Ever met anyone who likes waiting in traffic? No one has. One of the aims of nearly every infrastructure project is to increase capacity in some way to improve traffic flow. The big question that needs to be answered is how to improve traffic flow. The South Dakota Department of Transportation (SDDOT) is in the middle of the I-90 and LaCrosse Street Interchange Project, which has multiple objectives, including improving traffic flow. The project is taking place in Rapid City, South Dakota. The town – which has a population of just over 75,000 and is steadily increasing – is about 20 miles north of Mount Rushmore. Other tourist attractions such as Badlands National Park are also nearby; this draws loads of tourists. Besides these elements, Ellsworth Air Force Base, which has been growing, is about 10 miles northeast of the city.

Each of these elements has led to an increase in traffic. A study projected that traffic volumes for North LaCrosse Street will increase by 35 to 65 percent by 2035. The current average daily traffic at LaCrosse Street is 16,000 vehicles and it is expected to reach 23,000 vehicle in 2035. On the interstate, the average daily traffic raises from 33,000 vehicles to 52,000 vehicles. The area where the project is taking place is home to hotels and businesses, which draws local traffic, while the interstate has a good deal of truck traffic.

The issues related to driving in the area were included as part of the 2010 South Dakota Decennial Interstate Corridor Study, and the study determined that the I-90 Exit 59 was one of the top 10 existing interchanges on South Dakota’s Interstate System to target for improvement.

As is often the case when there’s an excess of traffic, the project area also has some safety issues. SDDOT did an Interchange Modification Justification Report that found, “This increased congestion has contributed to elevated crash rates, with the La Crosse Street Interchange ranking 5th out of 126 interchanges evaluated in the South Dakota Department of Transportation (SDDOT) Decennial Interstate Corridor Study (2010).”

There are two primary elements to the project. The first element the team concentrated on was adding auxiliary lanes to I-90 between exits 58 and 59. Over a one-mile span, the I-90 was expanded from two to three lanes in each direction. With this addition, drivers can get on the interstate, remain in the auxiliary lane and get off at the next exit. “In the past, drivers had to merge onto the highway and then quickly exit,” says Jesse Nelson, the SDDOT Project Engineer. “By eliminating the merges, we expect to see improved safety conditions and help keep traffic flowing.”

The second major element of the project is installing a diverging diamond interchange (DDI). This will be the first DDI in the state. The project team behind the Interchange Modification Justification Report looked at a variety of things during the interchange design, including the ability to handle traffic demands, constructability, environmental concerns, landowner impacts, and public input. It was determined that a DDI was the most suitable to handle the issues in the area.

The DDI will lead to fewer conflicting movements and therefore, fewer chances for vehicles to crash into each other. In addition, DDIs can handle increased capacity and reduce congestion. The way they increase capacity is by eliminating the left turn signal for traffic entering I-90 and allowing traffic to flow more efficiently through the interchange. This has been one of the biggest issues in the area, as traffic has been getting stuck on LaCrosse Street and backing up onto the interstate.

Other aspects of the project are related to the main elements. These include bridge modification and construction of a new parallel LaCrosse Street bridge to accommodate the DDI, on- and off-ramp improvements, widening of the I-90 Maple Street bridge to accommodate the I-90 auxiliary lanes, new traffic signals and lighting, pedestrian and bicycle improvements, and utility improvements.

An inherent challenge of working in the area is the short construction season due to the long winter. “It can be snowing at the beginning and end of project,” Nelson says, “so we need to get tons of work done in a short period of time.”

This means sequencing takes on great significance. “Despite the wintry conditions that typically arrive in October, we can’t simply not work then,” Nelson says. “We have to structure our work to accommodate weather challenges.”

For this project, the team worked in a unique sequence as it has many phases and is complex. Adding to this issue is the challenge of being constrained space-wise on the interchange element of the project. Despite the lack of space, the team is required to keep traffic moving while construction takes place.

Nelson credits the prime contractor, Complete Contract, Inc., who has completed many highly complex urban projects with SDDOT in the past. Because of their experience, the contractor has been able to find some efficiencies and move the project along.

“They are good at keeping projects on schedule and they are on top of their subs,” Nelson says.

The project began in February 2021 and construction expected to be complete by summer 2023. So far, the project is on schedule. The team has already completed the auxiliary lanes on the interstate and is currently working on the DDI and related elements.

The construction budget is $17.5 million, and it is being financed via the traditional 80/20 split with the Federal government paying the former and the state the latter. The project is on budget. Nelson credits the designers and planners. “The team thoroughly designed the project and there have only been some very minor change orders and no surprises.

Upon completion of the I-90 and LaCrosse Street Interchange Project, tourists and locals alike will experience less traffic in the area and improved safety. These standards are worth striving for.