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Public Roads - January/February 2012

January/February 2012
Issue No:
Vol. 75 No. 4
Publication Number:
Table of Contents

Montana's Roundabout Corridor

by Alan Woodmansey and Kirk Spalding

For the west-end entryway to Billings, the Montana Department of Transportation eschewed traditional signalized intersections in favor of something a little rounder.

A series of eight roundabouts, shown here from above, improve the safety and mobility of a busy urban street on the outskirts of Billings, MT.

As municipalities expand into rural areas of the country, transportation systems can become strained. In Billings, MT, for example, where the population is approximately 104,000, this was the case along a 4.5-mile (7.2- kilometer) stretch of Shiloh Road. The narrow section of two-lane road was becoming increasingly congested with traffic as the city grew westward into farmlands.

In 2002, the Montana Department of Transportation (MDT) and the Federal Highway Administration (FHWA) initiated an environmental assessment to explore options for improving Shiloh Road. Although the current corridor's traffic volume is less than 13,000 vehicles per day, MDT projects that it will approach 40,000 vehicles per day by 2027. Some of the cross streets also carry as much traffic as Shiloh Road.

Based on this traffic projection, MDT and the local agencies proposed widening the existing two-lane road to a four-lane divided highway as one option. However, community members expressed concerns regarding this solution, citing a need to create an identity and a corridor that was inviting and safer for all road users. In fact, balancing safety and mobility was a key consideration for developing a solution for the area's congestion.

During the time when MDT was developing the environmental assessment, FHWA had begun promoting roundabouts as an alternative solution for intersection design. Roundabouts potentially can reduce injury and total crash rates, facilitate higher capacity, and reduce delays compared to traditional signalized intersections. Therefore, MDT and FHWA proposed considering roundabouts in addition to signalized intersections for Shiloh Road.

As a result, the environmental assessment compared multilane roundabouts to traffic signals at eight intersections. The analysis determined that a corridor of eight roundabouts would offer a better level of service than would signalized intersections. In addition, the roundabouts would cut travel times, reduce crash and severity rates, enhance capacity, and limit the need for property acquisition. The roundabout option also would address access control and other community objectives more effectively and be less expensive overall than the entirely signalized alternative.

Demonstration Of Roundabouts

This aerial photo shows the geometry of the roundabout at Shiloh Road and Grand Avenue.

Although roundabouts looked promising from the engineering perspective, some area residents and members of the project's community advisory committee were dubious about installing multilane roundabouts. That is, they were unconvinced until MDT, in cooperation with Yellowstone County and the City of Billings, hosted a roundabout demonstration. Held in a large, empty parking lot in Billings, the demonstration involved passenger vehicles, large trucks, and several firetrucks completing various intersection movements for the first time in a full-scale multilane roundabout. Residents, local media, local officials, and MDT personnel attended to observe how roundabouts work.

The demonstration roundabout, which was laid out using traffic cones in a large parking lot, gave the advisory committee members and the broader community, via television news broadcasts and local newspaper articles, something tangible to look at, walk through, and ask questions about. Videos on roundabouts from other States and communities supplemented the live demonstration to show how multilane roundabouts look and function in the real world.

"This demonstration relieved the concerns of commercial stakeholders who use Shiloh Road on a regular basis," says Vern Heisler, P.E., deputy public works director at the City of Billings. "I was able to drive through the roundabout demonstration model to understand how the public would experience traveling through a roundabout. This experience helped me answer questions from the public regarding the roundabouts that were going to be constructed on Shiloh Road."

Sidewalks like this one alongside the approach to one of the roundabouts accommodate pedestrian movement in the renovated corridor, while landscaping helps provide way-finding for the visually impaired, enforces the curvatures located at the roundabout, and makes the area more visually appealing for all road users.

Another plus from the community's perspective: Roundabouts afford landscaping opportunities and an open feel, an advantage over signalized intersections.

The Preferred Alternative

In 2007, FHWA issued a Finding of No Significant Impact for the environmental assessment's preferred alternative, which included a series of eight multilane roundabouts. Although MDT had identified funding for only a portion of the estimated $40 million cost, the project was a priority for the State and the local community, so final design began on an accelerated pace.

Once the project design began, MDT held regular stakeholder meetings to keep officials and the public informed of its status. Due to the uncertainty of future funding, a decision from these meetings was to split the design into separate construction projects so the corridor could be built in three segments as funding became available.

In accordance with the locally approved plan for pedestrian and bicycling trails, the project also includes a 10-foot (3-meter)-wide asphalt multiuse path along one side of the corridor. The engineering firm included in the design at-grade pedestrian refuge areas in the roundabouts' splitter islands at pedestrian crossings, as well as one midblock crossing that featured a pedestrian-actuated warning system, a rectangular rapid flash beacon.

