Consultants present preferred 'skewed roundabout' design, funding plan for Towner/Tanner interchange

Snyder & Associates presented an update to the Johnston City Council on the Towner Drive interchange project along Iowa 141, recommending a skewed ‘‘dog-bone’’ or stretched roundabout interchange as the preferred alternative and outlining a funding strategy and schedule.

Wade Greiman, a transportation engineer with Snyder & Associates, said the project—s planning and environmental linkages (PEL) study is about 80% complete and will be submitted for DOT and Federal Highway Administration review; the team has already begun field work for the follow-up NEPA study. He said construction could begin as early as 2028 if sufficient funding is secured.

Greiman cited traffic and safety findings: the corridor carried roughly 25,000 vehicles per day in 2016 and about 33,000 in early 2023; the team projects between 50,000 and 62,000 vehicles per day by 2050 depending on growth scenarios. Crash data from 2019 to 2023 showed intersection and segment crashes, including one fatality, which the consultant said supported interchange improvements.

The preferred alternative is a skewed roundabout that the consultant said would reduce conflict points, improve operations relative to signals, and minimize impacts to the golf course on the east side. The design includes a grade-separated crossing (the roundabout would sit above the highway) and a 10-foot shared-use trail along relocated Towner for pedestrians and bicyclists.

Greiman presented a January cost estimate of $31.85 million and said an updated cost opinion is nearer $28 million; he cautioned that the numbers are estimates and subject to change. He also outlined funding sources identified to date: $3.4 million in Surface Transportation Block Grant (STBG) funds through the MPO, about $1.1 million from the Carbon Reduction Program, $1.205 million the city is obligated to via a predesign agreement with Iowa DOT, and $3.2 million pledged from the Iowa DOT per that agreement. The city additionally applied for a BUILD/BUILD-like grant request of roughly $15.492 million, submitted a $2 million community project funding request via the congressional office, and identified RISE and developer contributions as potential sources; the remaining local match was estimated in preliminary numbers.

Council members asked for clarification about what ‘‘funding secured’’ means; Greiman said funding approvals mean an award by the controlling agency but noted that many transportation funds are reimbursement-based and are not held in the city—s bank account until expenses are incurred and documented. He also said the project team has begun environmental and cultural-resource field work and expects the NEPA review to be streamlined because impacts are limited.

The consultant opened for questions and detailed next steps: submit the PEL, obtain DOT and FHWA concurrence, proceed to NEPA, and continue pursuing identified funding sources.