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Improved Operations & Safety Key to Corridor Study Focus

As communities surrounding the City of Des Moines continue to experience rapid growth, the roadways that carry traffic into and around the city have also seen increased traffic flows. Serving as a major freeway link between Des Moines and the outlying communities to the northwest, Iowa Highway 141 is one of those impacted roadways. One particular section of Iowa 141 directly east of the Town of Granger had been especially hard hit by increased traffic flows — experiencing an increase in the number and severity of crashes where it intersects with Northwest 121st Street.

In partnership with the Iowa DOT, Polk County authorities engaged Snyder & Associates to review traffic operations and the safety history along this busy stretch of the highway corridor. The study was designed to identify short-term traffic engineering solutions and alternatives that can be implemented to improve safety and operations. Because this stretch of corridor also intersects with the western-most termination point of Iowa Highway 415, this study also included long-term interchange, intersection, and corridor improvement needs to meet the forecasted increase in traffic demand.

Targeted Focus Area for Corridor Study Success

Before the proposed improvements, left-turn movements at the busy Northwest 121st Street intersection were extremely dangerous.

The primary focus area for this project was to address safety concerns related to the elevated crash rate at the Iowa 141 and Northwest 121st Street intersection. The east/west running Iowa 141 is a four-lane, high-speed highway divided by a wide median. Traffic attempting to enter from Northwest 121st Street faces a difficult and dangerous movement when crossing or turning left onto Iowa 141. Additionally, left-turning traffic was also subject to queuing issues in the median, leading to crashes. At the time of the study, this intersection was ranked 23rd statewide on the Iowa DOT Safety Improvement Candidate Location (SICL) list.

The secondary focus was to address all intersection operational issues within the corridor study area. In addition to the problems at the Iowa 141 and the Northwest 121st Street intersection, our traffic analysis also revealed that the intersection of Iowa 415 and Northwest Beaver Drive was functioning at poor levels of service for some traffic movements during the AM and PM peak periods.

Public Input Meetings Drive Solutions

Numerous public meetings were key to the success of this study. An initial meeting was held to update concerned citizens and stakeholders on the study background data and environmental constraints in the focus area. Also, the public feedback our team received regarding corridor safety and operations was used to advance potential intersection and roadway alternatives, and eventually assisted in the development of recommendations for improvements to the area.

A follow-up public information meeting allowed our team, along with Polk County and Iowa DOT staff, an opportunity to explain the study and some of the alternatives being considered. Residents and other interested parties were also allowed to express their opinions and ask questions regarding the project and any potential solutions.

Numerous Alternatives Provided for Consideration

To remedy the high crash levels at the Iowa 141 and Northwest 121st Street intersection, four viable alternatives were put forward for consideration. These alternatives included:

  • Alternative 1: Making the intersection four-way signalized
  • Alternative 2: Creating a J-turn intersection
    • Instead of motorists crossing fast-moving lanes of traffic to get to the opposing lanes, drivers at a J-turn intersection turn right in the same direction of traffic, merge into the left lane and then make a U-turn through the wide median and into the direction they intend to travel
  • Alternative 3: The closure of Northwest 121st Street north of Iowa 415 and the creation of an alternative north connector through the extension of Northwest 110th Court to the east of the Iowa 141/415 interchange
  • Alternative 4: Alternative 3 with the westerly extension of Iowa 415 to Northwest 121st Street, which also includes a reconfiguration of the interchange between Iowa 415 and Iowa 141

After weighing the extensive pros and cons of each alternative, the preferred option was alternative 3, which would drive traffic to the new north connector roadway. This in turn would encourage drivers to use the safer Iowa 141/415 interchange for travel further south. While this alternative had a longer timeframe for completion, it offered the best solution for the return on investment and increased safety benefits. It also paved the way for the future implementation of the more extensive alternative 4 improvements.

In the interim, it was decided the Iowa DOT would construct an Intersection Collision Warning System (ICWS) at the Northwest 121st and Iowa 141 intersection. An ICWS warns approaching drivers of an upcoming intersection. Additionally, an off-set right-turn lane for westbound Iowa 141 traffic was also included in advance of the long-term improvements deemed necessary by this study.

The intersection at Iowa 415 and Northwest Beaver Drive will also see improvements as a result of the corridor study.

The Iowa 415 and Northwest Beaver Drive intersection options included making the intersection four-way signalized, the construction of a roundabout, or constructing additional through lanes with a continued four-way stop. The addition of more through lanes was the ultimate best candidate for improving this location.

With an environmental study for the proposed north connector currently underway, the improvements to the corridor are progressing as planned. The upgrades identified by the Snyder & Associates corridor study will result in a considerable quality of life improvement for roadway users, as well as improved safety, efficiency, and traffic flow well into the future.