Conjuring 3,200 Square Feet to Solve Complicated Intersection Riddles
Written by: Amanda DeAmico, PE, MSA Professional Services
The intersection of East Main Street and Green Coulee Road in the City of Onalaska was host to a complexity of challenges. Traffic numbers were consistently on the rise, supporting over 20,000 vehicles per day along the cross-town corridor as well as traffic from 400 homes along a single-access residential road and influx/outflow from the adjacent US 53 interchange — the primary north-south corridor connecting the Onalaska-La Crosse metropolitan area to the far reaches of northwestern Wisconsin and Minnesota.
Traffic Congestion Woes
The intersection was struggling to safely and efficiently handle the increase in traffic, leading to long wait times at the prior stop sign, undue congestion, and risky driver decision-making — with resulting crashes. A busy business hub surrounded the intersection with a key Kwik Trip convenience store and the popular Coulee Golf Bowl — an 18-hole golf course, bowling alley, restaurant and event center. Available space for improvements was scarce.
Creative Engineering Solutions
After evaluating six intersection alternatives and working in close concert with the Wisconsin DOT and Kwik Trip owners, MSA transportation engineers proposed a new, two-lane roundabout as the best solution. The conundrum? There was simply not enough available space. MSA needed to get creative and looked to two large existing dual box culverts beneath Green Coulee Road and East Main Street which were currently accommodating a small stream. If connected with a new retaining wall, a portion of the channel slopes behind the wall could be filled in, thereby expanding the intersection footprint and gaining an indispensable 3,200 square feet of space needed to construct the new roundabout.
MSA modified the existing culverts, removing the large wing walls and tying the new retaining wall to the existing boxes and apron floor. Since the stream is located within a mapped flood plain, MSA also needed to arrange removal of sediment within the channel in order to assure the new retaining wall would not raise flood elevation levels. The wall was finished with a new crashworthy decorative parapet and railing to provide extra protection and safety.
Even with the additional space gained by connecting the box culverts, MSA engineers were still faced with challenges in the roundabout design. With very limited right of way and the need to accommodate the stream, businesses and access driveways, space to fit the new two-lane roundabout was tight. Engineers evaluated several iterations of the horizontal design, exploring varying impacts to different properties until the ideal location was identified. The design was pulled and stretched from the traditional circular shape to fit the available footprint, all while being mindful that the critical parameters of roundabout design like entry deflection, speed control, path overlap, and large truck accommodation must be met. The final design is an oblong shape that is approximately 140 feet across at the widest point, the size of which is more commonly seen in single-lane roundabouts.
Phased Construction and Innovative Methods
The construction of the roundabout was phased to allow full access to the Green Coulee neighborhood throughout and keep one lane of traffic open in each direction along East Main Street — the primary roadway — at all times. Green Coulee travelers were restricted to right-in, right-out access only, which allowed contractors to build the outsides of the roundabout while traffic utilized the middle of the roadway, then flip the traffic to the outsides to construct the middle of the intersection. In addition, MSA selected concrete as the new roadway material, which allowed contractors to install small concrete panels and piecemeal the intersection together, providing improved access and minimizing disruptions to the traveling public. One lucky homeowner even managed to get a large, 16’x35’ fiberglass pool delivered during the height of construction!
Positive Impacts
The resulting roundabout at East Main Street and Green Coulee Road immensely improves safety and efficiency at a busy regional intersection, slows vehicular speeds, reduces vehicle emissions, implements accessible two-stage crossings for pedestrians, and delivers intuitive, secure access to businesses throughout — all while boosting the well-being of local residents, guests and the local economy. As a finishing enhancement, the roundabout was landscaped with low-maintenance native plants and a white spruce, with electricity run to the central island to allow for decorative holiday lighting. This project was completed in the fall of 2021 and no injury crashes have been reported since the roundabout was opened for use.
About the Guest Blogger
Amanda DeAmico, PE, is a Transportation Project Manager at MSA Professional Services. She specializes in roundabout design, feasibility review, and operational analysis, and has worked on roundabout projects across the U.S. Amanda is also an experienced transportation design engineer and a graduate of the ACEC Wisconsin Leadership Institute. MSA Professional Services, Inc. is a 100% employee-owned, full-service multidisciplinary consulting firm. Based in 17 offices across Illinois, Iowa, Minnesota and Wisconsin, MSA partners with clients to help solve today’s complex and multi-faceted infrastructure challenges and specializes in the sustainable development of communities.