Interstate 84 (I-84) is the longest highway in Idaho, running from the southwestern part of the state to the southeastern part. Among the cities that I-84 connects are Twin Falls and Jerome in Southcentral Idaho, near the Snake River Canyon. The Idaho Transportation Department (ITD) and general contractor Western Construction Inc. are working on the I-84 Improvements South Jerome to Twin Falls Project to improve safety and mobility in the area.
Two projects are happening concurrently on I-84 in Southcentral Idaho. The first involves widening the route over a span of three years from two to three lanes in each direction and repaving the existing roadway.
The second project — the South Jerome Interchange Replacement Project — is focused on replacing an interchange with a new design, improving local roads, installing new traffic signals, adding a sidewalk and bike lanes to a local bridge, and more.
ITD is engaging in two significant projects on I-84 because it is a major corridor in the area with an average daily traffic of 22,000 on I-84 and 12,000 on Lincoln Avenue (the crossroad at the interchange), as of 2024. ITD expects the traffic count to double over the next 20 years.
"The interchange is a primary entry to the city of Jerome and is a primary commuter route for locals to and from Jerome," said Courtney Wagner, a Public Information Officer for ITD. "It's a focal entry to several commercial/industrial trucking destinations and companies and provides a stopping point for interstate traffic."
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The existing bridge on the interchange was showing signs of age. ITD crews were spending an inordinate amount of time addressing maintenance issues on the bridge that was part of the interchange.
"Since the interchange as a whole was not meeting current standards, it was decided to investigate full replacement versus continued maintenance," Wagner said. "The bridge ... did not meet current standards, the ramps were shorter than needed, and there was no space for future expansion of I-84."
Removing the bridge was not a rash decision. Bridge and interchange studies started in 2016, and the project was initially scheduled to begin in 2020, but it was delayed for additional analysis by the Federal Highway Administration. They concurred with ITD's analysis.
The previous interchange was diamond shaped, and it was built in 1966. The department determined that a new design would increase safety to the traveling public and reduce travel times. ITD considered a few interchange design options.
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"We presented the design options to the public in 2017 at two different public meetings," Wagner said. "The divided diamond interchange [DDI] was the preferred option of those in attendance and ITD decision-makers."
The decision-makers preferred this design because the DDI meets the goals of the project, which were to meet current design and engineering standards by removing the sharp skew of the existing bridge. This will reduce the need for additional right of way, provide space for additional lanes on I-84, and meet future traffic growth of the area. The DDI does all this and exceeds the expected traffic growth through 2045.
As part of replacing the interchange, the team is replacing the aging bridge structure and improving traffic functions with additional lanes and traffic signals. The team is also building retaining walls along the frontage roads and installing drainage systems and retaining ponds.
There are rocky conditions in the area, and the team needs to excavate the rock to build the DDI. Excavating the rock requires multiple days of blasting. Holes were drilled to the proper depth, loaded with explosives, and detonated to break solid rock into smaller pieces that could be removed via dozer and excavator.
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The team had to determine how to create a safe work zone while allowing traffic to flow along I-84 during the blasting. The contract does not allow the team to close I-84. However, I-84 is sometimes restricted to one lane in each direction at night. There have also been occasional daytime lane restrictions, as well as traffic shifts and lowered speed limits on I-84.
Outside help, however, has been the key to maintaining a safe work zone.
"The Idaho State Police have been a key factor in assisting with rolling closures on I-84 to accomplish safe blasting operations," Wagner said. “Idaho State Police sets up many miles back in either direction. Their presence encourages drivers to slow down or even stop traffic when necessary.”
This creates time in the blast zone for the blast and roadway clean up before the rolling closure arrives.
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ITD has also implemented a Variable Speed Limit (VSL) work zone to provide a safe work/traveling zone for workers and the traveling public.
"A VSL work zone allows for the lowering of the speed limit through the work zone, as conditions warrant, to allow for safer construction of the project," Wagner said. "Then, when conditions warrant, speeds can be raised to allow for faster travel through the work zone, i.e. weekends and holidays when workers are not present."
The National Highway Traffic Safety Administration’s (NHSTA) website says that VSL "may be appropriate on roads with significant variations in congestion throughout the day ... Sensors in the road detect congestion or weather conditions, then automatically lower the speed limit in stages to manage a more even slowing of traffic, delay of congestion onset, and smooth traffic flows. These changes plus lower speed have [the] potential to reduce crashes, including secondary crashes." The NHSTA also cites a study which found that VSL has reduced "crashes (especially rear-end crashes) and speeds in some cases, while improving flow capacity and travel time reliability in others."
The scope included installing a 48-inch irrigation line under the interstate. The team used a jack and bore to install the line. The trenchless method enabled the contractor to complete the work without interrupting traffic.
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Western Construction, Inc., is based out of Boise, Idaho, the state capital. The company was founded in 1961, aiming to be Idaho's source for aggregate crushing and mining operations. They have since evolved to focus on heavy grading and excavation, asphalt paving, rotomilling, and more. Yet, ITD has been particularly pleased with how the contractor has handled the rock excavation element of the project.
"This is a challenging project including rock excavation and sourcing of the large amount of fill needed for the construction of the new elevated divided diamond couplet," Wagner said.
She mentioned other elements that Western has excelled in, including subcontractor coordination, shareholder coordination, safety, and material sourcing.
The contractor has been able to progress the project, as designed.
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"Western Construction has been working in district 4 for over 60 years, and they know the area very well," Wagner said. "They are well aware of work site conditions, available aggregate sources, project-specific material suppliers, and details of the project that need to be accomplished."
The construction budget for the project is slightly below $54.5 million. The team is currently on budget, though the construction bid exceeded the engineering estimate. The federal government is the sole funding agent for the project.
Construction began in spring 2023 and is scheduled to be completed in fall 2026. It is currently on schedule. Wagner cites continual collaboration and coordination with all stakeholders and adjacent projects as the key reasons for keeping the project on schedule.
When the interchange project is completed, drivers will benefit from greater mobility and improved safety. The new interchange upgrades the roadway to current standards and will meet the growing traffic needs of the community and I-84. The lengthened on-ramps will bring them up to current speed and traffic needs. Finally, pedestrians will also benefit from the added sidewalk and bike path on Lincoln Avenue.
