A Recipe for Congestion?

Highway reconstruction can devastate intelligent transportation systems, which often depend on underground cabling for communications.

With more and more large-scale ITS deployments occurring around the country, a new challenge facing highway engineers is keeping those systems running during roadway con- struction or rehabilitation projects. Many systems, such as the Northern Virginia Traffic Management System, also serve critical regulatory functions, such as controlling access to reversible lanes of interstate roadway, and can tolerate very little down time.

This article explores methods that were used to keep ITS equipment operational and to manage traffic congestion during a major interchange reconstruction and details the lessons that were learned in the process.

The Northern Virginia Traffic Management System (TMS) is a computerized highway surveillance and control system that manages traffic along I-95, I-395, I-495 and I-66 in the Washington, D.C., area. The original system was constructed in the early 1970s and has since been expanded to cover a combined distance of 65 miles using an underground fiber optical cable communications system. The Virginia DOT controls the system from their “Smart Traffic Center” in Arlington, Va.

TMS operators and maintenance personnel cringed at the announcement that one of the area’s most notorious interchanges, the Springfield Mixing Bowl, would soon be reconstructed. Located about 15 miles south of Washington, D.C., the interchange sat directly over one of the TMS’ main trunk cables, which carried signals to approximately 25 CCTV cameras, 15 access control gate sets, 70 VMS and 60 detector stations on I-95.

Dubbed “The Interchange from Hell” by the Wall Street Journal, the Springfield Mixing Bowl carries 400,000 vehicles per day on 14 lanes that mix three interstates and one state highway together in a 3/4-mile span.

In addition to severing trunk communications, the reconstruction also would affect several traffic management devices within the interchange.

Another major concern to VDOT officials was managing traffic congestion during construction. Public acceptance and support for the new roadway interchange were critical given the major inconvenience that constructing the roadway would pose. VDOT awarded a design contract to HNTB Corp. for all aspects of the civil and ITS design, as well as the development of a congestion management program to limit the negative impacts the reconstruction would have on the area’s many commuters.

Maintaining existing equipment

Maintaining the existing equipment throughout the construction period was a daunting task. The trunk lines were scheduled to be severed during the first few months of the roadway construction, which meant several ITS-related activities fell on the critical path. These activities included rerouting the main trunk cable around the interchange, installing wireless communications to the existing equipment and implementing temporary equipment to replace existing devices that could not be maintained. Much of the existing and temporary equipment would then be replaced with new, more modern equipment as the final roadway took shape. The rerouting of the trunk cable and the installation of the wireless communications equipment and temporary field devices made up what came to be known as the Interim Traffic Management System. The Interim System would operate during the final construction of the interchange and then be replaced almost entirely by a Final System, which would communicate over a new fiber-optic cable installed as part of the reconstruction.

Perhaps the biggest challenge was maintaining communications to the myriad of equipment installed along I-95 over 19 miles south of the interchange. This meant re-routing the fiber-optic trunk cables around the interchange to the Smart Traffic Center (STC). Other communication alternatives were considered, including long-haul microwave links between each of three communication hubs and the Smart Traffic Center.

In the end, it was the reliability of maintaining a fiber link to the STC that influenced the decision to attempt what seemed to be a difficult and costly rerouting of the fiber. Installing the rerouted cable along arterial roadways and adjacent railroad right-of-way were promising options early in the design; however, a fiber installation on an adjacent corridor by a federal agency proved to be a coincidental stroke of luck. A resource sharing agreement was made that involved the federal agency’s contractor installing a dedicated fiber-optic cable for the TMS, bypassing the interchange entirely.

This contractor also was asked to run a spur off this cable to the top of a 15-story building adjacent to the interchange called the Springfield Tower. This cable would provide a link between wireless receiving equipment installed in the rooftop of the tower and the STC, providing a valuable means to keep existing equipment running throughout construction.

