Continuous flow intersections

January 2006 » Feature Articles
Running late? Chances are you've been stuck in heavy traffic or maneuvered your way through a congested intersection at least once during the past week. The number of congested intersections nationwide is on the rise, and congested intersections tend to get worse over time.

Gaining speed in the United States

BY MICHAEL G. BRUCE, P.E., AND PAUL W. GRUNER, P.E., P.S.

Running late? Chances are you've been stuck in heavy traffic or maneuvered your way through a congested intersection at least once during the past week. The number of congested intersections nationwide is on the rise, and congested intersections tend to get worse over time.

Traffic congestion costs Americans $63.1 billion annually, according to the Texas Transportation Institute (TTI).

Traffic congestion is wasting not only more of our fuel - 2.3 billion gallons annually from idling engines - but also more of our time. In 2003, travelers spent a total of 3.7 billion hours in traffic delays, and the average rush-hour traveler spent 47 hours in traffic delays, up from just 16 hours in 1982, according to TTI studies.

Long-term traffic congestion can erode communities, disrupt infrastructure, and lower our quality of life - even threaten our health. A 2004 New England Journal of Medicine study reported that being stuck in heavy, urban traffic can increase a person's risk of having a heart attack.

To relieve traffic congestion, engineers often widen roads, add through lanes or turn lanes, improve cycles and synchronization, or add signalization. But soon, traffic congestion builds up again, and engineers are forced to consider the ultimate fix: a grade-separated interchange.

But because this solution often proves too costly to implement, chronic traffic problems may persist. Additionally, if a gradeseparated interchange is built, other ramifications can result, including the following:

• existing businesses may be disrupted or may have to be bought out,
• existing businesses may lose one or more access points,
• visual blocks may be introduced, and
• congestion may be relieved on one road but typically not on both.

But an innovative, cost-effective solution is gaining favor among traffic engineers and local and state agencies.

Continuous flow intersections (CFIs) address one of the major reasons for logjams at busy intersections - conflicts created with left-turn lanes. By removing these conflicts at certain intersections, CFI proponents say, we can eliminate a major source of traffic congestion in our nation's cities.

What are CFIs?

A CFI is a reconfigured, at-grade intersection that eliminates one or more leftturn conflicts at a main intersection. As a result, more vehicles can move smoothly through the main intersection, thus reducing traffic congestion and delays. These atgrade intersections achieve traffic flows just like grade-separated interchanges, but they cost much less.

Instead of relying on left-turn lanes, CFIs rely on dedicated left-turn bays. Leftturning drivers are routed first to a left-turn bay - several hundred feet before the main intersection - and then across lanes of stopped, oncoming traffic. Because these vehicles are now left of oncoming traffic, they can turn left simultaneously with through traffic at the main intersection, on the same green light, without stopping.

(For a video demonstration of CFIs, visit www.abmb.com/cfi-testdrive.html.) CFIs are controlled at each conflict point by coordinated traffic signals, which are operated by standard signal control boxes.

Because a CFI includes a main signalized intersection plus various signalized left-turn bays, a CFI is actually two or more closely spaced intersections timed to operate as one unit. With CFIs, left-turning drivers can turn without worrying about oncoming traffic, oncoming traffic does not stop at the main intersection to accommodate leftturning vehicles, and only through traffic uses the main intersection.

History of CFIs

The CFI concept, developed about 20 years ago, has been included in research efforts by the Federal Highway Administration, Transportation Research Board, and other organizations. It also has been documented in publications such as the ITE Journal, the FHWA Traffic Control Systems Handbook, Transportation Research Record, and NCHRP Synthesis 225, Left- Turn Treatments at Intersections.

More than 50 CFIs are operating worldwide: two in the United States and the remainder primarily in Mexico. The U.S. prototype was built in 1996 at Dowling College on Long Island, N.Y.

Four years later, a high-volume CFI achieved successful results at the juncture of Maryland State Highways 228 and 210, approximately 15 miles south of oftengridlocked Washington, D.C. “Results from this project have shown that, depending on location, we can benefit by relocating left turns away from the main intersection,” said transportation engineer Sae'd Rahwanji of the Maryland Department of Transportation (DOT), State Highway Administration.

Acceptance of CFIs

Because the CFI concept didn't appear in the textbooks of today's practicing engineers, some professionals have not pursued CFIs, while others are waiting until more are built nationwide before they weigh in. But other engineers and public works officials are convinced that CFIs are a better solution to relieve chronic traffic congestion at certain intersections.

“CFIs seem to function where conventional designs won't,” said Dirk Gross, administrator for the office of roadway engineering for the Ohio DOT. “In many cases, CFIs can move traffic better than any option we have now.” Advanced modeling software has helped demonstrate the positive effects of CFIs and their ability to reduce traffic delays dramatically. “Most engineers accept that today's sophisticated traffic engineering models are good predictive tools,” said Blaise Carriere, who handles special engineering projects for the Louisiana Department of Transportation and Development (LDOTD). “I have great confidence in these modeling techniques and numerical analyses.”

Benefits of CFIs

CFIs result in a smooth, continuous flow of traffic, and considerable benefits include the following: CFIs are economical. CFIs cost less than roadway widening projects or gradeseparated interchanges - sometimes as much as 80 percent less. ABMB found that a recent plan to widen most of a 5-mile stretch of roadway from four lanes to six lanes with 11 signalized intersections, at a cost of $19 million, could be replaced with two-bay CFI designs at just four of the intersections. The CFIs not only would provide better long-term results but also cost less - just $10 million.

