The problem with intersections is that, well, they intersect. The challenge for the traffic engineer is to get all of the drivers where they want to go without (a) wasting too much time or (b) running into one another.
Until now, designers had a relatively limited tool box for solving intersection problems. Most U.S. communities have stuck with either traditional signalized intersections or, when the traffic volumes got too high, moved to grade-separated designs—ramps, bridges and underpasses that take up a lot of space and cost a lot of money. If you couldn’t afford a diamond interchange, there wasn’t much you could do.
Now, traffic engineers are proving viable options exist to grade-separated intersections, options that cost little more than traditional signalized designs. Continuous-flow intersections (CFI) move more cars more quickly with less potential for accidents than conventional designs.
Utah’s Department of Transportation (UDOT) is embracing the concept of CFI for a critical junction of 3500 South and Bangerter Highway in Salt Lake City. If the intersection works as well as they think it will, Utah could be building a lot more of these intersections—and so could many other states.
Quality green time
“In a nutshell, the continuous-flow intersection reduces the number of signal phases that you have to accommodate. It gives you more green time to spread around,” said Mike Worrall, project manager for the 3500 South and Bangerter Highway project for Carter & Burgess Inc.
The design focuses on eliminating conflicts from left turns. In conventional signalized intersections, left-turn cycles must operate separately from most of the traffic flow, giving a large amount of signal time to a relatively small number of drivers. Further, left turns often account for accidents when drivers try to slip across oncoming traffic after their light has turned red.
Continuous-flow intersections remove all left turns from the main intersection. “Continuous flow” refers to the constant flow of traffic straight through the intersection, going one direction or the other. Drivers turning left are shuttled off the main roadway several hundred feet before the intersection, crossing oncoming traffic with a signal. They continue in their own lane on the left side of the road to the intersection.
“Then they are actually released at the same time as the through movements, so you don’t need a separate left-turn phase,” Worrall said.
Diagrams demonstrating traffic flow show the different phases and can help clarify how the intersection works. Another good resource is an animated “Test Drive” through a CFI available online at the website of ABMB Engineers; go to www.abmb.com/cfi.html and click on “How it Works.”
CFI falls into a general category of indirect left-turn treatments, which include jug-handle and median U-turn crossover designs. All are intended to remove left-turning vehicles from the traffic stream without making them slow down or stop in a through-traffic lane. The advantage of CFI, said Michael Bruce, partner at ABMB Engineers, is that it makes sense to drivers.
“There are a lot of innovative intersection concepts out there, but the only one that is really intuitive is this one,” Bruce said. “That’s hard to explain to people who haven’t driven it. The worst way to look at CFI is from above, which is how it’s modeled. On the ground there’s no confusion. You don’t even know you’re driving through an unconventional intersection.”
University researchers have studied CFI against conventional designs and found the average delay reduced by 48 % to 85%, with the upper value applying to an oversaturated intersection. In fact, CFI becomes more efficient the higher the traffic levels. One study found that the point where a conventional intersection exceeds capacity is the point where CFI becomes most efficient. Of course, at a certain level, even CFI will fail, but research found the capacity of CFI nearly 50% higher than a conventional intersection.
CFI is not without drawbacks. Access to some corner properties may be more difficult. While less right-of-way is needed than for grade-separated interchanges, the footprint is nonetheless larger than that of a conventional intersection. Further, drivers and pedestrians might be confused by the unfamiliar layout of the intersection, and significant public information campaigns may be needed to educate the community.
The CFI concept was pioneered by traffic engineer Francisco Mier and was initially embraced by transportation designers in Mexico; today more than 40 CFIs operate in that country. Adoption north of the border is still limited; the first fully operational CFI in the U.S. is in Maryland at the juncture of State Highway 228 and State Highway 210 south of Washington, D.C. The intersection received a design award from the American Association of State Highway & Transportation Officials (AASHTO) in 2002. However, several states are actively considering CFI, with a project under construction in Louisiana and others nearing construction in Mississippi.
Outlandish turns outstanding
Innovative intersection design was the furthest thing from the minds of the staff at UDOT and their consulting team undertaking an Environmental Impact Statement (EIS) on 3500 South. During the EIS, the intersection at Bangerter Highway and 3500 South became the critical problem to solve. Also known as State Highway 154, Bangerter Highway is a major north-south thoroughfare parallel to I-15 through west Salt Lake City; at its north end, Bangerter enters Salt Lake City International Airport. Traffic levels on Bangerter are high, particularly at the intersection at 3500 South.
“We’re seeing heavy congestion in this area,” said Lisa Wilson, UDOT project manager. “It’s really bad right now, and it’s only going to get worse. The area to the west and south is really growing, and people are traveling through this intersection to get to and from work.”
