Prescott City studies SR‑89 redesign after repeated effluent‑line failures; widening, roundabouts debated

Prescott City officials and consultants told a City Council study session on April 8 that a failing effluent wastewater main and an undersized, weakened SR‑89 roadway require a coordinated construction plan and that three design alternatives will move to the 15% design stage for public review.

Lehi Roach, Prescott City Public Works director, opened the update by saying, “the effluent line in this right of way has broken over a dozen times in the last 6 or 7 years.” The consultant team from Kimley Horn and Associates presented traffic, geotechnical and constructability data and described the advisory‑group and technical‑committee process that produced three top alternatives to be further developed into 15% plans.

Why this matters: SR‑89 between the Willow Lake roundabout and the Fippin Trail roundabout — a little more than 2 miles inside the city limits — carries regional traffic from Prescott Valley, Chino Valley and other communities. The city and consultants said repairing the buried effluent main without also strengthening the roadway would leave the corridor vulnerable to repeated breaks, detours and safety risks.

Key findings and data

- Project limits and schedule: Consultants said the DCR covers SR‑89 from the existing Willow Lake roundabout north to the Fippin Trail roundabout. The team said the next formal presentation of 15% plans will be at a study session on July 8; the DCR will open a 60‑day public comment window anchored by two public hearings later in the summer. (Consultants gave exact public‑comment milestone dates during the presentation.)

- Repeated pipe failures: City staff and the consultant noted two major breaks in 2024 (July and December). The line is an Amaron, a bar‑wrapped concrete cylinder pipe the team said is subject to corrosion; field work also found angular rock and bedding conditions that have contributed to external pipe damage.

- Traffic and capacity: Counts from January 2025 show about 22,000 vehicles per day on the SR‑89 corridor, a level the consultants said already exceeds the SIMPO model’s projection for 2045. During the December closure, nearby Willow Creek Road recorded roughly 36,000 vehicles during the detour period, and counters on Williamson Valley Road also showed large increases. The consultant used travel‑time, origin‑destination and crash data to calibrate a traffic model and concluded that much of the corridor currently operates at level of service E (unstable flow) in peak and midday periods.

- Growth assumptions: The team used a historical vehicle growth rate of 2.75% (based on ADOT counters) for forecasting; under that growth the corridor would reach roughly 44,000 average annual daily traffic in a 25‑year horizon, exceeding typical 4‑lane capacity thresholds.

- Geotechnical and pavement structure: Ten borings found rock depths varying from about 3.5 to 9 feet. In the narrow section (borings 3–7) consultants found no discernible aggregate base beneath the asphalt. Consultants said the current structural section (about 3.5 inches of asphalt) is insufficient for existing volumes; a minimum structural section for current volumes would be about 6 inches of asphalt on 10 inches of aggregate base.

- Construction constraints: Ground‑penetrating radar, seismic refraction and bore data indicate localized shear zones and variable rock conditions. The consultant said a minimum 24‑foot work zone would be required to trench a new effluent main; the existing travelway in some narrow areas is about 42 feet, which limits options for maintaining two‑way traffic during construction.

Alternatives and advisory process

Kimley Horn said it evaluated 20 alternatives and used a 15‑member advisory group to weight 10 decision criteria; a separate technical committee scored the alternatives. The highest‑scoring options included corridor widening to five lanes (two lanes each way plus a 12‑foot median), with one or two roundabouts in selected locations to provide turn and turnaround access. Other alternatives the team reviewed included: no‑widening with intersection roundabouts and consolidation of driveways, barrier‑separated 4‑lane cross sections, flex‑lane concepts and a stacked (grade‑separated) option.

In describing the outcome, the consultant said the top scoring approaches would provide higher sustained level‑of‑service and improved safety. Andrew Baird of Kimley Horn told the council bluntly, “You will have continued failure until it's fully replaced, vetted properly, and you have a sufficient roadway structure over it to account for the traffic loading that's occurring.”

Community concerns and public comment

Public commenters at the meeting expressed a mix of priorities: several speakers urged preservation of the Granite Dells scenic corridor and opposed blasting or large‑scale widening through the Narrows; others emphasized safety and the foreseeable cost of inaction.

- Preservation concerns: Dottie Morris of the Granite Dells Preservation Foundation said, “I am very interested in preserving the look. This is a historical road,” and other residents described the Narrows as a defining scenic entrance to the city.

- Cost and rock‑removal worry: Advisory‑group member and commenter Mike Coffey warned about rock‑removal cost differences between widening and no‑widen alternatives, saying, “Google removal rock estimates, you'll find $200 a yard. That's $10,000,000 of taxpayer money.” The consultant and staff said detailed cost estimates will be developed with the 15% plans.

- Emergency response and traffic safety: Fire and police leaders at the meeting (and commenters) said added dedicated capacity or shoulders would help emergency access. Several residents and a contractor with long local road‑building experience argued the existing corridor’s service‑level problems present a public‑safety risk if not addressed.

What the council asked and next steps

Council members pressed the consultant on coordination with ADOT and other nearby projects (for example, SR‑69 widening), options to minimize rock removal, potential roundabout locations and whether future commercial or master‑planned communities would change travel patterns. Staff and the consultant committed to returning July 8 with 15% DCR plans and to open the public comment period; the team identified two later public hearings to complete the 60‑day comment window.

The consultant and staff advised the council that the city must reconcile three interdependent problems: the failing pressurized effluent main, an inadequate pavement structural section and limited travelway capacity. The team said combining offline widening and targeted intersection improvements may allow work on the utility while keeping at least partial traffic flow, but every option will require tradeoffs in right‑of‑way, rock removal and temporary maintenance‑of‑traffic impacts.

Ending

City staff will return with 15% plans and preliminary cost and construction schedules in July and will use an extended public comment period and hearings to refine a preferred alternative. The consultant emphasized that, absent full replacement and adequate roadway structure, crews should expect recurring breaks and traffic disruptions.