Drivers, meet the diverging diamond coming to Bloomington

Roundabout phobics, brace yourself for the newest traffic innovation: the diverging-diamond interchange.

Construction on a DDI, as its fans call it, has begun at Interstate 494 and 34th Avenue in Bloomington. The interchange, the third of its kind under construction in Minnesota and the first in the Twin Cities area, will open in the fall.

Diverging-diamond interchanges are designed to eliminate left turns in front of oncoming traffic, smoothing and speeding traffic flow. Traffic is controlled by signals, and right turns work just as they do at a conventional interchange.

But what works smoothly in practice looks like a nightmare on paper, with a tangle of spaghetti-like lanes weaving back and forth. Some traffic veers to the left, so drivers may get the vague feeling that they're driving on the wrong side of the road.

Fear not, engineers say. Drivers should just follow the lanes, which will be marked with small stripes to direct the unsure. Navigating a diverging-diamond interchange is much simpler in person than trying to figure it all out from a diagram, said Claudia Dumont, Minnesota Department of Transportation project manager for one being built near St. Cloud.

"Conflicts are eliminated, and you end up with more ramp-like movement," she said. "They have really good safety records and are very efficient."

For evidence, she points to a dashboard-level video of a car going through the U.S.'s first diverging-diamond interchange in Missouri. Traffic winds smoothly through the interchange with no trouble.

Dumont said French studies show that traffic delays are reduced up to 60 percent because the waiting associated with left turns is eliminated and signal cycles are shorter.

In Bloomington, the $7.5 million interchange is a joint project of the Metropolitan Airports Commission (MAC), MnDOT and the city of Bloomington. The I-494-34th Avenue interchange needed to be updated because heavy traffic on some ramps has been backing up onto the freeway, said Allen Dye, airport project manager for MAC. About 70 percent of the 48,000 vehicles going through the interchange are getting on or off I-494.

The DDI design was chosen for the site partly because it avoids the problem of having to build over or under the light-rail line that runs under I-494, said Jim Gates, Bloomington's deputy director of public works. The DDI's smaller footprint made it the cheapest option, too.

"Loops would have cost more because of the right-of-way, and we can't go into the [Fort Snelling National] Cemetery," Gates said. "It was the most cost-effective."

Bloomington already has a page on the city website devoted to information about the DDI. Construction will proceed at once corner of the interchange at a time, so there will be detours in the area into fall. Gates said the light-rail line may have to close in that area on some weekends, but buses will bridge the gaps for riders.

In St. Cloud, Dumont is taking no chances. MnDOT has been on a "PR blitz," she said, publishing construction fliers to show the stages of the project, how to drive the diverging-diamond interchange, and promoting video links that show how the interchanges work. Dumont has even talked to instructors who teach road safety classes to older drivers, showing them the videos so they can talk about diverging-diamond interchanges to their classes.

In a double whammy for timid drivers, the St. Cloud DDI is on the same corridor as roundabouts, Dumont said.

"There was a little learning curve with those, but people are getting used to them," she said. "I think [diverging-diamond interchanges] are more intuitive than roundabouts. You drive through it and you don't realize anything is different until you get through it."

MnDOT is paying 65 percent of the interchange project's $7.5 million cost, with Bloomington and MAC splitting the remainder.