CFIs are safe. While overpasses create visual barriers, CFIs provide clear lines of sight. And because CFIs reduce the number of conflict points at the main intersection, they're inherently safer.

Accident studies of the CFI at Maryland State Highways 228 and 210 from 2001 to 2004 revealed nothing out of the ordinary.

“We had no accidents that we felt were due to the CFI design itself,” Rahwanji said.

CFIs can be constructed faster than grade-separated interchanges. While constructing a grade-separated interchange can require 18 to 24 months, constructing a CFI takes about six months, or as long as a roadway widening project. Often, a CFI requires substantially less right-of-way than a grade-separated interchange, sometimes as much as 75 percent less.

CFIs save motorists time. Because CFIs allow more vehicles to pass through an intersection on a green light, drivers spend less time stuck in traffic. Studies by ABMB and Woolpert have shown that CFI designs reduce wait times at traffic lights by 50 percent to 90 percent compared with at-grade intersection designs. For example, simulated results for the CFI at Maryland State Highways 228 and 210 showed a 70.6 percent reduction in total delay on northbound 210 during the morning rush hour.

Some engineers are skeptical that CFIs decrease wait times so significantly, since conventional intersection improvements have produced only incremental benefits.

CFIs achieve exponential gains because they eliminate one or more signal phases in a cycle by removing left-turning drivers from the main intersection.

“Every time you have another phase, you have ambers, which are wasted time,” Carriere said. “If you can reduce the amber time and increase the green time, you'll reduce delay, and you'll become more efficient.” CFIs provide better service. CFIs can handle far more traffic than conventional at-grade intersection designs, which often require acquiring right-of-way and adding lanes to provide comparable levels of service.

Of course, in some cases, there is no at-grade solution that provides an acceptable level of service.

CFIs are well accepted by motorists.

A top-down view of a CFI intersection may appear confusing to some. But the driver's perspective - the view straight ahead - is not confusing. In fact, those who drive a CFI intersection say they don't notice anything different, and that driving a CFI is more intuitive than unusual.

“From the plan view, it looks like you are moving traffic in an unconventional way,” Rahwanji said. “But the driver sees the turn lane developing prior to the intersection and then makes the left turn. There's nothing unconventional about that.” CFIs are flexible. In many cases, CFIs can be adapted to an intersection's unique site conditions. And because CFIs can be installed in one-, two-, three-, and fourbay versions, more than one CFI design may be feasible at an intersection. Often, two-bay CFIs are constructed because they tend to avoid right-of-way and access issues that can exist with a four-bay CFI.

CFIs are environmentally friendly.

Because CFIs reduce congestion, they also reduce pollution from emissions. And because delays are reduced, drivers save on fuel costs.

CFIs have one potential drawback: If they affect site access to corner businesses, such as gas stations or convenience stores, these businesses may no longer be viable at their existing locations. In many cases, a frontage road can be built to improve access. But if no acceptable solution can be reached, corner businesses affected by the proposed CFI may have to be bought out.

CFIs in the works

Traffic engineers are exploring and designing CFIs, or managing their construction, at some of the nation's busiest interchanges in Ohio, Louisiana, Mississippi, Maryland, Utah, and Arkansas. For example in Ohio, design is under way following a study by engineering consultants ABMB Engineers, Inc., and Woolpert, Inc. The study proved the viability of a CFI for the heavily traveled intersection of Beechmont Avenue (State Route 125) and Five Mile Road in Hamilton County, an area with significant retail and commercial development. Study results using VISSIM, traffic and transit simulation modeling software by PTV America, Inc., showed that the CFI will reduce overall average delay at the peak afternoon rush hour from 73.6 seconds currently to 26.4 seconds. Engineers also say that the CFI will reduce accidents at this intersection since many are caused by left turns. The current average at this location is 45 accidents per year, which is among the highest recorded at any intersection in the county In Louisiana, construction of a two-bay CFI began for the Siegen/Sherwood intersection of Airline Highway, a highly congested urban corridor in Baton Rouge.

Afternoon rush hour drivers currently experience an average delay of 4 minutes at this intersection; the CFI will reduce this delay to about 30 seconds.

“We were able to apply this CFI concept within existing rights-of-way,” Carriere said. “Avoiding expensive real estate acquisition in an urban area was reason enough to move forward. Once this CFI is in operation, and we see how well it works, we'll plan more CFIs down the road.”

Getting started with CFIs

Each potential CFI site presents its own challenges. An independent analysis - based on traffic simulation modeling of existing conditions, conventional improvements, and the CFI alternative - is required at each intersection to determine if the CFI concept can be appropriately used, which approaches should incorporate the CFI design, and projected costs and impacts.

“You have to be sensitive to the location you're addressing,” Rahwanji said. “You don't want to introduce a CFI, reduce delay by 10 seconds, and create a propertyaccess issue, or relocate delay somewhere else. You must ensure you are reducing total delay for the whole intersection.”

Michael G. Bruce, P.E., is a founding principal of ABMB Engineers, Inc., which has offices in Louisiana and Mississippi. Bruce can be reached via e-mail at mbruce@abmb.com. Paul W. Gruner, P.E., P.S., is a vice president at Woolpert, Inc., a firm with offices throughout the country. He can be reached via e-mail at paul.gruner@woolpert.com.


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