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Superior's multi-lane roundabout at McCaslin Boulevard and Main Street is pictured south of the unique diverging diamond interchange with U.S. Route 36 and Marshall Road in Superior on Jan. 8, 2021. (Andy Colwell, Special to The Colorado Sun)

In the late 1990s, the city of Golden considered what then was a foreign concept: The idea of placing roundabouts, the circular traffic control configuration popular in Europe and Australia, along one of its arterials through a commercial center.

Although it had been in use in the U.S. for more than a century, Americans still hadn’t warmed up to the idea. A planned King Soopers store whose entrance would be on one of the proposed roundabouts along South Golden Road pushed back with concerns about how the unusual intersection might discourage customers.

Five years earlier, local businesses had shut down talk of using roundabouts to revamp the road, a broad swath that invited speeding, featured intermittent traffic lights and enough lanes to make pedestrian crossing an adventure. But this time, the city cut an unusual deal: If the grocery store didn’t hit its sales targets, the roundabout would be removed. 

It’s still there today — part of the early wave that, decades later, continues to gain traction in Colorado and around the country as both state transportation officials and especially municipalities embrace this once-unfamiliar solution that, in many situations, calms traffic and leads to fewer injury accidents.

Four years after Golden’s roundabouts opened, Alex Ariniello, then a traffic consultant on the project, published a study — “Are Roundabouts Good for Business?” — that’s still widely circulated as evidence that they not only enhance flow while reducing speed and result in improved safety, but also can contribute to commercial success.

“There’s always apprehension in communities that don’t have them, that haven’t had experience with them,” says Ariniello, now the public works director in Superior, where his influence is evident in the town’s several roundabouts. “We found that people just didn’t understand them or weren’t familiar with them. ‘Why are you forcing this on us? Why not a traffic signal?’ That was the typical reaction we’d get. 

“The general public didn’t understand the concept and why it’s a safer intersection type than a four-way stop or traffic signal. I spent a lot of time educating people.”

Though introduced to the state in Colorado Springs in the late 1980s, the roundabout concept has been improved upon over the years and began a resurgence in the following decade, as traffic engineers sought to regulate vehicle flow more effectively through trouble spots and to employ the inherent safety advantages of a system that lowers speeds and virtually eliminates broadside crashes.

There’s always apprehension in communities that don’t have them, that haven’t had experience with them…The general public didn’t understand the concept and why it’s a safer intersection type than a four-way stop or traffic signal.

Alex Ariniello, Superior’s public works director

As their popularity has grown, some states have legally designated roundabouts as the default option — meaning that they’re considered the optimal solution for intersections with the burden on other possibilities to be proven better. 

Ariniello, who serves on a subcommittee of the Vision Zero project seeking to eliminate traffic deaths, says he’s pushing for a “roundabout-first” policy in Colorado as well. He estimates he’s given his “Roundabouts 101” presentation to about 20 groups around the state as he pushes for their adoption. One of his clients was the town of Superior, where he helped install a few before leaving his consulting business in 2012 to direct public works. During his eight years in that position, four more roundabouts have gone in. 

He figures he’s been involved with “several dozen” across the state. 

The Colorado Department of Transportation estimates that about 140 roundabouts on the state’s highways have replaced traditional traffic signals or signs at intersections. Twenty-six of those have been constructed on or adjacent to state highways, including some that facilitate entry to or exit from interstate highways. 

Ben Kiene, CDOT traffic operations engineer for the region that includes Denver and surrounding counties, says that roundabouts have come to be regarded as one of the primary options for solving traffic problems across the state.

“Whenever we have a major interchange project, a roundabout is usually looked at as one of the alternatives,”  Kiene says, noting space constraints in urban areas can make roundabouts unworkable or, if they’d require a lot of property acquisition, too costly. “It’s not necessarily that there’s a designer preference or anything, it’s that they’re kind of in the mix.”

Last summer, CDOT even produced a roundabout education video, a birds-eye-view animation of how different types of roundabouts work.

YouTube video

“You don’t have what’s called lost time at a signal,” Kiene says. “Every time the signal changes, during that ‘clear interval’ nobody is supposed to enter the intersection — so that’s time that, with a roundabout, you have traffic flowing continuously. They’re very advantageous that way. And they do have an injury-crash rate that is pretty significantly lower.”

He adds that research has shown the average range is about a 70% reduction in injury crashes as a result of conversion of a conventional intersection to a roundabout. There’s some nuance to that — the number of lanes in the roundabout, which vary with traffic capacity at a given intersection, can have an impact on its safety performance. 

“When roundabouts first open,” Kiene says, “there might be a slight increase in property damage crashes that are mainly merely fender-benders as drivers get used to them. And then they drop off.” 

