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|Federal Highway Administration > Publications > Public Roads > Vol. 77 · No. 1 > Design at the Crossroads|
Publication Number: FHWA-HRT-13-005
Design at the Crossroads
by James McCarthy, Joe Bared, Wei Zhang, and Mark Doctor
Alternative intersection and interchange geometries are helping communities solve mobility challenges for a fraction of the price.
Meeting the safety and mobility needs of the Nation’s growing population poses an ongoing challenge for today’s transportation professionals, especially in light of budgetary belt tightening in the public sector. Annual travel delay in hundreds of U.S. urban areas has increased from 1.1 billion hours in 1982 to 5.5 billion hours in 2011, as reported in the Texas A&M Transportation Institute’s 2012 Urban Mobility Report.
At many highway junctions, congestion continues to worsen. At the same time, drivers, pedestrians, and bicyclists experience increasing delays and heightened exposure to risk when crossing busy intersections. According to researchers at the Federal Highway Administration (FHWA), today’s traffic volumes and travel demands often lead to safety problems that are too complex for conventional intersection designs to handle properly. Consequently, engineers are considering various innovative designs for intersections and interchanges as they seek solutions to alleviate congestion and improve safety.
Since 2004, FHWA has sponsored and conducted considerable research on alternative intersections and interchanges. In 2010, these efforts culminated in publication of FHWA’s Alternative Intersections/Interchanges: Informational Report (FHWA-HRT-09-060), the first national compendium on this topic. Since the report’s release, and with additional promotional support from numerous workshops and training sessions, FHWA’s research findings have inspired a proliferation of these new intersection and interchange designs across the country. Diverging diamond interchanges, displaced left-turn intersections, and restricted crossing U-turn intersections are among those gaining the most traction among State and local departments of transportation (DOTs).
Dozens of recently constructed junctions across the United States feature these and other innovative designs described in detail in FHWA’s report. “These designs are saving drivers thousands of hours of travel time and lowering the likelihood of crashes considerably,” says Associate Administrator Jeff Lindley of FHWA’s Office of Operations.
What follows are highlights from the report, including a description of four new intersection and interchange types, the rationale for choosing them, and the user and cost benefits they offer compared to traditional designs.
Diverging Diamond Interchanges
The diverging diamond interchange (DDI, also known as double crossover diamond) is a new design that is a variation of the conventional diamond interchange. The main difference between a DDI and a conventional diamond interchange is the crossing (or channelizing) of the traffic on the crossroad to the left side between the ramp terminals. The DDI design eliminates the need for constructing expensive left-lane bays or double left-turn bays by allowing left-turning vehicles on the crossroad to make a free turn left directly onto the onramp.
Because of its innovative way of connecting a freeway to a surface street, in recent years the DDI has become one of the leading alternative designs being implemented in the United States.
Traffic Safety and Cost Benefits
FHWA research and real-world experience in the United States and abroad with newly constructed DDIs demonstrate overall better performance than existing conditions and conventional designs. The DDI can operate with shorter traffic signal cycles and fewer phases than a conventional diamond interchange. When opposing traffic volumes are unbalanced (more than 60/40 percent) the DDI will reduce delay and stop times and shorten queue lengths compared to a conventional diamond design. Conversely, if through traffic is heavy and balanced (50/50 percent), a DDI might not perform as effectively because opposing movements cannot run simultaneously.
The DDI design also enhances the safety and operation of the intersections at a diamond-style interchange by reducing the number of perpendicular conflict points and by eliminating the need for a protected left-turn phase at the two intersections. A DDI has only 14 conflict points at the two crossing points compared to 26 conflict points in a standard diamond. Researchers at FHWA are studying the safety performance of 6 of the current 17 DDI deployments and will report on the results once that work is completed.
The earliest DDI project built in the United States, completed in 2009 on I–44 in Springfield, MO, showed an 80 percent reduction in injury crashes and 53 percent reduction in all crashes in its first year of operation. Preliminary studies of this and other DDIs show that this design holds promise for improving safety performance.
Design engineers at the Missouri Department of Transportation (MoDOT) noted that the interchange geometry also has traffic-calming features that reduce speeds while maintaining capacity. Another benefit is that the geometric design eliminates wrong-way movements onto ramps. According to MoDOT officials, these features result in fewer and less severe crashes.
Many of the DDIs are being built without replacing or widening the existing overpass structures. Highway agencies can reuse the existing overpass and underpass structures, resulting in cost savings of up to 70 percent per project compared to a single-point urban interchange. DDI projects that reuse existing bridges typically cost $2 million–$10 million, while the cost for most interchanges that require new bridge structures often exceeds $10 million. Given these proven advantages, use of the DDI design is growing rapidly nationwide.
“The [DDI] in Reno, NV, replaced a traditional transportation solution that would have been an order of magnitude more expensive, required right-of-way acquisition, [required] bridge reconstruction, and would have impacted the public for years,” says Adam Searcy, senior project manager with the Nevada Depart-ment of Transportation (NDOT). “Improvements to safety and capacity [were] achieved [in] a matter of months rather than years and for a pricetag [that was] a fraction of what yesterday’s solution would have cost. These types of innovations are what the future of this industry needs and the public should demand.”
Case Study: I–515 In Henderson, NV
A prime example of using the DDI to improve the operation of a failing standard diamond facility is the reconstruction of the I–515 service interchange at Horizon Drive in Henderson, NV. NDOT constructed the original interchange as a standard diamond. But, 15 years later, in 2010, the traffic volume on the ramps had grown by 300 percent, leading department and city officials to consider reconstructing the interchange.
