By: Jonathan Henderson, P.E., and Smith Siromaskul, P.E.
Travelers in Charlotte, N.C., already depend on I-85 as a vital cog in the Queen City’s transportation network.
Anticipating that traffic on I-85 will increase with the pending completion of the I-485 “Outer Loop,” the North Carolina Department of Transportation (NCDOT) is widening 7 miles of the highway from four lanes to eight. The I-85 project includes construction of two diverging diamond interchanges (DDIs) and associated “superstreet” improvements for two major roadways.
Contractor Lane Construction Corp. and designer HDR Engineering were awarded this design-build project in August 2010. Shortly after, NCDOT asked the team to investigate the feasibility of incorporating the DDI concept for the interchanges at Poplar Tent Road and NC 73. HDR conducted interchange and traffic analyses using VISSIM and collaborated with NCDOT during a three-day brainstorming session to determine whether the DDI concept would meet operational and site requirements.
The DDI immediately appeared to offer benefits; however, we needed to address existing issues with signal backups on the arterials for the DDIs to be effective. For the I-85 widening project, the team tested several design concepts for the arterials to help with the adjacent intersections, including displaced left turns, before settling on the superstreet design. The final design options were tested in microsimulation, and HDR worked with NCDOT to successfully incorporate changes.
Laying it on thick
With a shift required at each of the ramp terminals on the crossroad, another important consideration is maintenance of traffic during DDI construction. Maintaining traffic in its conventional pattern during construction of the two I-85 DDIs requires the use of temporary pavement and wedging to maintain ramp ties. A short closure will be required to shift into the final pattern.
Because the DDI concept is new to North Carolina, the design team paid particular attention to signing and pavement-marking designs and worked closely with NCDOT to develop statewide standards for this type of facility. It was determined that additional signing and pavement markings both through the interchanges and in advance would benefit drivers not familiar with this concept. Overhead signing is one element that typically costs more for a DDI than for a conventional interchange; however, most other standard items cost less. The biggest savings usually can be attributed to reductions in right-of-way (ROW) requirements and bridge area.
A number of factors need to be considered when determining if a DDI is appropriate at a given site. ROW, access control, nearby signals, turning-movement volumes and existing alignments all play a part in determining the best interchange concept for any site.
It’s a . . . superstreet
In the case of the I-85 interchanges, nearby signals posed a significant challenge to the DDIs. Both Poplar Tent Road and NC 73 had problems with queuing at the adjacent signals that would have backed traffic up into the DDIs. The design team used the superstreet concept to remove left turns from the intersections, eliminating those signal phases and improving safety by reducing conflict points. In this concept, where the arterial meets a minor cross street, traffic on the cross street is only permitted to make right-hand turns onto the arterial. A dedicated U-turn area then allows the cross-street traffic to reverse direction and either continue on the arterial or turn right at the cross street to resume the original course.
The superstreet accomplishes this within a limited footprint, which works well in high-density commercial areas such as the I-85 corridor. By improving operations on the arterial, the superstreet design mitigated the potential for backups that would have interfered with the adjacent DDI interchanges.
More than a one-trick pony
While the DDI/superstreet combination may be the star of the I-85 widening project, a unique approach to moving materials and equipment in and out of the construction site deserves its share of the spotlight. To minimize impacts to motorists on I-85 during construction, the project team devised a plan to install a temporary access bridge over the interstate with ramps to the median north of Poplar Tent Road. The ramps allowed hauling operations and construction site access and also were available to emergency-services personnel for incident response.
Lane Construction first considered the plan due to the high volume of daily traffic traveling through the project area. The goals were to preserve the safety of motorists, minimize impacts to traffic operations while providing necessary access for construction equipment and promote an efficient construction process. The temporary bridge and ramps allow ingress/egress for construction crews, equipment and materials to the I-85 median, as well as work areas on either side of the interstate. HDR designed the temporary structure and inspected it upon completion.
The first phase of the project was to build the median of I-85 behind concrete barrier walls. With the temporary access bridge and median ramps located near the center of the project, construction vehicles do not have to enter traffic to access the median.
Material staging occurs at a site immediately west of the temporary bridge. From there, the materials are transported into the I-85 median via the temporary access bridge.
Phase II comprises mainline construction and structure work at Poplar Tent Road and NC 73. Northbound and southbound traffic will be placed on the newly constructed median lanes while construction progresses on the outer portions of I-85. The median access ramp will be removed before shifting traffic; however, the temporary bridge will remain to provide access for construction of the northbound lanes. Access to I-85 southbound can be made directly from the material staging yard.
The innovative use of a temporary bridge and access ramps has been a resounding success, with construction progressing on schedule and limited impact to the 140,000 vehicles traveling this portion of I-85 each day.
Nice tie
Construction on the $125 million I-85 widening project began in the fall of 2011 and is scheduled to be completed in the fall of 2013. When finished, the I-85 interchanges at Poplar Tent Road and NC 73 will be the first operational DDIs in North Carolina.
Jim Dunlop, who oversees congestion management for NCDOT, commented recently that diverging from traditional concepts represents the future in roadway improvements.
“Poplar Tent Road—which from east to west incorporates superstreet design, DDI interchange, then continues as a superstreet to a roundabout—shows how everything can tie together to provide a more efficient solution than the traditional intersection/interchange concept. We have seen that the traditional diamond is not the long-term solution for conditions like this,” he said. R&B
About The Author: Henderson and Siromaskul are with HDR Engineering.
