U.S. Department of Transportation
Federal Highway Administration
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Federal Highway Administration Research and Technology
Coordinating, Developing, and Delivering Highway Transportation Innovations
This magazine is an archived publication and may contain dated technical, contact, and link information.
|Publication Number: FHWA-HRT-13-005 Vol. 77 No. 1 Date: July/August 2013|
Publication Number: FHWA-HRT-13-005 Vol. 77 No. 1
Date: July/August 2013
The Tar Heel State uses design-build-finance procurement, turbine interchanges, and superstreet intersections to speed up construction for tackling traffic congestion.
|This aerial photograph, shot in April 2013, shows construction of the I–85 and I–485 turbine interchange near Charlotte, NC, one of three major projects underway in the region. Traffic on I–85 is moving through the center of the picture, while construction on the final leg of I–485 is shown on the left. Traffic on the right is merging from the existing portion of I–485 to I–85 north and south.|
Like other booming urban areas, North Carolina’s largest city, Charlotte, was finding its interstates bursting at the seams. Mecklenburg County’s population had grown from 695,000 to 919,000 between 2000 and 2010. The 2010 Census reported that more than 1.7 million persons were living in the six-county region, including Mecklenburg.
The city needed three projects completed urgently to address its gridlock. Two vital corridors near Charlotte -- in northeast Mecklenburg County and southern Cabarrus County, I–485 and I–85 -- had separate but similar issues. The final segment of I–485, the beltway surrounding Charlotte, needed to be completed to link I–85 and I–77. The interchange connecting I–485 and I–85 had to be completed, as only one side of the loop had been constructed and was in use. And the existing lanes on I–85 from Charlotte to neighboring Cabarrus County needed expansion to handle future increases in traffic north of the reconstructed interchange.
As with most States, however, the funds needed to improve and expand the highway infrastructure were inadequate. So the North Carolina Department of Transportation (NCDOT) faced the quandary of not only doing more with less, but also each project was necessary to complement the others, yet two of the three were not slated to start until 2015, at the earliest. Meanwhile, motorists fumed.
Stepping out of its customary strategies, NCDOT found innovative solutions. By adopting design advances new to the State, the three-pronged effort would commence -- and even conclude -- several years ahead of the kickoff date initially projected. With accelerated starting dates in 2010, the I–485 beltway, the I–485/I–85 turbine interchange, and the I–85 widening projects are scheduled for completion in 2014 -- approximately a year ahead of when they were to be started under the old schedule.
|This map shows the location of the three Charlotte-area projects.|
In 2009, a competitive business environment among construction contractors promised savings of up to 20 percent on the project estimates developed by NCDOT engineers. At that time, however, only one of the three projects -- the widening of I–85 -- was scheduled for construction within a few years -- in fact, not until 2012. Because these projects related closely to each other, NCDOT wanted to fast-track all three so they could be built concurrently and with the favorable contract savings. To do this, NCDOT planned to introduce the design-build-finance procurement model.
Although other States such as Florida have employed this model successfully, these projects would be the first in North Carolina to use this concept. The design-build standard had been employed already in more than 30 projects statewide, including several in the Charlotte area. But addition of the financing component meant the contractor would contribute a portion of the project cost, presumably through bank financing. Future funds appropriated by the North Carolina General Assembly would reimburse the contractor. Under this model, NCDOT would fund completion of I–485 and the modification of the I–85/I–485 interchange.
This procurement method would yield several benefits. The projects would be synchronized to be built and completed at least 5 years ahead of schedule, while introducing the State to fresh innovations. Moving forward, NCDOT would team up with project contractors, design firms, and inspectors to simplify issues with utility coordination, environmental stewardship, and traffic control in work zones. Knowing that these projects were integral to the region, open communication and transparency would prove to be critical to their success.
“We knew close coordination would be crucial with the magnitude of work going on simultaneously,” says Barry Moose, P.E., retired NCDOT Division 10 engineer.
“These three projects are significant in moving people and goods through our State,” Moose added. I–485, the beltway surrounding the city, is one of the most important routes in the region. The loop totals 67 miles (108 kilometers) around Charlotte’s city limits and smaller communities in Mecklenburg County. Motorists use the loop to bypass heavily traveled surface streets and to reach outlying portions of Charlotte’s landscape. NCDOT has designed and built segments of the highway over the past three decades, with the remaining 5.7-mile (9.2-kilometer) section under construction since July 2011.
Upon completion in late 2014, this crucial piece will provide not only better mobility for residents in the northeast quadrant of Mecklenburg County, but also will link I–77 and I–85, two neighboring interstates that move goods, services, and more than 150,000 commuters daily. Having this final segment in place will reduce the need for motorists to drive other arterials to reach their destinations.
|This map developed for a public hearing on the projects shows a proposed diverging diamond interchange design at I–485 and Mallard Creek Road. The project will be one of the first diverging diamonds in North Carolina. Two more are currently under construction as part of the I–85 expansion project in Cabarrus County.|
The $139.5 million project will feature an eight-lane freeway, anticipated to carry more than 155,000 vehicles per day by 2035. NCDOT will rebuild the existing interchange of I–485 and NC 115 (Old Statesville Road) at the west end of the project. The agency also will build new interchanges in Charlotte’s university area. Although this project might sound routine, these interchanges will feature innovative designs, such as split-diamond and diverging diamond interchanges, that will better accommodate future traffic volumes, save money, and improve safety. Local roads currently bearing the brunt of heavier traffic will soon become calmer.
