Prescott council hears update on failing SR 89 effluent line, three design alternatives advance to 15% plans

Gwen Roach, public works director for the City of Prescott, told the City Council on April 8 that the effluent pipeline running in the SR 89 right-of-way “has broken over a dozen times in the last 6 or 7 years,” and that the city contracted Kimley Horn and Associates last November to produce a 15% design concept report evaluating alternatives for the roadway and the two wastewater mains.

The consultant team presented collected traffic counts, origin-destination analysis, geotechnical borings, crash data, and a scored evaluation of 20 initial alternatives that produced three recommended options (and one additional alternative for study) to carry into 15% plans. Andrew Baird, project manager with Kimley Horn, said the firm is “about 2 thirds of the way through their process” and will return on July 8 with the 15% design concept report and plans and the start of the 60‑day public comment period.

Why it matters: the corridor is both a functioning regional highway and a scenic, residential area. The project exists to replace a failing pressurized effluent main and to upsize a gravity wastewater main; at the same time the city must address deteriorating pavement and rising traffic demand that the consultant says already exceeds some previous 2045 projections.

Key findings and data

- Extent: the study limits run from the existing Willow Lake Roundabout north to the existing Fippin Trail Roundabout (a little over 2 miles); effluent and wastewater mains run slightly less than the full roadway limit.

- Traffic counts: Kimley Horn reported about 22,000 vehicles per day measured in January 2025 on SR 89; detour conditions in December 2024 shifted roughly 36,000 vehicles onto Willow Creek Road during a closure, and Williamson Valley Road saw an additional ~5,000 during that detour. The consultant used those counts plus Streetlight origin-destination data to model regional travel patterns.

- Growth and capacity: the team used a planning growth rate of 2.75% (based on ADOT counters and historical counts) and projects that, at that rate, SR 89 would exceed a failing threshold (about 28,500 average daily traffic) by roughly 2034; a 25‑year horizon (2050) projection under that growth rate reaches roughly 44,000 daily vehicles.

- Safety and crashes: collision data (2019–2023, with 2024 data pending) showed a high share of single-vehicle and rear-end crashes. The consultant noted that adding shoulders and reducing roadside hazards would lower crash risk and help emergency response.

- Geotechnical and utilities: field work included ground-penetrating radar, seismic refraction and 10 borings. Rock depth along borings varied from about 3.5 to 9 feet. In the narrow corridor the consultant found little or no engineered aggregate base beneath the pavement (the existing section was reconstructed roughly five years ago with about 3.5 inches of asphalt on a weak, old layer), so the current structural section “is not sufficient at all for existing traffic,” the presentation said. The existing effluent pipe (Amaron or bar-wrapped concrete as described in the presentation) is corroding and is exposed to angular rock bedding that contributes to repeated breaks; the December 2024 break undermined the roadway and required a full closure and repaving in that location.

Alternatives and committee process

Kimley Horn reported that it developed about 20 alternatives and used a two-group evaluation process: a 15‑member Design Review Advisory Group that weighted 10 evaluation criteria (vehicular safety, environmental/quality of life, traffic flow/capacity, viewshed impacts, constructability, maintenance impacts, etc.), and a five‑member technical committee that scored alternatives against those weighted criteria. Cost was intentionally weighted lowest among the advisory criteria; the advisory group’s pairwise weighting results placed vehicular safety highest (~14.2%) and cost lowest (~4.2%).

From scoring and constructability inputs the team presented three top alternatives for advancement to 15% plans: (1) five-lane widening through the corridor (two lanes each direction with a center median), with two new roundabouts proposed at locations chosen to minimize rock removal; (2) five-lane widening transitioning to a four-lane, barrier-divided section; and (3) a five-lane widening variant with one roundabout (differences primarily in where widening occurs and where roundabouts are placed). A fourth “no-widening” alternative (intersection improvements, consolidation of driveways, targeted roundabouts or turnouts) also remains under consideration as a lower‑impact option.

Construction and traffic impacts

The consultant said the new effluent main and gravity sewer must be constructed while the system remains operational, which constrains trenching locations and phasing. In narrow locations the team estimated a minimum 24‑foot work zone would be needed; the existing narrow section has only roughly 42 feet of total width to work with. Kimley Horn noted two kinds of detour/closure conditions were observed during 2024 breaks (one allowed the highway to remain open via a local loop; the other required a full closure and produced near‑gridlock on alternate routes). The team emphasized the need to coordinate timing with ADOT and other nearby construction (for example, widening work on Highway 69) to reduce compounded regional impacts.

Public comment and community concerns

Eleven members of the public spoke during the public comment period. Speakers representing the Granite Dells Preservation Foundation and other area residents urged minimizing rock removal and preserving the scenic Granite Dells corridor. Dottie Morris said the Dells are “like going into Zion National Park and saying, let’s blast those tunnels away.” Several residents asked the council to prioritize alternatives that preserve the visual character and limit permanent widening through the Narrows.

At the same time several commenters argued that the corridor is a regional arterial with growing safety concerns. Contractor and resident Michael Phan said bluntly, “Service level f means people die,” and urged council to select an alternative that restores reliable capacity and safety. Advisory-group member and commenter Mike Coffey urged the council to include a low-impact option among the three advanced alternatives and noted rock-removal cost risk; he summarized his view that parts of the process looked like “putting lipstick on a pig” and recommended preserving one no-widening alternative for detailed study.

Council discussion and next steps

Council members and staff discussed intersections on Willow Creek Road and Willow Lake Road, the feasibility of roundabouts versus signals, local right-of-way constraints, and the potential for destination and regional growth (including master-planned communities such as Deepwell Ranch). Fire and police officials noted that additional capacity or continuous shoulders would aid emergency response; the Fire Chief (name not specified on the record) said potential station locations being evaluated would sit near the Watson Lake and Fippin Trail roundabouts and that improved flow would benefit emergency access.

Kimley Horn’s next scheduled presentations: 15% design concept report and plans at the Prescott City Council study session on July 8 (which will open the 60‑day public comment period), a public hearing on August 20, and a final public hearing and close of the comment period on September 9.

What the presentation did not decide

The council did not vote or adopt any option at the April 8 study session; instead staff and the consultant will return with 15% plans for the three advanced alternatives (and accompanying cost, rock‑removal estimates and refined constructability) and open the city’s public comment period. No formal funding decision or construction schedule was adopted at the meeting.

Ending

Councilmembers signaled the issue is technically complex and politically sensitive; staff will use the geotechnical, traffic and cost details expected with the 15% package to refine a recommended course and to return to the council during the scheduled July study session.