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Diverging diamond interchange proposed for Greenwood

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A new Interstate 65 interchange could take some getting used to for Johnson County drivers.

One of the designs the Indiana Department of Transportation is considering for a new I-65 interchange in Greenwood is so novel that you won’t find it anywhere else in the state.

Drivers would briefly cross over to the wrong side of the road, something they normally wouldn’t do unless passing a slowpoke. But that configuration lets motorists merge onto the highway ramps without having to turn left in front of oncoming traffic.

France came up with the idea for the diverging diamond interchange, where vehicles crisscross into the opposite lanes before getting onto the highway ramps or just continuing straight. Missouri brought the highway exit design to the United States three years ago.

Diverging diamond interchanges have since been built in Utah, Georgia and Nevada. Indiana is poised to become the latest state to build them, and Greenwood could get one of the first, INDOT spokesman Greg Prince said.

The state is planning a diverging diamond on Interstate 69 in Fort Wayne and considering one at the new I-65 exit at Worthsville Road in Greenwood, which will be built in 2014, Prince said.

A diverging diamond design isn’t definite in Greenwood because the state also is looking at two more conventional cloverleaf design options and will pick the one that fares the best in an environmental study, he said. But state engineers said a diverging diamond would have advantages, such as cost savings and safety.

If the design is selected, the state might do a presentation about how to properly drive through the exit as part of a public education campaign, Prince said.

The Missouri Department of Transportation worked to educate the public about how drivers should navigate the new interchanges, spokesman Bob Edwards said. State officials made an animated simulation of how to drive through one and displayed it on the website and at public meetings.

They met with Rotary clubs and other groups to show how it differed from what they were accustomed to, Edwards said. They went door-to-door to nearby businesses, explained how it would work and asked them to relay that information to their customers.

State officials distributed brochures, issued news releases and went on local talk radio, Edwards said. They wanted to explain diverging diamonds to anyone who might regularly use the interchange, he said.

“A lot of people expressed doubts about ‘How is this stupid thing going to work?’” he said. “But we went over and over it again and again. People have overwhelmingly adapted to it, and it’s been well received.”

Such public education is important since drivers aren’t familiar with the traffic pattern, he said.

But diverging diamonds are also so well-marked that drivers will know where to go.

A concrete island separates the lanes, and they curve at an angle before traffic crisscrosses on the bridge over the interstate. Drivers would have to ignore all the signs and lane markings and make a full right turn before going into oncoming traffic, he said.

“We’ve not had any crashes related to the design,” he said.

Diverging diamond interchanges might seem unusual at first but are being built across the country because they’re efficient at handling traffic, generally safer since they cut out dangerous turns and also cheaper, Missouri Department of Transportation spokesman Robert Brendel said. Missouri saved $13 million on one project by building a diverging diamond instead of a cloverleaf because it didn’t have to buy as much land.

“It’s a lot cheaper,” Brendel said. “Originally when it was announced, there was some trepidation because it seems kind of convoluted to cross over and drive on the left side of the road. But once the first one was in place, people quickly adapted.”

Traffic flows faster through the intersection, and drivers can worry less about getting T-boned while trying to turn left onto a ramp, Brendel said. People also can safely walk on sidewalks that run right down the middle of the road.

Diverging diamonds separate traffic with two stoplights at either end of the bridge over the highway.

A driver headed east on Worthsville Road could merge directly onto the southbound ramp by turning right at the first stoplight. They’d be off to the side in a separate turn lane and have to obey a yield sign before merging.

If they wanted to go north on I-65 or straight on Worthsville Road, they’d cross over into the left lane once they had a green light.

Drivers headed north on the highway would merge left onto a ramp. Motorists continuing straight on Worthsville Road would cross back into the right lane after passing the second stoplight.

The left-hand lanes would be separated from the right-hand lanes, and pedestrians could walk between them.

Drivers in Springfield, Kansas City, Joplin, Branson and the St. Louis area have been using the exits without any problems, Brendel said.

Missouri has been building them across the state and adding them at existing interchanges instead of going to the costly expense of replacing bridges, he said. The retrofit projects have cost as little as $2.6 million in some places.

Greenwood could potentially save $1 million to $2 million on construction costs if the state goes with a diverging diamond interchange, city attorney Krista Taggart said.

The city has pledged to pay up to $11 million of the project cost in order to get the new interchange built in 2014. The project is currently estimated to cost about $22 million, but that would vary depending on the design picked and how much land has to be purchased, INDOT engineer KimberLee Peters said.

Two other cloverleaf designs are under consideration. One features looping circular ramps on two sides of the interchange, while the other would have ramps on all four sides.

Both would cost more than a diverging diamond since the looping cloverleafs would take up a bigger area and require more land to be purchased, Peters said.

No estimate was available for how much land each option would require.

Cost is one factor, but the predominant consideration will be which design fares best in an environmental study, Prince said. The state likely would choose the one with the least environmental effect after looking at factors such as erosion, noise, archaeological value and habitat preservation.

Greenwood City Council member Mike Campbell said he thought a diverging diamond would have the least effect on the environment and surrounding farmland, since it would have the smallest footprint. He said he also thought it would be safest, move traffic the fastest and give the most favorable impression of Greenwood to visitors, he said.

But he said the city should consider doing some public education about how to drive them, given the novelty.

“There might be a learning curve,” he said.

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