Changing lanes

Transportation engineers at North Carolina State University have designed a road 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, a professor of civil, construction, and environmental engineering, and recent master’s graduate Meredith L. Harris designed the new four-level interchange, called the ‘nanointerchange’, that saves about 10 to 30 acres of land as compared to a conventional four-level interchange.

A 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 US.

'The four-level interchange works well, but we were interested in seeing whether we could reduce the amount of land it takes up,’ Hummer said.

Harris, as part of her graduate research, compared two versions of the nanointerchange 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 nanointerchange,” and her modification, the “parallel nanointerchange,” to the conventional four-level interchange.

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 nanointerchange designs divide each highway into two levels. The north-south highway, 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 highway. With the main lines arranged in this manner, the ramps can be direct turns.

The bad news is that both the reverse and parallel designs are more expensive to build than the conventional four-level interchange. At a 55-mph ramp speed, the construction costs are $272 million for a 67.5-acre reverse design, $179 million for a 70.3-acre parallel design and $150 million for the 100.8-acre four-level design.

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 highway 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 nanointerchange does cost more to build than the four-level interchange, in dense urban areas where open land is scarce and very expensive, it may be the best and least expensive option. Hummer believes that the nanointerchange would be an especially good choice for large, dense cities in developing countries.

One possible disadvantage to the nanointerchange is that it uses left-hand exits and entrances and most drivers are not used to them.  One of Hummer’s doctoral students will be investigating the safety issues involved this summer.