Like roundabouts and zipper merging, drivers face learning curve with 'diverging diamond'

Roundabouts and the zipper merge are problematic enough for some drivers.

Now Northern Colorado has its first diverging diamond interchange and, well, driving could get even more interesting along Interstate 25.

The diverging diamond intersection, located at Colorado Highway 60 (Johnstown east exit) and I-25, opened to one-lane traffic in each direction Monday, according to a Colorado Department of Transportation news release. It is part of the North I-25 Express Lanes project.

"Opening up one lane is like putting on training wheels; it allows drivers get a feel for the configuration,'' said Jared Fiel, CDOT's Northeast Region spokesperson.

A second lane in both directions and completion of the interchange is expected by mid-2023, Fiel said.

Colorado is home to a handful of diverging diamond intersections, the first of which was built in 2014 in Grand Junction.

Here is a look at how these intersections work and why, in places, they help traffic flow smoother and safer.

How diverging diamond interchanges work

When entering the interchange from the through-highway, which in this case is Colorado 60, you can either get directly on the I-25 on-ramp or proceed to the traffic signal. Once the arrow turns green, follow that lane across oncoming traffic lanes, equipped with a traffic light to stop oncoming traffic, to the opposite side of the road. Instead of oncoming traffic being on your left, it will now be on your right. Think England.

From here, you can either turn left onto the interstate without a traffic signal light and without stopping for traffic or you can proceed to the intersection. Here, there is another traffic signal to direct when you can again proceed across oncoming traffic lanes and back to the customary side of the road. 

What takes getting used to at diverging diamond interchanges

It feels strange to cut across oncoming traffic lanes — even though vehicles are stopped at a signal light — and end up with oncoming traffic lanes on your right instead of your left.

"It will feel like you are in England for the width of I-25," Fiel said. "It’s more of perception thing because all of a sudden you are on the left side of oncoming traffic. The easiest thing to to do is get in the lane of where you want to go in the beginning and follow the signs.''

Don't worry, there is a wide buffer while on the overpass between oncoming traffic lanes.

Why is the diverging diamond concept being used at this intersection?

Fiel said diverging diamond interchanges are most useful when through-traffic is not as voluminous as turning traffic. In this case, there are fewer drivers going across I-25 on Colorado 60 than turning onto I-25 from Colorado 60.

He said that is not the case at an interchange like U.S. Highway 34 and I-25, so such a configuration wouldn't work there.

However, he said CDOT will look at interchanges in the north I-25 area for opportunities where the diverging diamond interchanges might make sense.

Roundabouts and zipper merge:You might dislike them but they work

What are advantages of diverging diamond intersections?

National studies show crashes were reduced by an average of 37% overall and 54% for fatal crashes after these interchanges replaced traditional ones at 26 interchanges across the country.

The main reason is left turns are eliminated, which minimizes T-bone-type accidents.

Also, these interchanges move high volumes of traffic through an intersection without increasing the number of lanes and traffic signals, according to studies.

More: Most dangerous intersectionsWhat Fort Collins is doing to reduce crashes