While the entire corridor was being studied, this particular intersection needed help fast. The team started generating ideas. One obvious answer was grade separation, but the team saw quickly it wasn’t viable financially. UDOT simply didn’t have the budget to throw more than $40 million at this one intersection. That’s when the team started to research other, more innovative approaches and came up with CFI.
Not surprisingly, UDOT was initially skeptical. “Our team came up with this outlandish, progressive idea—, Now we had to prove it would work,” Worrall said. Team member firm Fehr & Peers began by modeling the intersection and demonstrated that a CFI could still operate at a passing level of service at 2030 predicted traffic levels. “We’re at that level of service now under the current conditions,” Wilson said.
Next, the team organized a field trip. Working with staff at the Texas Department of Transportation’s El Paso District, UDOT staff and consultants flew to the border and visited Juarez to see CFI in operation. It was an eye-opening experience.
“We spent a good part of a day driving it and walking it,” Worrall said. “We had concerns going in, but realized after we saw it none were really a big deal.”
For example, one concern of the team had been pedestrian access. What they found was that pedestrians can move easily across a CFI, although they must pause at the middle of the intersection between cycles. “It’s actually very pedestrian friendly,” Worrall said. “With enough information and a good design, it’s readily apparent what you’re supposed to do and where you’re supposed to go.”
Bruce said watching CFI in action convinces most people. “Once you see it on the ground, you realize it really works.”
The Mexico trip was so successful that UDOT decided to separate the 3500 South/Bangerter Highway intersection from the overall corridor EIS and start immediately on a CFI design for the junction. It was a big commitment to a big idea.
One of the first steps in the new project was to team up with ABMB Engineers, a Baton Rouge-based firm pioneering the use of CFI in the U.S. “They understand the subtleties of this design—the finer points that really make it work,” said Worrall.
The next step was to determine how to best apply CFI to this specific location. The final design modifies the CFI concept, applying it only to the Bangerter Highway drivers; that is, only left-turn drivers on Bangerter Highway cross oncoming traffic to their own dedicated lane. This decision made sense because Bangerter has a high percentage of left-turn drivers and South relatively few.
Additional right-of-way will be required for the new design, “but not as much as you might think,” Worrall said. “Actually the overall width will not be much wider than it is now. And it certainly wasn’t as much right-of-way as other alternatives would have required.”
One major emphasis of the new design will be to slow the traffic entering the intersection. “Bangerter is a fairly high-speed highway. With the complexity of the intersection, we thought it was important that the intersection operate at a slower speed,” Worrall said.
Another aspect of the project will be an aggressive public information campaign to educate drivers about the intersection. The team proposes to use public service announcements, broadcast and print ads, a website and variable message signs to explain the new design to the public. “Nobody has even seen one of these before, so you need to be sensitive to that and let them know what it’s all about,” said Worrall.
They’re all watching
A lot is riding on the Bangerter Highway/3500 South intersection. If the CFI design proves successful, UDOT would like to implement the concept at other locations. “If this intersection is as successful as we think it will be, you’ll probably see quite a few more along Bangerter Highway,” said Wilson.
That puts pressure on the project team to make the intersection work, said Worrall. “They’re talking about it all over,” he said. “So it’s important that we make this project a success.”
Meanwhile, other states will be watching the intersection. The same problems plague locations around the U.S.:— too much traffic at intersections, too few funds to build a grade-separated alternative.
“CFI is a congestion solution,” said Bruce. “When an intersection gets to the point that the only solution is an interchange, CFI is able to come in and solve the congestion problem without bridges and without spending tens of millions of dollars.”
Worrall thought it was interesting and ironic that such a creative solution came out of Mexico. “People rolled their eyes when we said we were going to Mexico to look at a traffic solution,” he said. “That was a fairly typical, prejudiced and narrow-minded response. They’ve got a number of extremely unique and resourceful intersection designs. It’s amazing the things they’ve tried.”
When he talked to traffic engineers in Juarez, “they told me, we don’t have any money, and necessity is the mother of invention,” he said. “There’s something to be learned from that. Other countries are way ahead of us on this.”
“With the tight dollars every transportation agency is facing, CFI has a tremendous potential to solve congestion problems that are currently being ignored,” said Bruce.
UDOT expects the new intersection to be under construction by the summer of 2006. In the meantime, the UDOT team will be exploring the boundaries of intersection design, a task many more agencies may be soon undertaking.
“In this country we’re reaching the limit of current funding for transportation,” said Worrall. “The gas tax is flat, and yet our needs are growing. I think we as transportation professionals are going to have to look to creative, unconventional solutions to traffic problems.” TME
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