries. The “Main Street Roundabout” in Middlebury, VT, serves as an example of a legitimate roundabout that may require some minor design modifications to lower operating speeds, thereby improving its level of safety. This intersection has also been added to the State’s HCL listing with a reported 41 crashes and 4 injuries on its eastern approach.


Figure 1.

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This image is of a conversion of a Rotary into a Roundabout in Kingston, NY, off of I-87. Here the size difference between the two traffic devices is clearly indicated; where an approximate rotary diameter is greater than 600 feet, an approximate roundabout diameter is less than 300 feet.

“Modern Roundabouts” and their Improvement upon Intersection Safety

A “Modern Roundabout” is a one-way, non-signalized circular intersection in which traffic flows around a center island. Roundabouts have specific design and traffic control features which ensure low travel speeds and efficient traffic movement, while meeting the needs of all road users, including; passenger cars, large trucks, buses, pedestrians, and bicyclists. A roundabout is a unique traffic device, different from a Traffic Circle, Circulator, Rotary, or traditional intersection.


Figure 2.

A roundabout reduces vehicle-to-vehicle conflict points from 32 (at a conventional four-way intersection) to 8 (See Figure 2). Among those conflict points eliminated are left-hand turns, which can result in more serious crashes at traditional intersections. Since roundabout traffic only enters and exits via right-hand turns, occurrence of severe crashes is greatly reduced. Small angle collisions may occur as a result of right-hand turns, but these are typically less severe types of collisions. According to the Federal Highway Administration (FHWA) Office of Safety, roundabouts have a greater than 90% reduction in fatalities, a 76% reduction in injuries, and a 35% reduction in all crashes when compared to conventional intersections (FHWA, 2008).

Roundabouts are not Traffic circles. Traffic Circles (or rotaries) operate under different traffic rules and have experienced operational and safety problems. Older style traffic circles (such as those found in Boston, Washington, D.C., Paris, and London) are significantly larger than roundabouts, and were designed for high speed entry and multi-lane weaving. They generally suffered high crash rates and operational problems causing many to fall out of favor in the US during the 1950s and 1960s. Roundabouts are commonly referred to as “Modern Roundabouts” in order to distinguish them from these older style traffic circles (Click here for more details).

Many traffic circles require circulating vehicles to grant the right of way to entering vehicles, whereas modern roundabouts generally follow the “Yield-to-Left” rule where vehicles in the roundabout would have the Right-of-Way. There are three safety design features of a roundabout; 1. Yield Control of entering traffic, 2. Channelized Approaches that deflect traffic into the proper one-way, counterclockwise flow, and 3. The Geometric Curvature of the circular road and angles of entry to slow the speed of vehicles. These three features are critical to the success of a roundabout because they effectively decrease driving speed to 15-25 miles per hour (-Transportation Research Board ~ NCHRP Report 572, 2007).

Good roundabout design results in slower traffic speeds. Some potential safety benefits of slower traffic speeds in roundabouts include; 1. Reduction of crash severity for pedestrians and bicyclists (including older pedestrians, children, and impaired persons), 2. More time for entering drivers to judge, adjust speed for, and enter a gap in circulating traffic, 3. More time for all users to detect and correct for their mistakes or mistakes of others, and 4. Less frequent and less severe collisions. Additionally, roundabouts are safe when the power is interrupted. Where traditional intersections can become a safety concern during outages (especially in higher volume intersections), roundabouts only need electricity for nighttime illumination, requiring a slight reduction of driver speed to retain their level of operation during darkness.

A good functional example of a modern roundabout employed in Vermont is the Keck Circle roundabout in Montpelier. Constructed in 1995 (on VT State Route 12), Keck Circle is Vermont’s first modern roundabout. It is also the first modern roundabout constructed on a State Highway within the United States. Since its construction, this 13,000 vehicle per day intersection has had a 69% reduction in injury accidents, and only 4 reportable accidents within the first 10 years of its operation (-NE Roundabouts, 2008). The Project Summary and the post-construction Public Opinion Survey on the Keck Circle details a mostly favorable response from local residents and Department of Public Works.

Size, Utility, and Context of Roundabouts

While the radius of a Modern Roundabout may seem too small, they are actually designed for the largest reasonably anticipated vehicles on the road. A component of the roundabout called the “Apron” (between the circulatory roadway and the central island) serves as a mountable curb built of a rough textured material (e.g. brick or cobble stones), in order to discourage routine use by smaller vehicles. Additionally, just as with tradition intersections, traffic is required to not to enter a roundabout when an emergency vehicle approaches it upon another leg. Once having entered, vehicles should clear out of the roundabout if possible, facilitating queue clearance in front of the emergency vehicle. Since roundabouts are designed to slow all traffic when entering and eliminating left-hand turns, safety is improved over traditional intersections.

Roundabouts are designed to take up as little space as necessary while meeting engineering design standards. An approximate rotary diameter is greater than 600 ft., while an approximate roundabout diameter is less than 300 ft. In some cases, roundabouts can take up less space than a traditional intersection. Landscaping in the center of a roundabout can help minimize visual impacts and help to incorporate the facility into the surroundings. Roundabouts can also provide an attractive gateway into a community.

Pedestrian & Bicycle Accommodation

Roundabouts generally are safer for pedestrians than traditional intersections. In a roundabout, pedestrians walk on sidewalks around the perimeter of the circulatory roadway. If it is necessary for pedestrians to cross the roadway, they cross only one direction of traffic at a time. A roundabout reduces vehicle-to-pedestrian conflict points from 16 (at a traditional four-way intersection) to 8 (See Figure 3). Additionally, crossing distances are relatively short, especially where “Splitter Islands” provide refuge for pedestrians where crossing is permitted, and traffic speeds are lower than at traditional intersections. Roundabout design discourages pedestrians from crossing to the center island.


Figure 3.

Bicyclists may take the full travel lane and follow the same “yield-to-left” rule as an automobile. Since traffic speeds are slower at a roundabout, bicycles fit into the normal traffic flow. Bicyclists may also choose to dismount at the roundabout and use the sidewalks and crosswalks to navigate to their desired exit point as a pedestrian.

Conclusion

Roundabouts are usually considered as one of several options for traffic control at intersections. The Vermont legislature passed Act 141 in 2002, which included a section requiring consideration of roundabouts at intersections with high crash rates.

“The general assembly finds that the installation of roundabouts at dangerous intersections in the state has been cost-efficient, and has enhanced the safe operation of vehicles at these locations. The agency of transportation is directed to carefully examine and pursue the opportunities for construction of roundabouts at intersections determined to pose safety hazards for motorists.” (H. 764, Sec. 37, 21 Jun 2002)


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