Diverging Diamond Interchange Provides Creative Solution to Busy I-80 Intersection in Lincoln
What began as a six lane expansion of a 40-mile stretch of Interstate 80 between Omaha and Lincoln, Nebraska in 1999, eventually became eight different projects. The original timeline for overall completion was set for 2012, but efforts were pushed back due to the 2008 economic downturn. Now in the last 3 miles of that original 40-mile improvement, the Nebraska Department of Roads (NDOR) has employed a creative solution to a future challenge - the diverging diamond interchange (DDI).
In a DDI, the two directions of traffic on the non-freeway road cross to the opposite side on both sides of the bridge at the freeway. Sometimes referred to as a double crossover diamond interchange, the DDI is unusual in that it requires traffic on the freeway overpass (or underpass) to briefly drive on the opposite side of the road from what is customary for the jurisdiction. Like a continuous flow intersection, the diverging diamond interchange allows for two-phase operations at all signalized intersections within the interchange. This contributes significantly to safety, as it improves traffic capacity, eliminates conflicting traffic movements, requires less land to build and is a less costly configuration. The DDI was listed by Popular Science magazine as one of the best innovations in the engineering category in 2009. The first DDI constructed in the U.S. was in Springfield, Missouri, and prior to 2009, the only known diverging diamond interchanges were in France.
Initially constructed in 1961, improvements to this section of I-80 have been maintenance-related projects like pavement and/or shoulder replacement or overlay. The existing cross section is a four-lane highway consisting of two 24-foot roadways divided by a depressed turf median. Each direction has 6-foot inside shoulders (of which 4 feet are surfaced) and 12-foot outside shoulders (of which 10 feet are surfaced). I-80 traffic volumes, including heavy trucks, have continued to increase within this project location, with heavy traffic volumes at 21.4 percent of total traffic in 2009. An NDOR study indicated that upgrading I-80 to six lanes (building three lanes in each direction) would facilitate estimated future traffic volumes.
A Multi-Phase Project
The I-80 DDI project includes eight phases: phases one and two were for work on the bridge crossing over 48th Street, for concrete paving of portions of the interchange and temporary access areas, as well as for culvert, electrical and grading work. The bridge required taking a four lane divided interstate with a depressed grass median and turning it into a six lane road. "It had to be constructed and widened in order to proceed with the DDI below it," said Brian Johnson, Civil Engineer for the Interstate Roadway Design Section of the NDOR. Construction began in July 2014.
Phases three through six were completed throughout 2015 and early 2016, "The portion of NW 48th that encompasses the DDI is about 3,200 feet long, or 6/10 of a mile. This is a significant section of the project," noted Johnson. He says the contract states that the DDI will be complete in July 2016. The crew is currently completing phases seven and eight, with October 2, 2016 as the scheduled completion date for the entire project.
Following what is known as a Calendar Day project schedule, the number for all eight phases of the project was 797 days. "We had an agreement with the contractor for this number of days to finish the project, regardless of weather," said Johnson. However, there is a clause in the contract that if there's a weather declaration by the Nebraska Governor, the crew may be entitled to an extension. "Even with the abnormally large amount of rainfall we had during the spring of 2015, this crew has been able to stay close to schedule," he commented. As weather improved in the spring, a higher demand was required of the workforce. "During wintertime, the only tasks that were completed included bridge work performed under very large insulated blankets with portable heaters," noted Johnson.
With a $36 million budget for the overall I-80 DDI project, project funding came from Federal, State and local dollars and included 2 miles of I-80 and 1 mile of NW 48th Street.
The Federal Highway Administration (FHWA) requires an Interchange Justification Report for any alterations involving an interstate interchange. NDOR hired engineering firm Olsson Associates to look at a variety of alternatives. "While there was not a great deal of development in the immediate area at the time, they forecasted traffic volumes 20 years out from date of construction." The 2012 report showed that 12,000 vehicles per day traveled from the north through the NW 48th Street interchange in 2009, with a predicted number of 58,000 per day by 2034. In comparison, from I-80 traveling east to west at that same point, traffic volume was at 35,000 vehicles in 2009 and projected to be at 92,000 in 2034.
"When six lanes were initially built going east to west, we had no idea there would be so much traffic coming through NW 48th Street. The justification report looked at several different interchange options and finally pointed to the Diverging Diamond Interchange. When the report came in, there were only 10 DDI around the country. Olson Associates said a DDI would handle the project efficiently and save us $1 million, which we initially couldn't believe. So, we went through all the numbers with a fine tooth comb, and they were correct," recalled Johnson.
"Most DDI's are reactionary, built as a result of traffic," he remarked. "Our choice was to be proactive in this case. When we decided to go with the DDI, we began with environmental studies, including the impact to wetlands, to local habitats and to area communities." Since the DDI required a smaller footprint, the design was approved. NDOR felt it was a perfect solution, because by 2012, there was new federal requirements for storm water management (MS4 - Municipal Separate Storm Sewer System) in areas with a population greater than 100,000 people. "The DDI solution changed the way the entire structure was designed to help the water flow because it shrinks the footprint, so there are more outside areas for the water to flow. With partial clover leafs there are different quadrants where water can collect, as opposed to stand on the roadway," said Johnson.
Preparing for Increasing Traffic
After selecting the DDI solution, NDOR then had to design for the future projected numbers from the study but limit the interchange to properly operate for the current traffic. "Since the numbers were so high based on what was predicted in the coming years, we made some adjustments and a phasing of the construction. The 2034 traffic numbers warranted four lanes for the NW 48th Street Diverging Diamond design and assumed a complete build-out of the surrounding land by developers and residences. An agreement was made between FHWA and NDOR to construct the NW 48th Street Diverging Diamond in a "phased" approach. NW 48th Street will be built with three lanes but provide the necessary grading and right-of-way to allow for a fourth lane to be constructed when traffic numbers exceed the "phased" design. "Not only did we have to design for what we ultimately needed but for what was actually going to happen in the near future. There were no sidewalks in the area and since the City of Lincoln is building sidewalks, we ultimately decided to incorporate pedestrian connectivity through the interchange to local business and biking/hiking trails."
Technology helped the project as well, as all surveys, grading and paving were done with the help of GPS. "Things are much more seamless using the GPS to build the necessary models," noted Johnson.
According to Joe Kuehn, Highway Project Manager for NDOR, the General (or Prime) Contractor was Hawkins Construction of Omaha, Nebraska, which handled all concrete road paving and bridge construction, storm sewer and culvert pipe work. Hawkins holds the main contract and has 29 subcontractors on the job that handle grading, hauling materials, guard rails, electrical, seeding/erosion control, and traffic control (signs, barrels, barricades, safety). Over 250 contracted employees have worked on the final I-80 Diverging Diamond Interchange at NW 48th Street project.