An unanticipated benefit of involving the public throughout the environmental assessment and design process was that a large landowner along the corridor decided to fund most of the costs for an unplanned pedestrian tunnel beneath a section of Shiloh Road. The tunnel connects the landowner's undeveloped property on both sides of the road as well as the corridor's multiuse path and sidewalk.

Since MDT had no prior experience with multilane roundabouts, the department asked its prime consultant, a design engineering firm, to retain another firm with extensive roundabout experience to complete a peer review when the design was approximately 30 percent complete. The peer review reassured the State that the design was on the right track, and only minor modifications were necessary.

The Design Takes Shape

The largest roundabout in the Shiloh Road corridor is at King Avenue, which has a 221-foot (67.5-meter) diameter and two entering and exiting lanes on all four approaches. The engineering firm designed this roundabout in such a way that it easily could be expanded to three lanes on the Shiloh Road approaches if growth expectations are met.

Large trucks are common on the road corridor. The roundabout designs accommodate WB-67 trucks -- those interstate semitrailers with a 67-foot (20-meter) wheelbase commonly used to deliver goods to businesses -- traveling through the roundabouts alongside passenger vehicles. Most of the roundabouts have two entering lanes with gore striping (striped area between two travel lanes of the same direction), which separates the adjacent lanes by as much as 10 feet (3 meters) and provides for the large, sweeping turning movement of the WB-67 trucks without infringing on the adjacent entry lane. This design feature also helps achieve the desired entry speed, which is less than 20 miles per hour, mi/h (32 kilometers per hour, km/h). The road surface in the roundabouts is asphalt, but the interior truck aprons (for rear axles of truck trailers to off-track) are raised, stamped red concrete.

Recovery Act Funding

By early 2009, when MDT advertised the first segment for construction, the agency had secured funding for only one segment of the corridor. When the U.S. Congress passed the American Recovery and Reinvestment Act (the Recovery Act), and the President signed it in 2009, MDT allocated Recovery Act funding to the project's remaining two segments. In fact, the first Recovery Act project MDT advertised for bidding was within the Shiloh Road corridor. All three construction projects for the corridor thus were awarded within a 7-month timeframe in 2009.

Recovery Act funds provided almost $15 million for construction of the final two segments, which enabled MDT to complete the project years ahead of the original timeline. Plus, the contractors bid the two Recovery Act projects for 11 percent less than the engineer's estimate, which equated to $1.7 million in savings. The entire corridor was fully opened to traffic in the fall of 2010.

"The Recovery Act helped bridge a funding gap, allowing for the Shiloh Road projects to be let to contract in two construction seasons rather than the many years it would have taken utilizing traditional funding sources," says MDT District 5 Administrator Stefan Streeter.

Early Signs of Success

In June and July 2011, the prime engineering firm that designed the project conducted preliminary postconstruction studies of the corridor to assess the effectiveness of the new roundabouts. The results are promising. The posted speed limit for most of the corridor is 45 mi/h (72 km/h), with advisory speeds of 15 mi/h (24 km/h) signed for the roundabouts. The engineering firm recorded the average speed for the corridor to be 37 mi/h (60 km/h) with a stop delay of less than 5 seconds. Travel time to traverse the 4.5-mile (7.2-kilometer) corridor averages 7 minutes and 20 seconds, with a variation of only 10 seconds regardless of time of day.

The longest observed queue was eight vehicles. Although observed queues were rare, those that did occur continued to move forward releasing the queue in platoons or individually as gaps in circulating traffic occurred. Prior to the reconstruction, motorists experienced significant congestion and delay during peak periods of the day at several of the major intersections, and the travel time could easily exceed 15 minutes to traverse the corridor.

Overall, MDT officials report that the as-constructed traffic conditions at the roundabouts are generally meeting expectations with few exceptions, and travel times are better than expected. "The Shiloh Road corridor is a beautiful gateway for the City of Billings," says "allowing virtually unimpeded travel from Zoo Drive to Rimrock Road."

Gore striping helps separate the two travel lanes and provides additional space for large trucks as they pass through the roundabout at Shiloh Road and Hesper Road. Painted arrow markings on the pavement in the two travel lanes instruct motorists on how to move through the roundabout.


Alan Woodmansey, P.E., is an operations engineer in the FHWA Montana Division Office. Woodmansey was the Federal oversight project manager for the Shiloh Road project. He has a B.S. in engineering from the United States Military Academy and an M.S. in engineering management from the Missouri University of Science and Technology.

Kirk Spalding, P.E., is an associate principal with Sanderson Stewart, an engineering consulting firm headquartered in Billings, MT. Spalding served as the project manager and lead designer for the Shiloh Road project. He holds a B.S. in civil engineering from Montana State University and is a licensed P.E. in Colorado, Idaho, Montana, and South Dakota.

For more information, please contact Alan Woodmansey at 406-441-3916 or, or Kirk Spalding at 406-656-5255 or