The first new I–485 interchange will be a split-diamond interchange, with a one-way service road connecting the interstate’s entrance and exit ramps. Six roundabouts also will be included in the finished project. Modern roundabouts differ from the older style traffic circles in that they are smaller, have reduced traffic speeds, and offer safety benefits to pedestrians and bicyclists. In recent years, NCDOT has built roundabouts in several locations, including the intersection of I–485 with West Moores Chapel Road in west Charlotte.
The other new I–485 interchange will feature one of three diverging diamond interchanges associated with the three projects, and it will be the first of this type of interchange to be built in North Carolina. According to the Federal Highway Administration’s (FHWA) technical brief, Double Crossover Diamond Interchange, the concept is relatively new to the United States, with the first diverging diamond interchange opening at I–44 and State Route 13 in Springfield, MO, in 2009.
As noted in an NCDOT newsletter from February 2011, I–485/I–85 Improvements, for interchange ramps that carry a high volume of vehicles, a diverging diamond interchange improves turning movements, traffic flow, and safety. The design eliminates the signalized left-turn phase at the two intersections within the interchange by shifting the crossroad traffic to the left side of the roadway between the ramp terminals. This change in the crossroad configuration improves safety by reducing the number of traffic conflict points and improves traffic flow by reducing the number of signal phases. The interchange is less costly to build because it uses less right-of-way. Signing and pavement markings help drivers become acclimated to the interchange layout.
The location of this interchange requires the reconstruction and alignment of several existing roads. This aspect of the project has provided the opportunity for citizens to be engaged in renaming some of the new alignments, with assistance from the city of Charlotte and Mecklenburg County.
NCDOT has divided the plan into three sections: west, central, and east. This division has proven helpful in determining the controlling operations in each area and defining action items for the agency and the project’s contractor.
|A new roundabout at West Moores Chapel Road and I–485 improves traffic flow and allows safe navigation for vehicles both large and small.|
NCDOT held citizen information workshops in 2009 to present proposals for improvements to the I–485/I–85 interchange in northeast Mecklenburg County. Initially, plans included building a trumpet interchange, along with a modified half-cloverleaf on a separate level above the trumpet interchange, creating a four-level “stack” interchange to connect the two interstates. The plan, dating back more than 20 years, was not an ideal model for accommodating the impending traffic volumes.
About a month before the deadline arrived for the submittal of final designs, NCDOT engineers introduced a configuration -- the turbine interchange -- new to North Carolina. The plan proposed a two-level alignment with directional ramps that curve around a central bridge in a circular pattern. A bird’s-eye view of the design gives the appearance of swirling ribbons, but this innovative design would save millions and complement the environment because it uses less right-of-way, contains smaller bridges with smaller columns, and reduces the impact on traffic as no long, offsite detours are required. Traffic safely transitions from one road to the other at speeds of up to 65 miles per hour (mi/h) (105 kilometers per hour, km/h).
Preliminary estimates from NCDOT approached $124 million. The State awarded the project in September 2010 for $92.2 million, about $11 million less than the next bid, and 26 percent less than NCDOT’s projected cost estimate.
The turbine design, under construction since September 2011, provides several advantages in addition to cost savings. The design has 19 smaller bridges, of which 10 are single-span structures. These bridges have smaller spans and bents than would have been used in the trumpet design, making their construction easier. Also, the majority of bridge construction with the turbine design occurs without affecting traffic, simplifying traffic control measures. The overall design leaves a smaller footprint, is simpler to maintain, and improves sight distance for drivers.
The I–485/I–85 interchange also includes replacing the existing bridge taking Mallard Creek Road over I–85, located north of the turbine’s location. The bridge allows construction of new entrance and exit ramps connecting I–85 to the new interchange. The bridge opened in the fall of 2012.
|In this diagram of the proposed $92.2 million turbine interchange at I–85 and I–485, a separate structure to the right will replace an existing bridge that takes Mallard Creek Road over I–85.|
NCDOT designed the interchange for projected 2035 traffic volumes ranging from 142,000 to 190,000 vehicles per day. The interchange modification will improve local and regional travel, relieving congestion on parallel roads in north Mecklenburg County. The department anticipates completion of the turbine interchange by summer of 2014.
The third piece of the puzzle straddles the Mecklenburg/Cabarrus county line. The I–85 corridor routinely bottlenecks from Charlotte’s university area well into the city of Concord, as the number of general-purpose lanes narrows from four to two in less than 3 miles (5 kilometers). Additional lanes were needed to accommodate traffic merging from the turbine interchange to the south, providing a higher level of service on I–85. However, existing interchanges would have had to be rebuilt in order to accommodate these additional lanes. This project presented another opportunity to showcase design firsts and innovative construction methods.
