How to navigate a diverging diamond interchange

Iowa now has three “diverging diamond” interstate interchanges, one of which recently opened on Interstate 380, and more are planned in the state.

Although the diverging diamond’s traffic pattern is new to most Iowan drivers, it’s fairly simple to navigate, according to Chris Rolwes, a driving instructor and the owner of Safe Driver Driver’s Education, a driving school in Cedar Rapids. The key is to pay attention to signs and be sure to follow directional indicators on the road.

“As you approach it, if you’re reading the signs and you're calm and watching your speed, it's not that big of a deal,” Rowles said. “It looks to me that once drivers get used to it, it’s going to be much safer.”

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Rolwes recently took two students through the new diverging diamond interchange at Tower Terrace Road and I-380, bridging Hiawatha and Cedar Rapids, for the first time. Both students performed well, although they both commented they felt worried about crossing over to the other side of the road.

The diverging diamond is a twist on the traditional diamond interchange, in which cars turn right to enter a highway going one way, or left — across oncoming traffic — to enter a highway going the other way.

A diverging diamond, in contrast, features an intersection with a stoplight on either side of the highway. At the intersections, vehicles are directed onto the left side of the road while they cross over the highway below, then back onto the right side of the road. Cars turning right to enter the highway make the turn before crossing to the left side of the road, and cars turning left to enter the highway can make the turn while on the left side — so no one has to turn in front of oncoming traffic.

“I watched a video about it first, and I can definitely see why departments of transportation across the country are moving to these. It really takes away that situation where cars are turning in front of other vehicles to get on the interstate. I think it keeps traffic flowing better,” Rowles said. “As you come to it, you can tell this is something different. The signage, I think, is very good, from what I can tell.”

The signage at the Tower Terrace interchange is clear enough that Rowles said he doubts someone could end up on the wrong side of the road unless they were intoxicated — a situation that has happened before at a diverging diamond interchange in Iowa.

The first diverging diamond in Iowa opened in 2015 at Grand Prairie Parkway and Interstate 80, on the border of Waukee and West Des Moines. In 2016, a drunken driver — Benjamin Beary, 25 — entered I-80 going the wrong direction from the diverging diamond interchange. The Des Moines Register reported Beary was driving more than 100 mph and crashed into a Des Moines police vehicle, killing two officers — Susan Farrell, 30, and Carlos Puente-Morales, 34 — and a detainee they were transporting — Tosha Hyatt, 32. Beary also died in the crash.

Family members of Farrell filed a lawsuit in 2018 against the state and both cities involved in constructing the interchange, as well as several contractors. The suit alleges that the diverging diamond interchange had insufficient lighting and signage, which contributed to Beary entering the interstate the wrong way and causing the fatal crash.

An appeals court ruled in favor of the cities in 2021, but that decision was overturned in 2022 by the Iowa Supreme Court. The case is now ongoing, with a jury trial scheduled for Sept. 16, 2024.

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Despite the lawsuit, diverging diamonds are becoming more common in Iowa. Three more are planned in the Corridor: The First Avenue interchange with Interstate 380 in Coralville, with an expected opening in 2024; the Wright Brothers Boulevard interchange with I-380, east of The Eastern Iowa Airport in Cedar Rapids, which could start in 2024; and the Boyson Road interchange with I-380 in Hiawatha, expected to start in 2024.

The nation’s first diverging diamond interchange was built in 2009 in Springfield, Mo., according to the Missouri Department of Transportation, and there are now more than 150 across the United States.

The Iowa DOT has looked at national crash and safety data when deciding what kind of interchanges to use for the projects in the Corridor and other interchanges across the state, according to Catherine Cutler, Iowa DOT transportation planner in the District 6 office in Cedar Rapids.

“The diverging diamond keeps traffic flowing well, it reduces crashes, and it doesn’t have to be retrofitted when the area grows. It’ll just keep handling traffic,” Cutler said.

The diverging diamond design is meant to handle large amounts of traffic, while taking up less land than other designs, like the common clover interchange.

“It's a very small footprint, so we don't have to buy as much right of way, which is good. We don't want to take excess land if we don't have to. It allows two lanes of traffic in either direction and some really good handling of left turns,” Cutler said.

Comments: (319) 398-8328; emily.andersen@thegazette.com