Superior Public Works & Utilities Director Alex Ariniello poses for a portrait in front of Superior’s multi-lane roundabout at McCaslin Boulevard and Main Street on Jan. 8, 2021. Ariniello was a traffic engineering consultant previously, and has been a strong advocate for roundabouts over the years in Colorado. (Andy Colwell, Special to The Colorado Sun)

All-in on traffic signals

So what took Colorado — and the United States in general — so long to warm up to roundabouts?

Brian Walsh, chair of the Transportation Research Board’s standing committee on roundabouts as well as an operations engineer at Washington state’s transportation department, says the reasons stem from the country’s decision, early on, to go all-in on traffic signals.

“We just decided that was what we’re going to do anytime we had an intersection issue from a capacity standpoint, or a safety issue,” Walsh says. “So consequently, I think we have 300,000-plus traffic signals in this country, and what was missed a little bit with that was the idea that you can build intersection control that was self-regulating and also was dirt cheap, meaning you didn’t have to put anything out there, other than a circle. And the Europeans had figured that out.”

Oddly enough, American designer William Phelps Eno largely gets credit for the invention of the roundabout. Built in 1905, Columbus Circle was his handiwork, though it doesn’t quite conform to today’s definition. Walsh notes that what was missing from that circular traffic pattern in New York City was one essential concept: the yield rule, which states that vehicles currently in the circular pattern always have the right of way.

Meanwhile, in the 1960s, both the United Kingdom and Australia figured out that roundabouts using that simple rule of the road could maximize both safety and capacity. 

“Then we had a couple Americans give it a shot and said, ‘Hey, this makes sense. Why haven’t we had more of these?’” Walsh says. “So now you have a culture that’s very biased towards one thing — and the other thing looks like it’s a foreign thing. It’s an us-against-them type of thing that’s in play with two intersection-control devices.”

Eventually, Walsh’s Washington state and about a half-dozen others became early adopters of the most recent wave. Colorado was in that mix — particularly locations like Colorado Springs, Vail and Golden. Still, those Colorado experimenters acted primarily on the municipal agency level to address specific problems. 

In particular, Golden stood out, in large part because traffic engineer Ariniello, its consultant on the project on South Golden Road through the heart of town, not only designed four roundabouts to regulate traffic flow, but also continued to study their effectiveness — and then published his research paper on the results. 

Walsh says he would often hold out Golden as proof of concept, presenting to interested groups aerial photos of the town as he explained how the roundabouts maintained traffic flow through the business district — without unnecessary stops — while eliminating left-turn hazards.

Ariniello sees the strategy as even more relevant now, as Colorado’s population expands and developers construct new housing and commercial centers. 

“If a low-volume road turns into a high-volume road with massive development coming, it’s not that hard to incorporate roundabout concepts,” he says. “It’s not a panacea, but for situations where it makes sense, it’s an option to look at.” 

Sometimes, Walsh says, the decisions can come down to having the right decision-makers in the right places at the right time, when it comes to considering roundabouts as solutions to problems traditionally addressed by traffic signals.

“I think there’s a progressive piece in there,” he says. “I think that you come from a state that’s fairly progressive and that’s why you see some of that in some of those towns as well as maybe segments of the DOT.”

Vehicles approach a roundabout intersection in Golden where U.S. Route 6 meets 19th Street and Elm Street on Jan. 11, 2021. Construction of several roundabouts in Golden in the late 1990s helped pave the way for broader use of the traffic solution in Colorado and nationwide. (Andy Colwell, Special to The Colorado Sun)

Simple as they appear, roundabouts remain constantly in a state of flux as engineers consider ways to make them both easier to navigate and safer. Walsh notes that he’s currently involved in a $750,000 research project to “whittle away” at concepts like the angle at which vehicles enter the roundabout. He adds that roundabouts in Colorado Springs, home to what he calls the “first generation,” may look and perform differently than what’s being constructed now, in what he considers the third or fourth generation guided by more recent research. 

Whether it’s entry angle (which largely avoids T-bone style crashes), placement of a sign, rolled curbs to facilitate large truck traffic or how crosswalks are installed, engineers continually assess details that can influence how well roundabouts perform. There’s also the matter of driver familiarity. Signal lights have bred some bad habits — particularly in recent years when a red light might as well mean “time to check your texts.”

Roundabouts may pause traffic, but they depend on a higher level of attention in order to work.

“You’ve got to actually drive your car through a roundabout, there’s no messing around,” Walsh says. “I mean, people are expecting you to be on your game, meaning you better take the gap when it’s there, and you better keep moving because I didn’t plan on you doing a text here.”

What Walsh calls gap tolerance evolves over time. He ventures that in Golden, for instance, motorists probably navigate the roundabouts on South Golden Road fairly aggressively because they’ve been there for years. But newer roundabouts are more likely to see more drivers approach them with uncertainty. 

With experience, they learn to trust the system.

“The math is there that there should be enough gaps for a certain number of cars to get through,” he says, referencing the calculations that enter into traffic flow. “But we kinda are counting on you as a human being with that great computer called a brain to, you know, take the gap there.” 