The proposed $125.2 million project will expand more than 6 miles (10 kilometers) of I–85 from four to eight lanes from south of Bruton Smith Boulevard -- the location of two major attractions, Concord Mills shopping mall and Charlotte Motor Speedway -- to NC 73 in Concord. Interchanges at Poplar Tent Road and NC 73 will be converted from two- to four-lane divided roadways and will be among the first diverging diamond interchanges in the State.
“Building these interchanges will save the taxpayers about $50 million,” says Davis Diggs, P.E., NCDOT’s resident engineer overseeing this project and the turbine job. “They require less right-of-way to build and retain their level of service longer over the life of the structure.”
Another distinct element of this project will be a superstreet intersection, also known as a restricted crossing U-turn intersection. With the superstreet design, vehicular traffic from the minor street may not proceed straight through or make a direct left turn. Instead, these movements are accomplished by a right turn onto the major road, followed by a U-turn at a median crossover downstream of the main intersection, then proceeding back to the main intersection for a right turn (to complete the original through movement) or continuing straight (to complete the original left-turn). This reduces conflict points at major crossovers, minimizes travel time, and increases safety. The Poplar Tent Road interchange will feature a superstreet, and the interchange at NC 73 also will contain a superstreet.
“Putting these elements together results in moving traffic more efficiently,” says Louis Mitchell, P.E., NCDOT’s Division 10 engineer. “We’re moving away from conventional concepts to incorporate ideas providing better solutions for years to come.”
The hefty responsibility of keeping travel lanes open on I–85 during construction set the stage for another new idea -- utilizing the median. Delays on this segment were the norm, as more than 118,000 vehicles per day would clog the travel lanes.
The prime contractor used materials from previous projects to build a temporary work bridge north of Poplar Tent Road with ramps on both sides, enabling heavy equipment to haul materials from offsite into the median without interrupting interstate traffic. While local residents are pondering the possibility of a new interchange being built near Poplar Tent Road, the primary use of the temporary bridge has been solely to provide access to the project.
|Currently, traffic on I–85 south is shifted to a new inside lane and median shoulder, while the original lanes are rebuilt on the outside. Traffic is using this temporary exit ramp until work on the diverging diamond interchange at Poplar Tent Road is completed.|
The temporary bridge idea already has proven itself to be beneficial, as motorists have consistently had two travel lanes in each direction to use, with the exception of scheduled nightly lane closures. The entrance ramp to the south end of the project already has been removed; once the project is completed in 2014, the bridge will be dismantled.
The contractor has erected a temporary barrier wall within the project limits to keep workers safe from interstate traffic while the new lanes are being built in the median. With limited access to the shoulders, motorists encounter a “cattle chute” effect while driving through the corridor. The contractor created seven emergency pulloff sites for motorists so that travel lanes would not be blocked in the event of a crash. The contractor installed additional “No Parking” signs to discourage motorists from abandoning vehicles, which would pose a safety risk. Traffic engineers with NCDOT also issued ordinances to reduce the speed limit on I–85 to 55 mi/h (89 km/h), canvassing both the interstate widening and turbine interchange projects, which are literally overlapping. NCDOT reduced the speed limit to protect workers present in the median, as well as the public. The speed reduction will remain in effect until the project is completed.
NCDOT officials have been meeting regularly with county and municipal emergency responders to improve communications and incident clearance in the work zone. Furthermore, the North Carolina State Highway Patrol has maintained a strong presence in the area, targeting aggressive drivers and speeders in the work zones.
The engineering advances of these projects, coupled with the efficiency of their scheduling, have received recognition from State and FHWA representatives. In February 2012, FHWA Administrator Victor Mendez visited Charlotte to recognize the progress of these high-profile jobs, complimenting NCDOT’s investment in its infrastructure, and raising the bar for other States to follow.
|Traffic has been shifted to the new lane and median shoulder on I–85 south on a segment of NCDOT’s expansion project in Cabarrus County. The contractor erected a barrier wall in the median to protect project workers and the traveling public.|
These projects are tied together by creative problem-solving and innovative thinking that will bring lasting benefits to North Carolina’s transportation system. These plans will help fulfill NCDOT’s mission of “connecting people and places safely and efficiently, with accountability and environmental sensitivity.”
|The contractor built this temporary bridge over I–85 in Cabarrus County using materials from previous projects. The crews use the bridge to bring materials from offsite to the median of I–85 without interrupting interstate traffic.|
Instead of beginning work in 2015, these projects will be at or nearing completion at that time. Collaboration among NCDOT, the contracting community, and FHWA successfully moved the timeline forward to accomplish significant engineering successes. In the near future, these projects, known as “the Big Three,” will become North Carolina’s transportation trifecta.
Jen Thompson is a communications officer with NCDOT. She has a communications degree from Barton College and serves as NCDOT’s point of contact for media outlets in the greater Charlotte region.
For more information, visit http://www.ncdot.gov/projects/CharlotteOuterLoop, or contact Jen Thompson at 704–507–3262 or firstname.lastname@example.org. For more information on alternative intersection and interchange designs, see FHWA’s Every Day Counts Web site at www.fhwa.dot.gov/everydaycounts.