What is going on with Interstate 17? Massive expansion underway north of Phoenix

The massive expansion of Interstate 17 from Black Canyon City to Sunset Point is tough to miss, with hundreds of workers and dozens of pieces of heavy machinery grinding around the clock to add new lanes.

I-17 connects approximately 5 million people in metro Phoenix with cooler weather and recreation amid pine trees in places such as Flagstaff and Prescott, and offers residents in those northern communities the quickest route to the state's largest city.

Work began last year, and travelers heading north from Phoenix on I-17 now see the first signs indicating they are in for 23 miles of construction as they approach the community of Anthem, at milepost 228.

The smell of fresh asphalt is apparent by the time motorists hit Table Mesa Road 8 miles farther.

People stopping for fresh pie or to stretch their legs at Rock Springs might notice the median packed with saguaro and ocotillo removed from the area and packaged for transplanting.

The most dramatic work appears at milepost 245 as I-17 heads sharply uphill, and heavy machinery can be seen clinging to the cliff side and chiseling away at the rock.

By the time travelers pass the last saguaro and top out at the windswept grassland around the Sunset Point rest area, the scope of the project becomes apparent. There, workers crush boulders from rock piles larger than houses.

For travelers heading south, Sunset Point is where the 23-mile construction zone begins.

Here's what to know about the project, when it will finish, and what exactly is being built.

Is the Interstate 17 project on schedule? When will the Interstate 17 project be finished?

The project is scheduled to be finished in 2025. The Arizona Department of Transportation does not yet estimate which month it will be done.

When should I avoid travel on I-17 if I can?

The interstate frequently shuts down in both directions south of Sunset Point from 10 p.m. to 11 p.m. to allow crews to blast and clear away rock. If possible, motorists should avoid travel around those times. Those closures require crews to begin narrowing the roadway to one lane in both directions at 7 p.m.

While ADOT tries to limit the closures to one hour, the department warns that the blasting could actually take place from 10 p.m. to 4 a.m.

The work also requires frequent lane restrictions, which can lead to traffic backups. These also affect both the northbound and southbound lanes.

ADOT sends weekly emails for people who want to plan trips around these restrictions. And it posts the closures online. People can subscribe to the updates and track closures and restrictions at www.improvingi17.com.

The blasting is expected to continue into mid-2024, said Annette Riley, Interstate 17 Improvement Project manager for ADOT.

What are they doing on Interstate 17?

ADOT is adding a lane in each direction between Anthem and Black Canyon City, replacing two bridges and widening 10 others. Then between Black Canyon City and Sunset Point, ADOT is adding 8 miles of "flex lanes."

What is a flex lane? Are the new I-17 lanes going to go north or south?

The flex lanes are a separate, two-lane roadway that can carry traffic north or south, depending which is needed most at the time. For example, the lanes likely will flow north on Fridays when the majority of traffic is from people heading out of the Phoenix area. Then the lanes can carry traffic south on Sundays as travelers head home.

The two lanes will be next to the southbound I-17 lanes and will be separated from those lanes by a concrete barrier.

The flex lanes will actually use much of the existing southbound lanes while new southbound lanes are built next to them.

This sounds complicated. Why didn't they just add one lane going each direction?

ADOT officials considered simply adding additional northbound and southbound lanes to this section of road but decided against it because the existing northbound lanes are in such rocky, hilly terrain that adding a single lane would have prolonged the entire project and caused substantial traffic delays and closures during the work.

ADOT also considered paving the rough, dirt road that runs parallel to I-17 from Mayer to Bumblebee to provide a relief route to the interstate. But that idea was scrapped because that road is too curvy for big trucks to navigate.

What if people go the wrong way? How will people know how to use flex lanes?

ADOT officials have said that overhead signs indicating when the lanes are available to motorists will be clear, and that the design will ensure travelers can use the flex lanes without confusion or fear of traveling the wrong direction at the wrong time.

The system of flex lanes also will include "a slew of gates" and a barrier to prevent drivers from entering the lanes in the wrong direction.

"Although this flex-lane concept on a freeway is new to Arizona, it is not new nationally," Riley said. "Many other states have implemented this system, actually more in an urban setting, which has even higher traffic volumes and a daily switch (in the direction of traffic)."

A gantry system, where signs and other equipment are supported by large U-shaped poles over the roadway, will support a net barrier to stop or slow drivers who don't heed the overhead signs indicating the lanes are closed and somehow get beyond the series of gates, she said.

"We take this very seriously," she said.

ADOT has a video explaining how this works online at www.improvingi17.com/flex-lanes.

Why is this project needed?

I-17 is the main connection between metro Phoenix and Flagstaff, Prescott, the Grand Canyon and Page/Lake Powell.

The main challenge on I-17 is an approximately 45-mile stretch immediately north of Phoenix between Anthem and Cordes Junction. Traffic is lighter north of that point because some motorists head west on State Route 69 to the Prescott area from there.

Southbound traffic is heavier south of Cordes Junction because it is the point on I-17 where travelers from both the Flagstaff and Prescott areas converge on the way to Phoenix.

When there is a crash that blocks traffic in that area, ADOT is forced to recommend drivers either detour east through Payson, or west through Prescott and Wickenburg to get between the Phoenix area and the north country.

Both options are hours longer than I-17, but traffic also can be stopped or backed up for hours, making those long detours the best option, other than staying put.

If drivers find themselves stuck behind an accident in the stretch from Anthem to Cordes Junction, there are no alternative routes and few places to even turn around and head the other direction.

How many cars travel on I-17?

About 40,000 cars travel both directions from Anthem to Cordes Junction daily, and on weekends that number hits 55,000 to 60,000. Holiday weekends bring even more drivers.

Increased traffic brings increased crashes and delays.

Most of the troublesome crashes are on the southbound lanes between Sunset Point and Anthem, ADOT officials have said previously.

How much does this project cost?

ADOT's current budget estimate for the I-17 project is $522 million.

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What happens to cactus and native plants that are removed from the construction site?

Many native plants and trees can be seen stored in the median around the construction site. ADOT plans to transplant as many of the native plants as possible around the roadway once the work is finished.

ADOT also will reseed the disturbed areas with native plants.

How many people are working on this?

The primary contractor is the Kiewit-Fann Joint Venture, a partnership between Kiewit Infrastructure West, a subsidiary of an Omaha, Nebraska, company, and Fann Contracting of Prescott.

They have about 150 people working, Riley said.

Additionally, ADOT has about 45 people working on-site, and subcontractors move in and out as needed for specialty work, she said.

What about all of the rock they are removing? Where's that going?

ADOT intends to use all of the material being removed to make way for the new road surface within the project for embankments, protection for bridges and other needs, Riley said.

Some of the large rocks are being crushed to the appropriate size to be used in pavement, she said.

The project is referred to as "balanced" because the source material is all on-site, meaning no hauling rock in and out of the work site.