The application of roundabouts to traffic issues isn’t confined to urban or even small-town examples. In rural areas, high speeds — 50 miles per hour or above — can combine with high volume on main highways at certain times to diminish the number and duration of safe gaps in which drivers from other arteries can access or cross the highway. 

“And so what happens is you start to see fairly serious crashes, considering the speeds involved,” Walsh says. “So we slap what we call a single-lane rural roundabout out there, with really great geometry and advanced channelization. And we basically solve our safety problem with a roundabout.”

Essentially, the roundabout slows down the primary thoroughfare. He explains to skeptical rural audiences that the slowdown doesn’t significantly impact travel times. But it does allow for more traffic to flow safely onto that highway.

“It forces them into a much slower speed through the intersection so that the people entering on the side street can get on at very slow comparable speeds and that’s what the whole magic is,” Walsh says. “If you can bring all the speeds to the same threshold within 15 to 20 miles an hour, you get really great safety.” 

Ariniello compares that experience on a rural highway to traditional traffic signals, and how drivers instinctively approach them.

“Going through a roundabout, you need to find a gap. You’re focused on other drivers, where the other car is, and whether there’s time to get into the roundabout,” he says. “But coming up to a light, you’re concentrating on the green light, and whether, if it turns yellow, you want to speed up and make it through the intersection.

“It’s a different philosophy.”

A vehicle drives through a roundabout intersection at 15th and Pine streets in Boulder on Jan. 8, 2021. This basic version of a roundabout has signs that remind motorists of the cardinal rule: yield to traffic already in the circle. (Andy Colwell, Special to The Colorado Sun)

Vail’s domino effect

The mountain resort town of Vail knew it had traffic issues as far back as the late ’80s, but they really came to a head in 1993. The crush of skiers coming and going via Interstate 70 grew so bad that the town took to placing people at intersections to direct traffic — which Greg Hall, Vail’s director of public works and transportation, recalls as a labor-intensive and totally inadequate solution.

“Not to say we were desperate,” he says, “but doing research we came upon roundabouts, and we did some more research. Our council felt that was a solution we should try, and so we worked with the Federal Highway Administration and Department of Transportation to get approval, and built a big one in 1995.”

At that time, roundabouts weren’t an easy sell. 

Hall recalls transplants from the East Coast who hated that region’s “traffic circles,” an early and less efficient iteration of what eventually would evolve into roundabouts. By then, that evolution involved the more up-to-date European design, but the idea still riled residents — including renowned ski filmmaker Warren Miller, who railed against roundabouts.

There weren’t that many in the United States set in circumstances similar to Vail’s. And none of them offered a clue how they’d perform in a snowy environment. Plus, Vail would become the first in North America to incorporate a roundabout with an interstate interchange. 

There was opposition, but at the same time we would have considered it a win just to get safety officers out of the road and have a more attractive entry to town than before.

Greg Hall, Vail’s director of public works and transportation

“There was opposition,” Hall says, “but at the same time we would have considered it a win just to get safety officers out of the road and have a more attractive entry to town than before.”

So Vail plowed ahead with its project, which — lo and behold — appeared to work. Traffic flowed more evenly. Injury accidents dropped by 85%. Better traffic flow paved the way for revitalized development. But even as the benefits accrued, some residents initially remained skeptical.

Hall clearly recalls the chorus: The locals may get used to it, but wait till the tourists arrive. They work now, but what about during the busy holiday season? But the holidays — and the tourists — both came and went smoothly. 

But it hasn’t worked during a snowstorm. The town went so far as to interview experts in Norway to get an idea how to plow roundabouts so they’d work effectively. And then, two days after Christmas, a blizzard struck — and the roundabout still proved a success.

“That’s finally when everybody said, maybe these work,” Hall recalls. “All it took was that first holiday season. It handled our holiday traffic and a snowstorm. People used to sit for 30 minutes to get out of Vail. Traffic was pretty nasty during holidays to that point. It was kind of night and day.”

The town made a video about how to drive roundabouts and put it on public TV, published brochures and did everything it could to bring folks up to speed. 

“There was always a ‘Yeah, but…’” Hall says. “But it worked.”

Two years later West Vail faced a similar traffic problem. Hall says when the town announced it would explore alternatives, “people said, ‘No, you need to build  a roundabout.’” 

Now, he adds, whenever the town considers constructing a major intersection, roundabouts are the default option.

Vail’s adoption proved contagious, all along the I-70 corridor. Now, 26 years after the town built its first, others have been constructed in Frisco, Avon, Edwards, Eagle, Glenwood Springs — all the way to Grand Junction.

“Everybody looked and said, ‘We’re glad you went first, but we’re glad to copy,” Hall says. “Originally we did get calls about ours. But we’re the ancient people now.”

Kevin Simpson is a co-founder of The Colorado Sun and a general assignment writer and editor. He also oversees the Sun’s literary feature, SunLit, and the site’s cartoonists. A St. Louis native...