Possible solution for I-77 bridge: Diamond design has many facets

While it's hard to immediately visualize how a diverging diamond overpass works, people in the Show Me State have seen their way clear to start building the nation's second such traffic management system.

The first, on the north side of Springfield, Mo., has been in operation for less than a year. It's worked so well — despite the fact diverging diamonds make both directions of traffic swap lanes so that everyone's driving on the left side of the road, as if they were in London — that the Missouri Department of Transportation is going to build another one on the south side of Springfield close to key services like hospitals.

Cornelius officials are considering a diverging diamond to improve traffic across the Interstate 77 overpass at Exit 28. The cost is considerably less than a traditional intersection between a state road and an interstate, especially when the bridge itself is as wide as it can be without costly improvements.

“It's working seamlessly, with no major hiccups,” said Ryan Mooney, vice president of business development at the Springfield Chamber of Commerce. Springfield is a city of about 156,000 people with a vibrant business community, plenty of traffic and an aging infrastructure. With funds tight, the city opted for an innovative and inexpensive approach to improving an increasingly busy overpass.

The cost was about $2.9 million vs. about $10 million for a full re-do of the intersection. A local newspaper said: “Yes, it catches first-time users by surprise, but regular users should attest that it works fabulously in moving traffic.”

Bob Edwards, a spokesman for the Missouri Department of Transportation, said the I-44 interchange with Route 13 had become a real bottleneck on the northwest edge of Springfield.

“We did not have the money to really do a huge project to add lanes. The bridge needed extensive repairs and, to make a long story short, our MODOT folks in Kansas City started working on this design … They were casting around to see what they could do with ours and gave it a try here,” Edwards said.

“It has really worked slick,” he added.

MODOT organized a major public relations campaign to help get the public ready for a radical new design. There was a simulation of traffic flow on the new interchange on their web site, as well as frequent visits to local service clubs and community groups to educate residents.

They did a bang-up job. In fact, there have not been any accidents attributable to the new design, Edwards said. “Most people have liked it because it moves the traffic through.”

Cornelius Town Commissioner Lynette Rinker says the diverging diamond could get under way if and when NCDOT engineers determine it is feasible. “We pull the trigger assuming the cost estimate is in line with the dollars we have,” Rinker said. Up to about $4 million in funding is already set aside for improvements to the I-77 overpass, not nearly enough for a traditional interchange, along with widening the bridge. That approach could run a whopping $30 million. But $4 million is more than ample for a diverging diamond. The one in Springfield, Mo. cost all of $2.9 million.