News Release
NC State Engineers' Interchange Design Minimizes Right-of-Way
Media Contact(s)
Kathi McBlief, College of Engineering, (919) 515-2283
June 11, 2007
FOR IMMEDIATE RELEASE
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The nano-interchange relies on all-directional connections, combinations of left-and right-hand entrances and exits and four levels of freeway and ramp structures to minimize right-of-way. Image courtesy of Meredith L. Harris. |
Transportation engineers at North Carolina State University have designed a “nano-interchange” that requires less real estate, making it a good alternative for high-density urban areas where land is scarce and expensive.
Dr. Joseph E. Hummer, professor of civil, construction, and environmental engineering, and recent master’s graduate Meredith L. Harris have designed a unique four-level freeway-to-freeway interchange that saves about 10 to 30 acres of land as compared to the conventional four-level interchange. The nano-interchange relies on all-directional connections, combinations of left- and right-hand entrances and exits and four levels of freeway and ramp structures to minimize right-of-way acreage. Hummer calls it the “world’s smallest free-flowing interchange.”
The conventional four-level interchange accommodates all ramp speeds and consists of one freeway crossing over another, with two additional levels of connector roads that also cross. It is a common urban interchange design and can be found throughout the United States.
“The four-level interchange works well, but we were interested in seeing whether we could reduce the amount of land it takes up because open land is getting scarce and more expensive and people are more concerned about the environmental impact of such construction,” Hummer said.
Harris, as part of her graduate research, compared two versions of the nano-interchange to the four-level interchange, evaluating a number of factors, including construction costs and right-of-way requirements. She compared Hummer’s original design, the “Reverse Nano-Interchange,” and her modification, the “Parallel Nano-Interchange,” to the conventional four-level interchange at 35-, 45- and 55-mph ramp design speeds. At all ramp speeds, both the reverse and parallel designs required less acreage because the ramps, instead of swinging out and around to grab more real estate as in the conventional four-level design, are just direct turns.
The nano-interchange designs divide each freeway into two levels. The north-south freeway, for example, is split so that the southbound level is the lowest of all four levels and the northbound is the highest. Crossing, but sandwiched between these two levels, are two more levels that make up the east-west freeway. With the main lines arranged in this manner, the ramps can be direct turns.
Both the reverse and parallel designs are more expensive to build than the conventional four-level interchange at all ramp design speeds. In a comparison of the three interchange designs at a 55-mph ramp speed, Harris showed that the construction costs were $272 million for a 67.5-acre reverse footprint, $179 million for a 70.3-acre parallel footprint and $150 million for the 100.8-acre four-level footprint. Of the two nano-interchange designs, the reverse has the smallest footprint but is more expensive to build because of the way in which one level of freeway is stacked on top of the other. Harris improved the reverse design by staggering levels. Her parallel design cut the construction costs of the reverse design almost in half.
Although the nano-interchange costs more to build than the four-level interchange, in dense urban areas where open land is scarce and very expensive, the nano-interchange may be the best and least expensive option. Hummer believes that the nano-interchange would be an especially good choice for large, dense cities in developing countries, such as Seoul, Cairo, Rio de Janeiro, Bombay and Shanghai.
“All over India and China there is a need to build infrastructure, build freeways,” Hummer explained. “The cities are so crowded and dense. There is not much available real estate, and it is quite expensive. In the United States, we are not building too many freeways anymore, but in India and China they are and some day in Africa they will be. This interchange should be an option for these places.”
One possible disadvantage to the nano-interchange is the left-hand exits and entrances because most drivers are not used to them. One of Hummer’s doctoral students will be investigating the safety issue this summer.
Hummer said, “We will see what he comes up with. We would like to get the nano-interchange in the hands of designers and agencies. Get them to kick the tires.” When they have a freeway project, such as North Carolina’s upcoming interchange projects at I-40 and I-77 in Statesville and I-40 and I-26 in Asheville, Hummer would like for them to consider the new design.
- mcblief -