The most critical devices to be maintained within the limits of the interchange were three sets of gates controlling access to the reversible roadway lanes. Communications with the gates was not only critical to opening and closing them at the appropriate times, it also was necessary for the operation of a safety interlock system that ensured no conflicting gate sets could be open at the same time. Because continuous multipoint RS-232 communications are required for the interlock system, spread spectrum radio was chosen to maintain communications to the gate controllers when the interchange construction severed the existing fiber-optic cable.

In addition to providing information about roadway conditions, dedicated VMS also are used in a regulatory capacity, controlling access to the reversible HOV lanes of the roadway. Maintaining control of these signs was planned using cellular telephone communications over a standard dial-up connection to the STC. Dial-up connections for the signs were chosen both for their economy and their simplicity.

Sequence of construction

The sequence in which ITS components were added and removed from the traffic management system during the roadway construction was carefully choreographed in order to keep the existing system running while providing the necessary regulatory control for the reversible HOV lanes. This demanded careful design coordination among all the civil design aspects of the project. Every roadway construction activity, such as field grading or the opening of a detour ramp, had to be analyzed for its effect on the existing TMS and on the Interim System. For every roadway construction milestone in the project, a corresponding ITS milestone also was planned to meet the goal of minimal system down time.

After the reroute of the existing trunk  early in the project, the construction sequence planning focused on the reversible roadway access control gates and VMS. A number of temporary gates and signs were installed throughout the project to replace existing equipment demolished by the roadway construction and to facilitate any shifting or rerouting of HOV traffic during the construction of the roadway. As traffic was shifted to detours and detour ramps, the temporary gates and signs were moved accordingly to continue regulating entry to the reversible lanes.

When a certain amount of down time for the gate controllers was inevitable at various points during the roadway construction, the contractor was required to open and close these gate sets by hand.

Congestion management

HNTB and VDOT embarked on an aggressive and comprehensive congestion management program very early in the project. Four distinct working groups were set up to explore various congestion relief measures, with each group reporting to a steering committee consisting of high-level county, state and federal officials.

The Transportation Demand Management Group explored a variety of alternative commute programs to give commuters more choices in avoiding the construction delays.

An Incident Management Group implemented extensive measures for decreasing incident response times and improving the incident response capabilities.

The Local Area Network Operations Group examined the local streets and recommended spot improvements to handle the additional traffic volume created by motorists diverting to avoid construction.

Finally, a Communications Group worked in concert with VDOT’s communications director and a private public relations firm to disseminate commuter information to area motorists. A website (www.springfieldinterchange.com) was developed providing commuters with upcoming construction phasing, detour routes and real-time traffic information to help them make informed decisions about their commuting options.

Lessons learned

With more and more traditional highway and bridge projects involving a substantial ITS component, significant challenges are being created for ITS engineers and managers to keep existing systems running and to coordinate the civil and ITS aspects of the design. With TEA-21’s emphasis on mainstreaming ITS into general highway funding programs, these combined projects will become more and more common.

The following lessons learned focus on challenges created by combining these types of projects.

The relative contract value of roadway construction versus ITS deployment in these types of combined projects can create numerous problems in itself. For the Springfield Interchange, the total construction value of the ITS component of the project was only 5% of the overall contract value.

As such, contractors tended to place little importance on the ITS aspect of the project, despite the fact that ITS work was on the critical path. For such projects, the contracting agency is placed at the mercy of the prime contractor in choosing the subcontractors that work on the job, because their subcontract bids have little effect on the overall bid price.

The lesson here is to split out as much of the ITS construction as possible into a separate contract, but keep the ITS and civil design task intact to ensure that the necessary design coordination takes place. The designers should determine which parts of the ITS design should be separated into a distinct ITS contract. The ITS contract should be let as far in advance of the roadway project as possible.

Another important lesson learned was the value of a carefully planned sequence of construction. No matter how much ITS work is accomplished in advance of the roadway construction, complex systems usually require more ITS work during the roadway construction.     TME