Motorists are driving into more and more foreign-born traffic designs in Colorado neighborhoods and highways, put there to more safely handle increasing volumes of cars and trucks.

Traffic circles and roundabouts began emerging in the mid-1990s and have been grudgingly accepted by drivers, planners say. But commuters should be prepared for the introduction of an even bigger change: the first “diverging diamond interchange” in Colorado.

If plans are approved, the McCaslin Boulevard overpass at U.S. 36 — at the border of Superior and Louisville — will be reconfigured to force drivers into a crisscross pattern after a traffic signal, putting them on the left side of the road for a stretch. From there, drivers have the choice of making an unimpeded left turn onto a highway ramp or, after a second traffic signal, crossing over again to the right at the end of the overpass.

“At first, it makes people uneasy because drivers are driving on the wrong side of the road,” said Jonathan Grano, assistant public-works director in Springfield, Mo., where the first diverging diamond in the U.S. was built in 2010. “But once you do it once or twice, it comes natural for most.”

Springfield’s interchange is similar to designs in Europe. Missouri is building its fourth diverging diamond. Utah is adding them at virtually the same pace. Grand Junction traffic planners are contemplating a diverging diamond too.

Utah and Missouri engineers said the interchanges increase “green time” — the amount of time motorists are moving and not stopped at a traffic signal. The design also cuts the number of “conflict points,” where traffic flows cross each other, thus eliminating most accidents.

It’s the same reason traffic planners use roundabouts, which can cut the number of crashes at previously signaled intersections by as much as 75 percent, according to some studies.

But holdouts find that roundabouts — developed in England and New York to move traffic in one direction around a central island — are more confusing than a good, old-fashioned signal light.

“Based on the feedback I get, roundabouts are a mixed bag to residents,” said Loveland Police Chief Luke Hecker.

Loveland has about 20 roundabouts.

“Some people think they are great and do a good job in traffic flow,” Hecker said. “Some don’t like them and don’t believe they ever will like them.”

Preliminary design

Superior and Louisville have each committed nearly $400,000 to develop a preliminary design for the diverging diamond, a concept the cities selected

over two other less-costly proposals to ease congestion on the Mc-Caslin overpass, where traffic counts reach about 41,000 vehicles daily.

One now-discarded proposal called for a roundabout on the north side of the bridge and dual left-turn lanes onto westbound U.S. 36.

In all, the diverging diamond could cost $12.4 million to build, said Louisville planning director Troy Russ.

In addition to unsnarling private-vehicle traffic, the improvements would smooth ramp operations and improve travel times for Regional Transportation District buses that heavily use the intersection, Russ said.

RTD buses would get their own ramps in the interchange. Pedestrians and cyclists could also cross U.S. 36 using the interchange or pedestrian crossings, Russ said.

“It’s the best solution for this intersection,” he said.

Planners are hoping to have the interchange included in the Phase II widening of U.S. 36 being built by the Colorado Department of Transportation.

“This really is our best shot at getting this done as soon as possible,” Russ said.

Transportation planners are generally pleased with how traffic circles and larger roundabouts have reduced the number of serious accidents in Colorado.

“Roundabouts are definitely a safety improvement,” said CDOT traffic and safety engineer Steve Hersey. “Where you used to see intersections where there were severe injuries and fatalities, those numbers basically go away.”

They have also found that with proper planning and design, larger vehicles such as 18-wheelers can share roundabouts with smaller vehicles.

There are, however, limits to what roundabouts can do, Hersey said.

“The more lanes you have, the more complicated it gets for drivers,” Hersey said. “You have to be able to plan well and make sure that a roundabout fits in well with the area. Sometimes a signal works best.”

Monte Whaley: 720-929-0907 or mwhaley@denverpost.com