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Beginning in the mid-1990’s, the 12-mile section of US Highway 6 extending from Interstate 35/80 to Adel was reconstructed from a two-lane highway to a four-lane divided highway. The plan included an urban section from I-35/80 through the city limits of Waukee, transitioning to a rural expressway from Waukee to Adel. A team of Snyder & Associates transportation engineers designed the new highway which represented our firm’s first major roadway design project for the Iowa Department of Transportation (DOT).
Construction on the widening project was done in several phases and spanned many years, wrapping up in the early 2000s. Policymakers were hoping the expanded corridor would help reduce the congested roadway and spur development in rural Dallas County through improved access and mobility. The project performed beyond all expectations. Dallas County quickly became the fastest growing county in Iowa and the 5th fastest growing county in the nation with a growth rate exceeding 30%.
The initial traffic study, which led to the highway widening recommendation, predicted 31,000 vehicles per day would be using the corridor by 2029 based on the trend at the time. However, after the reconstruction project was completed, the residential and commercial growth along the corridor accelerated. A subsequent traffic study conducted by Snyder & Associates in 2000 determined the original usage prediction would be realized a full decade sooner than expected.
The initial Highway 6 widening project would eventually lead to over 50 additional projects during the next 20 years to help maintain an acceptable Level of Service (LOS) benchmark along the corridor. These projects included turn lane additions, traffic signal timing upgrades, traffic studies, and multiple other projects.
To help manage access to the Highway 6 corridor, in 2003 a Corridor Master Plan was agreed to by the Iowa DOT and the Cities of Urbandale, Clive, and Waukee. The goal of this master plan was to provide guidelines that promote safe and efficient traffic flow to support economic development. The four main guidelines established in the plan include:
For the most part, this plan has been followed in the intervening years. Most deviations from the plan were allowed for existing conditions at the time the master plan was adopted.
By 2016, the overwhelming residential and commercial growth along and west of the corridor created a need for another traffic study. Snyder & Associates reviewed 5.5 miles of Highway 6 west of Interstates 35/80, through the cities of Clive, Urbandale, and Waukee. The study included traffic counts (both turning movement and daily counts), crash analysis, existing and future operational analysis, and recommendations for corridor and traffic management strategies. Taking the operation of both signalized and non-signalized intersections into consideration in addition to the rest of the data collected, it was determined that corridor improvements would be necessary.
Traffic volumes are expected to continue growing into the future. Historical traffic trends indicate up to 3% annual growth in Waukee alone, which is the area with the most developable land along the corridor. Annual growth rates were applied to existing traffic volumes to determine geometric needs for each intersection and the overall corridor. Our roadway improvement recommendations included:
Also, the operational analysis indicated that transforming US Highway 6 from four lanes to six lanes would be beneficial for the entire corridor study area. To achieve six through lanes, two options were considered:
Both options for converting US Highway 6 to six through lanes can be constructed with minimal traffic impacts. However, each would require widening the bridges over Walnut Creek.
Beyond converting US Highway 6 to six lanes, the study determined that all of the intersections along the corridor would require improvements to operate safely and efficiently in the future. These improvements range from dual left turn and single right turn lanes on all approaches to the installation of traffic signals or movement restrictions at others.
Another option recommended as a result of the study was to convert numerous traffic signals along the corridor to use Adaptive Signal Control Technology (ASCT). This technology uses sensors to gather real-time traffic information and monitor the system’s performance along an entire corridor. This information is quickly evaluated and signal timing improvements are developed by an algorithm. The ASCT implements the updates on a continual basis, sharing information between signals to make adjustments to timing. These constant adjustments accommodate traffic patterns that are different from the peak periods (during which the standard signals were designed to operate) and help keep traffic flowing smoothly.
With standard signals, retiming might only occur every few years and is reliant on time-consuming, manually collected data. Even then, standard time-of-day signal timing plans don’t accommodate for variable and unpredictable traffic patterns. This results in more delays due to congested traffic, excess fuel consumption, and frustrated drivers.
Implementing ASCT along Highway 6 at major intersections beginning at NW 156th Street and moving east into Windsor Heights was highly recommended. The growth along the corridor has been substantial enough that it warrants adding ASCT to improve performance now and in the future.
As part of the corridor study, Snyder & Associates also provided cost opinions with a breakdown of Iowa DOT and local agency cost-sharing options. In addition, we provided a summary of potential funding assistance opportunities. Since completion of the study in 2018, our firm has assisted the Cities of Clive, Urbandale, and Waukee in obtaining Iowa Clean Air Attainment Program (ICAAP) and Transportation Safety Improvement Program (TSIP) funding for improvements to numerous sections of the corridor, including:
Maintaining this active and critical roadway is an ongoing effort to meet the needs of an ever-growing region. In conjunction with our firm, all agencies are committed to implementing improvement projects along the corridor as funding becomes available in the future. Snyder & Associates is proud to have been associated with this extensive undertaking for over a quarter of a century — and looks forward to continuing our relationship with the communities and organizations we’ve served.