It’s an unfortunate truth that congested intersections tend to become more congested over time—despite the traditional remedies available to transportation professionals. Additional turn lanes and signalization may relieve congestion for a while, but sooner or later adding another turn lane or through lane--or perhaps prohibiting some movement—may be needed. In some cases, the transition to a grade-separated interchange may seem like the only solution. There is, however, another alternative, one that is increasingly attractive considering the property impacts and financial burdens tied to grade-separated interchanges. It’s called the Continuous Flow Intersection (CFI), an at-grade intersection design that provides comparable level of vehicular flow to grade-separated interchanges at a fraction of the cost—and dramatic, long-term improvements over conventional at-grade approaches. An Elegant, Effective Idea CFI is based on one of those flashes of insight that seem obvious in retrospect. When you remove points of conflict at an intersection, through movements get more green time and intersection capacity rises. Since one of the biggest conflicts at many intersections occurs when left-turning vehicles cross oncoming traffic, modifying intersection geometry to eliminate the left-turn conflict can significantly reduce delay to through vehicles. That’s what the inventors of CFI did, proposing a solution so elegant and original that it consistently outperforms both traditional and nontraditional intersection designs. CFI Moves to the Mainstream Over the years, CFI has gained a small but growing following. Over 40 CFI intersections have proven their value in Mexico over the last decade, where they have reduced congestion and are well-accepted by motorists. In the United States, a prototype was built at Dowling College in New York, followed by a full-scale project in Maryland, at the juncture of State Highway 228 and State Highway 210, approximately 15 miles south of Washington, D.C. In 2002, this intersection, which was constructed under the auspices of the Maryland State Highway Administration in association with CFI design expert, Francisco Mier, received the prestigious Francis B. Francois Award for Innovation from the American Association of State Highway and Transportation Officials (AASHTO). "We have been overwhelmingly happy with how it's run, and the flow has minimized backups that were pretty significant prior to making this move," said David Buck, a spokesman for the Maryland State Highway Administration. Currently, additional CFI intersections are receiving serious consideration in Arizona, Arkansas, California, Louisiana, Maryland, Mississippi, Nevada, and Ohio. Ramanujan Jagannathan, of Virginia Tech. and Joe G. Bared, of the Federal Highway Administration, are presenting a paper at the 83rd Transportation Research Board Annual Meeting that should contribute to the growing acceptance of CFI. They used VISSIM to model three different variants of CFI intersections, providing all the values necessary to enable highway engineers to apply these models to problem intersections in their jurisdictions. In addition, they ran the numbers on CFI, comparing it to traditional designs. Their conclusion: “CFI consistently outperforms the conventional intersection even at low traffic volumes. The reduction in the number of phases on approaches having [CFI] geometries results in tremendous vehicular delay savings as well as considerable increase in the capacity of the intersection.” Stripping Out the Left-Turn Conflict The objective of CFI is to move the left-turn conflict out of the main intersection. In a typical CFI intersection, this is accomplished with a signalized left-turn bay placed several hundred feet before the intersection. The left turn leg feeds a special CFI leg, which in turn empties into the cross street at another signalized intersection. The signals at the left-turn bay, CFI crossover, and main intersection are all operated by a single controller and coordinated to provide smooth traffic flow. If you were in a car planning to make a left turn at a CFI intersection, this is how you might proceed: Step one. You enter the left-turn bay and stop at the first signal (most drivers don’t realize that this location is different from a conventional intersection). Step two. The signal at the CFI intersection is coordinated with the signal at the main intersection to optimize traffic flow. The signal at the end of the left-turn bay turns green (shortly after the cross traffic at the main intersection gets a green light). You cross the oncoming traffic lanes and proceed up the CFI leg. The right-turn lane merging with oncoming traffic will be on your left and the oncoming lanes themselves will be on your right Step three. Shortly before you reach the end of the CFI leg at its intersection with the cross street, the cross traffic gets a red light. The signal at the end of the CFI leg turns green and you complete your left turn onto the cross street without having to stop The beauty of this arrangement is the opposing traffic no longer has to be stopped to accommodate left-turning vehicles, eliminating a signal phase and increasing the amount of through traffic moving during green time. Although this design is new, drivers adapt easily. The authors of a study of the Dowling College intersection, Paul Abramson, Charles Bergren, and Reuben Goldblatt, conclude that “the intersection is easily negotiated by drivers who are initially unfamiliar with the design and that after a short learning curve, nearly all drivers are familiar and comfortable with the [CFI] intersection.” These conclusions have been echoed by those familiar with the 40 other CFIs in use. Breathe New Life into Old Intersections Ultimately, the true test of CFI intersections is how much they boost the capacity of an intersection. ABMB Engineers, a Baton Rouge, Louisiana engineering firm specializing in transportation, has made detailed studies of a number of intersections using traffic modeling software. These studies have shown that CFI outperforms conventional alternatives dramatically. In case after case, ABMB engineers found that CFI produced extraordinary improvements in levels of service under existing traffic loads and reductions in average intersection delay of 90% or more. They also determined that these advantages persisted decades into the future as traffic volumes continue to grow [show tables for MD 355]. CFI also saves money. ABMB is currently conducting a study of planned improvements for a five-mile, four-lane corridor with 11 signalized intersections. The current plan, which calls for widening the road to six lanes for most of the corridor, has an estimated price tag of $19 million. The preferred CFI alternative, with two-legged CFIs at the four major intersections, would cost just $10 million and provide significantly better long-term performance. The other seven signalized intersections will operate more effectively without any new construction because of better signal timing allowed by the CFIs. In fact, ABMB was so impressed by CFI’s advantages that 18 months ago it teamed up with CFI expert Francisco Mier to explore the use of CFI throughout the United States. Mr. Mier has designed over 40 operating CFI intersections worldwide. This relationship has led to the potential and ongoing projects noted previously. The Flexible Alternative CFI is certainly a flexible alternative. The Continuous Flow Intersection lives up to its name: it provides a continuous flow of traffic at all times, which is readily evident when viewing one in person or when observing an accurate traffic model. CFI can be deployed in a variety of geometries, including one-, two-, three-, and four-legged versions. Another attractive attribute is that it doesn’t require substantially more right-of-way than a traditional at-grade intersection and substantially less than an interchange, and although CFI can reduce access to existing businesses near the main intersection, the flexibility inherent in the CFI design means that these issues can be addressed in most circumstances. In any case, access restrictions are much less onerous than with a grade-separated interchange. Most of all, for state highway and transportation officials, CFI provides a high-performance, less expensive alternative to traditional remedies for congested intersections, and with congested intersections on the rise, that’s a good thing. For additional information, please contact Michael Bruce at email@example.com. References R. Jagannathan and Joe G. Bared, Design and Operational Performance of Crossover Displaced Left-Turn (XDL) Intersections (Also Called Continuous Flow Intersections (CFI)), Presented at the 83rd Transportation Research Board Annual Meeting, January 11-15, 2004, Washington, D.C. P. Abramson, C. Bergren, and R. Goldblatt, Human Factors Study of the Continuous Flow Intersection at the Dowling College NAT Center, (June 1995).
Source: Michael G. Bruce - ABMB Engineers, Inc.