Plans to Improve Mobility & Congestion through Transportation Corridor Study
US Highway 6 from Interstate 35/80 to Adel was reconstructed from a two-lane highway to a four-lane divided highway between 1997 and 1999. A team of Snyder & Associates transportation engineers designed the new highway, which was the firm’s first major corridor design project with the Iowa DOT. The highway design included an urban section from I-35/80 through the City limits of Waukee, then a rural expressway from Waukee to Adel. Policymakers hoped the new corridor would spur development in rural Dallas County. The project performed beyond all expectations, as Dallas County became the fastest growing county in Iowa and 5th fastest growing county in the entire nation with a growth rate exceeding 30 percent.
During the design process, the traffic study predicted 31,000 vehicles per day by 2029. However, that volume has been realized a decade sooner. The success of the original reconstruction project created a need for a corridor study to determine additional improvements to accommodate rapid growth that’s projected to continue.
Studying & Evaluating Existing Corridor Conditions
Typically a corridor study is the first step in developing a plan for the future of a transportation facility. Defining the needs of a corridor helps focus land planning efforts on the most significant issues.
Safety Performance & Crash Analysis
The first step of the corridor study was to evaluate the existing conditions in regards to operational and safety performance.
Five years of corridor crash data (2010-2014) was reviewed, and crash rates were calculated and compared to statewide averages. The statewide average crash rate for similar corridors during the same time frame was 253 crashes per hundred million vehicle miles traveled (HMVM). The average intersection crash rate is .8 crashes per million vehicles entering.
Overall, the entire corridor is below the statewide average. The Raccoon River Valley Trail, a well-used multimodal facility, is located north of US Hwy 6, and there have been 14 bike/pedestrian-related crashes along the corridor. There was one fatal crash during the crash period under review, but it was not trail related.
There are two corridor segments that exceed the statewide crash average, located between Warrior Lane and 156th Street. These segments have the most undeveloped land along the corridor, and over half of the bike/pedestrian-related crashes occurred along these segments.
Most intersections have a crash rate below .5 crashes/MEV at unsignalized intersections. Signalized intersections tended to have higher crash rates, including three that had crash rates at the statewide average. Signalized intersections were also the site of the most bike/pedestrian-related crashes and eastbound-left or southbound-right turning vehicles were often involved.
Existing Operational Analysis
Iowa DOT turning movement counts collected in 2012 were used as the primary source of data for operational analysis. The counts were supplemented with 2014 turning movement and 24-hour daily counts by Snyder & Associates. The data was analyzed using Synchro 8 software and Highway Capacity Manual (HCM) 2010 methods.
It was determined that approximately 2,000 vehicles per hour (vph) utilize the corridor from 142nd Street to the I-35/80 interchange in the peak hour direction. It was also observed that peak hour traffic volumes decrease further west along the corridor to approximately 1,200 vph in the peak hour direction at Warrior Lane in Waukee.
From the traffic data gathered, the intersection operational analysis identified three signalized intersections that fail to operate at acceptable levels during peak hours. The intersections were with Alice’s Road, 142nd Street (pm only), and 128th Street, where significant queuing occurs.
Other signalized intersections were observed to be operating at acceptable levels. There were also non-signalized intersections found to be operating below the preferred operational level, which is typical for an arterial roadway like US Hwy 6.
Taking the operation of both signalized and non-signalized intersections into consideration, it was determined that corridor improvements are necessary.
Corridor Study: Recommended Improvements to Improve Traffic Safety & Operation
Since adoption the Corridor Master Plan has been followed by all three communities, with a few deviations. Traffic operations and safety don’t appear to have been impacted by the deviations, which occurred before the plan. However, a couple of modifications to the plan are recommended:
- The plan listed the Peachtree Drive intersection as a future signalized location, but there’s limited connectivity to the street from the south. Westgate Drive has more connectively to the south and still maintains 1,000-foot signal spacing. This allows for the installation of another signal east of Peachtree Drive while maintaining one-quarter mile spacing.
- The plan also lists 149th Street as another intersection for a future traffic signal. Development occurred around this intersection without connection to/from the north, so it’s suggested that a traffic signal isn’t needed due to the lack of connectivity.
- Outside of the signalized intersections, there are several areas in between that need to be monitored for safety and operational performance. If performance decreases due to increased volume, it’s recommended that consideration be given to restrict movements. For example, restricted movements from the side street were implemented east of 128th Street at a local auto dealership. At this location, there’s a partial median closure that restricts left turn and through movements from the side street to significantly reduce conflicts at the median opening.
- Consider implementing a single speed limit of the entire corridor to provide consistency. Currently, the speed limit varies from 45 mph to 55 mph.
- Consider installing permanent dynamic message signs to alert drivers of traffic incidents and corridor conditions. The signs could be operated by the Iowa DOT through their Traffic Operations Center.
Future Traffic Projections & Operational Analysis
Traffic volumes are expected to continue growing into the future. Historical traffic trends indicate up to three-percent 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 operational analysis indicates that transforming US Hwy 6 from four lanes to six would be beneficial for the entire corridor. To achieve six through lanes, two options were considered:
- Reducing lane widths from 11.8 feet to 11 feet and reallocate the existing pavement. The only widening and construction needed would be needed at intersections requiring additional auxiliary turn lanes and replacement pavement markings. This option would locate pavement markings off of joint lines and leave visible scarring from the removal of the existing markings.
- Minimal widening to maintain the full, 11.8 feet lane width. This option would leave most pavement markings on joint lines and possibly maintain the use of the existing storm sewer system.
Both options consider widening bridges over Walnut Creek.
Beyond converting US Hwy 6 to six lanes, each intersection will require improvements to operate safely and efficiently in the future. Intersection improvements range from dual left turn and single right turn lanes on all approaches to the installation of traffic signals or movement restrictions. We also recommended using adaptive traffic signals.
Both options for converting US Hwy 6 to six through lanes can be constructed with minimal traffic impacts. As part of the corridor study, we provided cost opinions with a breakdown of Iowa DOT and local agency cost sharing and provided a summary of potential funding assistance opportunities.
To conclude the corridor study, we reviewed 5.5 miles of US Hwy 6, west of Interstates 35 and 80, through the cities of Clive, Urbandale, and Waukee. Our 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.
Since completion of the study in 2018, we’ve assisted the Cities of Clive, Urbandale, and Waukee in applying for and receiving Iowa Clean Air Attainment Program (ICAAP) and Transportation Safety Improvement Program (TSIP) funding for improvements to:
- US Hwy 6 & NW 128th St by 2021
- Intersections of US Hwy 6 and Alice’s Rd in 2019
- Implementation of adaptive signal controls from NW 156th St east beyond Interstates 35 and 80 in 2019
- Implementation of automated traffic signal performance measures in Waukee in 2020
All agencies intend to continue constructing improvements along the corridor as funding becomes available.