Left turns get special treatment on Santa Fe Drive in Douglas County as traffic engineers aim to reduce congestion

The left turn is about to get white-glove treatment in Douglas County.

This week, the first of three “continuous-flow intersections” opens for business on a nearly three-mile stretch of U.S. 85 that’s notorious for aggravating traffic backups — made worse when crews started work on expanding the roadway, better known as Santa Fe Drive, more than two years ago.

The initial continuous-flow intersection, also known as a displaced left-turn intersection, was set to go live at Highlands Ranch Parkway on Thursday, while the second will open at Town Center Drive next week. The third — a more burly version of the configuration that’s designed to smoothly move drivers onto westbound C-470 from northbound Santa Fe Drive — opens in April.

They’re all part of a $130 million effort to expand Santa Fe from four lanes to six lanes, from Highlands Ranch Parkway to just north of County Line Road. The project, which has turned this stretch of road into a maddening maze of orange barrels and construction equipment since its debut in October 2022, is expected to fully wrap by late July.

“Hopefully, it helps move traffic faster,” said Ben Rabinoff, a bartender at Living the Dream Brewing, a watering hole located right in the middle of the construction zone. “Hopefully, it will reduce accidents.”

On both counts, traffic engineers say the continuous-flow design will help by reducing conflict points between left-turning vehicles and oncoming traffic. It guides left-turning drivers to the other side of the road and into a dedicated “left-turn bay,” where they drive on offset pavement well before arriving at the intersection.

The maneuver eliminates vehicles stacking up in the middle of the main road at the intersection itself, which can often result in turning vehicles sitting through more than one light cycle and spilling over into the through lanes, gumming up the entire roadway. Instead, left-turn drivers queued off to the side can turn while through traffic zips across the main intersection.

“A CFI typically provides anywhere from 30% to 70% more capacity,” said David Millar, traffic engineering program manager for the engineering firm HDR Inc., referring to the acronym for the design. “I’m confident it will be able to handle traffic on Santa Fe for the next 20 years — and well beyond.”

Millar heads up the design efforts on the U.S. 85 project, which is being funded with federal, state and Douglas County money. The developers behind Sterling Ranch and the Solstice communities are reimbursing the county for some of the project’s cost.

The highway segment sees average daily traffic volumes ranging from 48,000 vehicles at its north end, near C-470, to 30,000 around Highlands Ranch Parkway, according to state traffic data.

According to the Federal Highway Administration, delays at a partial continuous-flow intersection drop between 30% and 40% compared to a conventional configuration, while delays go down between 50% and 80% at intersections that get full treatment.

At a continuous-flow intersection in Baton Rouge, Louisiana, the FHWA found that crashes overall dropped 24%, while fatal and injury crashes went down 19% in the two years after the redesign was completed.

Increasing adoption of design

Continuous-flow intersections are not new to Colorado, but they aren’t common, either.

The first was built in Loveland in 2010 at Madison Avenue and East Eisenhower Boulevard. They can now be found in Colorado Springs and Durango, with two new ones coming soon to Wheat Ridge — at west 44th and 38th avenues — as part of a yearslong effort to improve Wadsworth Boulevard.

Metro Denver drivers faced the fastest-increasing traffic delays in the nation over the past year, according to a recent analysis by the global transportation data firm INRIX. Drivers on metro roads last year lost an average of 44 hours to traffic jams, up from 37 hours in 2023.

By comparison, residents lost an average of 102 hours in traffic in New York City and Chicago. In Los Angeles, the nation’s second-largest city, they lost 88 hours.

The stretch of Santa Fe south of C-470 is a critical corridor to improve, said Art Griffith, Douglas County’s transportation capital improvements project manager. The other major north-south highway in Douglas County is Interstate 25, nine miles to the east.

“It’s not like there’s a grid here that would allow alternate routes to spread things out,” Griffith said.

On a breezy morning this week, Millar and Griffith toured a reporter through the yet-to-open redesigned intersections on Santa Fe. The two at Highlands Ranch Parkway and Town Center Drive are partial CFIs, meaning they are designed to provide left-turn relief only to drivers turning east into Highlands Ranch.

On the west side of Santa Fe Drive is a largely industrial and commercial slice of land, bounded by Chatfield Reservoir, that doesn’t see large volumes of traffic and doesn’t need the alteration, Griffith said.

Hundreds of feet ahead of the intersection of southbound Santa Fe with Highlands Ranch Parkway, a sign directs drivers into two left-turn lanes controlled by an overhead signal. When the light turns green, drivers cross in front of oncoming vehicles that have been stopped by their own signal. They enter a dedicated left-turn bay and proceed to the intersection on what feels like the wrong side of the road, until they hit another signal — where they take their turn.

“The first time you drive through it, it feels odd — then it was no problem,” Millar said. “You drive it again and it’s absolutely no problem.”

Millar said the signals will be carefully timed to avoid having turning vehicles sit through more than one light cycle.

“That’s generally considered failing if you have to wait through more than one cycle of lights,” he said.

Data show improvement in Durango

A decade ago, the intersection of U.S. 550 and U.S. 160 in Durango, in southwestern Colorado, was starting to see “long traffic-flow delays, backups and congestion,” said Lisa Ann Schwantes, a spokeswoman for the Colorado Department of Transportation’s southwestern Region 5.

A continuous-flow intersection was seen as a solution to the growing problem at the major regional crossing.

“The Durango CFI project was constructed out of necessity to keep the intersection operating at an acceptable level of service,” she said.

And data from before the 2014 rebuild of the intersection to the years since show a left-turn crash reduction of almost 50% at the intersection, Schwantes said. Mike McVaugh, who helped oversee the project in Durango when he headed CDOT’s Region 5, said he recently heard from the city’s police department, which said the intersection “went from one of their higher-crash locations to one that has a low-crash frequency now.”

“We had a lot of skeptics who did not believe this would work,” McVaugh, now with HDR, told The Denver Post. “Many of those same people today tell me it works and they would not go back to the old intersection.”

Millar is determined to keep everyone using the road happy with the changes, along with the shops, restaurants and businesses that have been trying to operate in a corridor beset by construction challenges.

“If you can get 30%, 40% or 70% more capacity in the same footprint, absolutely — that’s something you want,” he said.

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