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Florida has a gem of an idea to move traffic on and off I-75. It's called diverging diamonds

A diverging diamond interchange on Interstate 75 at the Sarasota-Manatee county line is the biggest among 90 in the nation. Two more of the innovative designs are coming to the area, one in Tampa and one near Wesley Chapel. [LUIS SANTANA    |   Times]
A diverging diamond interchange on Interstate 75 at the Sarasota-Manatee county line is the biggest among 90 in the nation. Two more of the innovative designs are coming to the area, one in Tampa and one near Wesley Chapel. [LUIS SANTANA | Times]
Published May 21, 2018

TAMPA — Driving on the wrong side of the road isn't just for the British any more under an innovative plan for moving traffic through three local Interstate 75 interchanges.

Construction will soon begin on a $37 million, three-year project to turn I-75 and State Road 56 near Wesley Chapel into a "diverging diamond" interchange. The crisscross road design briefly diverts traffic in both directions onto the opposite side of the road as it passes under or over a freeway.

The Florida Department of Transportation has the same plan for I-75 and Martin Luther King Boulevard in Tampa, a $78.6 million project. The new interchange design, originally dreamed up by French traffic engineers, will be added at other major Florida interchanges in the next five to 10 years.

The double-crossover design helps traffic flow and reduces accidents and congestion, road planners say. By switching traffic to the other side of the road, motorists turning left onto the freeway don't have to cross oncoming traffic. The switch-over happens after the entrance ramp for drivers making right turns.

If that sounds too complicated, think of it this way:

Instead of as many as six different cycles of green lights, the intersection will need only two.

"It is one of many innovative interchange designs," said FDOT spokeswoman Kris Carson.

They're called diverging diamond interchanges, or DDI. There are more than 90 of them in the United States, but only one open to traffic in Florida. It's the biggest in the nation, though, at I-75 and University Parkway on the Sarasota-Manatee county line.

The $74 million interchange opened last year and has proven popular with commuters who for years crawled through a stretch of road clogged with red lights and traffic from the sprawling Lakewood Ranch community.

How are motorists coping with driving on the side of the road same side as their steering wheel?

Tami Vaughan of Parrish uses the diamond twice a week on her way to her job as a layout designer in Bradenton.

"It was very confusing at first," she said. "Now, I'm sort of getting used to it and it doesn't feel as scary. It's still weird driving on the wrong side of the road."

Even FDOT officials admit it takes a little getting used to.

"Some drivers might drive a little more slowly the first time they go through the DDI," Carson said. "Otherwise, drivers feel more like they are driving on a one-way street than driving on the left side, so there is little to no adjustments needed."

[Click on the image to enlarge]

Transportation officials say the diverging diamond can be a cheaper option than widening existing interchanges.

But the complex design can mean construction takes longer and is more expensive, said Pei-Sung Lin, a program director at the Center for Urban Transportation Research at the University of South Florida.

The project will also have to include a public education effort to explain the new road configuration so drivers know what to expect when it opens, he said.

"There is a learning time for drivers," he said. "Education is very important."

For University Parkway, FDOT's driver education included videos posted on Youtube.

FDOT does not typically measure traffic flow after a project is completed provided the new road is performing as planned. That is the case at University Parkway, Carson said.

"Operationally, traffic is running smoothly through the DDI even through the peak periods of the day," she said. "Public reaction has been overwhelmingly positive."

The intersections are designed to make it easy for motorists to navigate. Road markings lead drivers through the crossover. Some designs include low concrete barriers to prevent a wrong-way turn.

FDOT will wait until it has five years worth of crash data before it conducts a safety analysis. But typically DDIs have reduced overall crashes by about 40 percent and severe crashes by nearly 70 percent, Carson said.

Early data seems to bear that out.

Florida Highway Patrol records shows that there were 20 crashes involving 43 vehicles at the University Parkway DDI during its first year of operation. None were fatal.

That compares to 34 crashes involving 65 vehicles the previous year.

The design is not a fix for every intersection, said Gilbert Chlewicki, a transportation consultant with American Consultant, which is based in Maryland.

It's best suited for interchanges where a high volume of traffic is turning onto an interstate like I-75 and Dr. Martin Luther King Jr. Blvd., but will not work as well where the bulk of traffic is going straight through.

And drivers who take the wrong exit off the freeway will probably have to make a U-turn to get back on it.

"If it's built in the right location and designed correctly, it's really a win," Chlewicki said.

Vaughan, the Manatee county motorist, said traffic does seem to flow faster through the interchange. But that doesn't necessarily mean she arrives at work earlier.

"Even if you get through the diamond faster, you still get stuck at the next light," she said.

Contact Christopher O'Donnell at codonnell@tampabay.com or (813) 226-3446. Follow @codonnell_Times.

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