The Idaho Transportation Department (ITD) embraced technology on the new $11.2 million Thornton Interchange with U.S. 20, south of Rexburg, and has no regrets.

"What's special about this project is we did a very detailed 3-D model of the interchange," says Mike McKee, Technical Engineer with ITD. "Then we released that model to all of the contractors at the time of advertising the bid. It worked out very well for us."

The Technology

The 3-D model included construction elevations, piping, utility locations, right of way, fencing and ditches, according to Reed Hollinshead, a spokesman for the department.

The ITD required the contractors to use automated machine guidance systems, with GPS, on the earthwork portion of the project and prevented the bidders from using traditional surveying and slope stakes. The department did not slope stake the job, saving about $100,000.  

"They built it 100 percent from the GPC models," says Wade Allen, ITD Project Engineer.

Not only were the contractors better prepared when bidding, Ryan Day, Lead Designer on the project, says it eliminated some of the risk, so the department got better prices on the project. Additionally, he says, it saved the contractor having to pay someone to develop a model to use with the automated systems. The automated machine guidance on the heavy equipment also ensured a more accurate and timely job.

"The automatic grade control allowed the contractor to build the road to a tight tolerance on every side, the slopes or top layer," Allen adds. "It was built close to our original design, so there was little waste. It was efficiently built."

Both Day and Allen report the automated machine control provides a smoother riding surface.

"We've gotten quite a few complements on the ride of the project," Allen says

Day says the contractor was familiar with working with modeling and automated machine control, but it was a learning process for the department. The contractor provided the department with a GPS rover to spot check and verify elevations. Inspectors also used the rover to collect data for payment measurements, such as lengths, areas, and volumes, Hollinshead says.

ITD used the project as a pilot to see how modeling and GPS grade control would work, and were impressed, McKee says. The department has now let two more projects using models and automated machine guidance.

Project Goals

The project, converted a traditional intersection with left turn lanes into a diverging diamond, with U.S. 20 now running above the county road. The diamond eliminated left turn conflicts, improving safety and reducing delays. The department designed the intersection to reduce accidents and fatalities, and it has worked. This was the seventh and final interchange rebuilt on a 34-mile stretch of U.S. 20 in eastern Idaho, between Idaho Falls and Sugar City, from 2000 to 2016, after U.S. 20 was widened to four lanes. Cost for all seven was $85 million.

"We've been focusing on replacing at-grade interchanges with diamonds," McKee says. "As we've widened U.S. 20, on the county roads that crossed over, we were finding a lot of crashes taking place, coming from the county roads and making left-hand turns."

The improvements have reduced serious-injury crashes by 75 percent, despite a 115 percent increase in traffic volume.

"We're also seeing a steep reduction in fatalities," McKee says. "Fatalities have been reduced to about one per year."

Railroad Company Reduces Costs

Western Construction of Boise, Idaho, began work on the project at Thornton in March 2016 and completed it in November 2016. In addition to the road and bridge construction for the diverging diamond, the scope of work included building access roads for farms, demolishing several structures, realigning county roads, rerouting irrigation facilities and wildlife fencing, and working with utilities to reroute lines.

The project team saved the department $350,000 at the Thornton site when the Eastern Idaho Railroad agreed to let Rocky Mountain Power run its power line in the rail right-of-way, after Day contacted the short-line railroad. Hollinshead explains, ITD had planned to pay two-thirds of the $1.3 million cost of rerouting the power line.

"With the railroad company's concession, the cost of moving the line was cut to $800,000, of which ITD's share was $415,000, Hollinshead says.

The bridges have concrete girders, spanning 74 feet, and mechanically stabilized earth walls. Contractors could use traditional surveying on the bridge structures.

Family owned and operated, Western Construction, founded in 1961, excavated 75,184 cubic yards, brought in 442,585 cubic yards of granular borrow, placed 73,440 cubic yards of 3/4-inch base, and placed 42,500 tons of asphalt pavement.

During construction, ITD required the ramps be built wide enough for two lanes of traffic. Once the first one was complete, the department shifted traffic and built the next set of ramps. The second set was done in time for the peak summer travel season, allowing two sets of ramps with traffic in each direction. The contractor had the middle to build the bridges.

The department also implemented a Bluetooth traffic-control system. Bluetooth detectors at each end of the project could tell when a cell phone or Wi-Fi signal in a car passed through and how long it took. By identifying slow times, officials could look at conditions and take remedial action. Average traffic speed was about 57 mph.

"The delay from normal travel time was less than a minute," Allen recalls.

Western Construction placed four mobile cameras around the job site, so ITD officials could view the images taken by them from inside the office. One camera was a 360-degree, zoom and three were fixed in an area with work progressing. One camera provided images of the project to the public.

"We could see the work as it was transpiring," Allen says.

From design through construction, various technological advances helped the job remain efficient and on time and on budget. The project received an "America's Best" award as a small category project from the American Association of State Highway and Transportation Officials, the AAA motor club and the U.S. Chamber of Commerce. 

"It was built very well," Allen says. "We're trying to learn as technology and information becomes available and apply it to our projects as fast as we can."