Just about any roadwork causes some disruption to traffic. The U.S. 12 and 21st street Intersection in Lewistown, Idaho, which is located near the Idaho/Washington border, was recently reconstructed. Referred to as “Central Idaho’s busiest intersection," traffic concerns and congestion were a major consideration on the project. Upon completion, the project enabled improved safety and traffic flow.

What makes the U.S. 12 and 21st street intersection so busy? The intersection serves the biggest industrial area in the region (which includes the Clearwater Power Co. and the Idaho Forest Group). Therefore, there’s significant commercial traffic in the area. Average daily traffic in the area is 25,000 to 30,000.

Over the course of the project, the intersection was redesigned, a new, larger storm sewer trunkline was installed through a partnership with the city, and the road was reconstructed. The project began in April 2019 and the plans called for it to be completed in three phases. The entire project was complete in late September.

Time-consuming Turns and Traffic Concerns

Before the redesign, two left turns from to get to East Main Street street were unsignaled. Instead, there were stop signs, and commuters – along with large commercial traffic – had to wait for gaps in traffic. “Prior to the work, the level of service was rated F,” says Curtis Arnzen a Resident Engineer with the Idaho Transportation Department (ITD). Arnzen was part of the design team that managed the consultant and served as the resident engineer during construction. Upon completion, these primary movements will be controlled by a signal, eliminating times when traffic waiting to turn caused delays for through traffic. 

The time-consuming turn led to several issues. “There were back-ups on to the Clearwater River Memorial Bridge and safety was an issue as the area had a number of low severity accidents,” said Arnzen.

During construction, 21st street was shut down. In order to keep traffic flowing, ITD took multiple measures. Informing the public of the upcoming work happened in a timely fashion, “We started with a pre-construction open house where the city of Lewistown talked about the improvements,” said Megan Sausser, a Public Information Specialist for ITD. “The event was well attended, and we collected attendees’ emails to provide them updates and details.”

In addition, the detour that was set up worked well according to Arnzen who credits its success to good planning. Ultimately, IITD received many compliments and few complaints from the public about the handling of the traffic.

A Multi-Phase Project

The project was completed in three phases. During phase one, the longest of the three at five months, 21st street was rebuilt. The pavement, which had been made up of a plant mix, was in poor shape due to all the commercial traffic it had to withstand. It was replaced with 12 to 14 inches of concrete in order to withstand the heavy loads.

This phase also included stormwater improvements. The previous stormwater trunk line was at a 90-degree angle, which resulted in friction loss at the manhole. It was also undersized and resulted in occasional issues of water over topping the system. “The new stormwater trunk line is a 60-inch system that can handle a 20-year event and will mean less issues,” says Arnzen. “It’s also in-line with the system as has less friction loss.”

Phase two of the construction lasted approximately a month. It saw work completed on other streets in the area and the closing of an intersection. Parts of a local park were also closed. The final phase of the project, which took another month, included reconstruction of section of the highway near Clearwater Memorial Bridge.

Scheduling of the project was well conceived as evidenced by it being completed six weeks ahead of schedule. Arnzen credits the contractor for this success. “There was a clause in the contract that provided the contractor an incentive for each day early and loss per day if late. It was important to them and they took advantage of it,” says Arnzen.

To meet this incentive, the contractor had two crews – a morning crew and a late-night crew – working through phase one. Arnzen also felt the contractor did a good job of scheduling subcontractors who responded well.

Construction Challenges

Arznen identified a couple of challenges involved with the project. The trunkline was placed 13 feet below grade line while a water line was placed beneath that. “There were a number of underground utilities including a water line and storm drains that we had to work through,” says Arnzen. 

During the excavation, fine grade soil was removed and replaced with solid granular gravel, a better construction material since it helps with drainage, compacts better, and there’s no need to worry about settlements.

Proximity to a beloved local park, Locomotive Park, was also challenging. The park has a historic train, many different species of trees, and plays host to Winter Lights, an extravaganza that lasts from Thanksgiving to New Year’s Day. Due to the construction, the size of the park was slightly reduced and some trees had to be moved. Plans were made to impact the park as little as possible.

Previously, the intersection was highly irregular, with opposing movements offset rather than coming into a single point. Today, it’s much more basic and resembles a standard intersection, ensuring more movements can be allowed concurrently and safely. “With the redesign, we boosted efficiency enabling the intersection to handle large volumes of traffic,” says Arnzen. He also notes that the expectation is the new construction will last 40 to 50 years.

The $7.5 million contract came in on budget. Arnzen credits this to accurate design and good construction inspection.

With the redesign of “Central Idaho’s busiest intersection," travelers in the area have a safer, quicker, and smoother commute while ITD has less maintenance work. Let the industry in the area keep on flowing.