With the goal of relieving congestion at the ramp terminal intersections, NDOT evaluated three alternatives: (1) addition of dual left-turn lanes on Horizon Drive and the off-ramps, (2) installation of a DDI using the existing crossroad pavement, and (3) construction of a single-point urban interchange (in which all through traffic on the arterial street, as well as the traffic turning left onto or off the interchange, can be controlled from a single set of traffic signals).
The alternative analysis and traffic study confirmed that the DDI would accommodate existing and future traffic demands effectively and would not require replacement of the freeway overpass. In the evaluation, the dual left-turn option and the DDI promised similar benefits in terms of performance, with the DDI offering slightly less delay during all time periods studied. The primary difference in the construction of these two options was that the dual left-turn option would use seven lanes across the overpass bridge, while the DDI required just five lanes. The DDI alternative, therefore, would leave room for future expansion and provide superior accommodation for pedestrians and bicyclists.
Although the single-point urban interchange also offered improved performance compared with existing conditions, NDOT determined that it was not competitive with the DDI for a number of reasons. First, the single-point urban interchange would have delivered less improvement than the DDI and would have caused significant work zone delays on I–515. Specifically, it would have required completely replacing the overpass, resulting in lane closures and other work zone traffic control on the I–515 freeway mainline. In addition, the time to construct a single-point urban interchange would have been significantly longer (up to twice as long) than the relatively minor DDI project, which would take only about 9 months. Further still, at an estimated $20 million, the single-point interchange vastly exceeded the DDI’s more modest $2.25 million pricetag. Ultimately, NDOT opted to modify the original diamond interchange to a DDI scheduled for construction in late summer 2013.
According to FHWA estimates, if 25 percent of existing diamond interchanges in U.S. urban areas -- about 2,140--were converted to DDIs rather than reconstructed as single-point urban interchanges or widened conventional diamonds at the end of their useful lives, the cost savings would be staggering. At an estimated savings of $15 million per conversion, the savings would amount to $32 billion.
Displaced Left-Turn Intersections
A second emerging alternative design is the displaced left-turn (DLT, or continuous flow) intersection, which literally displaces the mainline left turns in advance of the intersection. Upstream of the main intersection, left-turning traffic crosses the median and opposing traffic, with signal control, and travels parallel to through traffic then turns left at the crossing with the side street. The relocation of left-turning movements in opposing directions eliminates the left-turn signal phase, thereby increasing the effective green time for all movements. Although new, this alternative is gaining acceptance for use at high-volume intersections.
Researchers at FHWA have analyzed the performance of both full and partial DLT intersections. (A partial DLT intersection is one in which some, but not all, of the legs feature displaced left turns. Each roadway radiating from an intersection is called a leg.) The studies show that DLT intersections offer operational improvements over conventional intersections, even at relatively low traffic volumes. Even greater benefits accrue as traffic volumes increase. The reduction in the number of signal phases minimizes delay and increases the intersection’s capacity considerably.
Under a range of operating conditions, full DLT intersections can reduce delays by 19–90 percent. For partial DLT intersections, including some installations in Utah, the reduction in delay related to traffic control was 30–39 percent. Another benefit: According to FHWA evaluations of the full range of DLT designs, simultaneous movement of left-turning and through traffic promotes improved progression of traffic platoons (faster-moving groups of vehicles) on the arterial and increases vehicular throughput.
Because relatively few DLT intersections existed when FHWA conducted its research for the report, limited data were available to evaluate the safety performance of this type of intersection design. However, the researchers concluded that, based on its design and operational characteristics, a DLT intersection offers at least one key safety advantage compared with conventional intersection designs: A reduction in conflict points. Specifically, a full DLT cuts the number of conflict points by 4 (from 32 for a conventional intersection to 28), while a partial DLT reduces conflict points by 2 (from 32 to 30).
To date, at least 19 DLTs have been constructed and are operational in the United States.
Case Study: West Valley City, UT
In 2006, during an environmental study for the Bangerter Highway corridor in West Valley City, UT, the Utah Department of Transportation (UDOT) began investigating the DLT design as a potential solution for the intersection of 3500 South and Bangerter Highway. Due to the high volume of traffic and recurring congestion, the conventional solution would have been a tight diamond interchange. But several challenges at the existing intersection required UDOT to look at alternative solutions.
First, traffic analyses revealed that simply widening the existing intersection would not adequately meet the capacity needs in the corridor. A grade-separated interchange would have cost about $35.1 million, which UDOT deemed prohibitively expensive. Further, that kind of interchange would have required the relocation of 20 residences and 3 businesses, as well as significant modification to access and mobility near the intersection, particularly on the crossroad to Bangerter Highway. The interchange solution also would have changed the character of the project area, including having negative impacts on nearby land uses. In the end, UDOT settled on a DLT intersection and completed construction in 2007.
Initial findings show an increase of 11 percent in intersection green times due to the DLT’s more efficient signal timing. The design also resulted in a 31-percent increase in capacity and a 50-percent reduction in delay at the intersection. Plus, UDOT achieved this improvement in performance for less than 20 percent of the cost of its standard practice of replacing this type of intersection with an interchange!
Restricted Crossing U-Turn Intersections
Another innovative design is the restricted crossing U-turn (RCUT) intersection, which prohibits left-turn and through movements from side street approaches. Instead, the RCUT intersection accommodates these movements by requiring drivers to turn right onto the main road and then make a U-turn maneuver at a one-way median opening 400 to 1,000 feet (122 to 305 meters) after the intersection. In cases of heavy left-turning traffic from the major road, the DOT might choose to restrict that left-turning traffic to continue through the intersection and then make a U-turn downstream, followed by a right turn after returning to the main intersection.
Also known as the superstreet or J-turn, the RCUT can be implemented on four-lane divided roadways at rural and suburban intersections at a fairly low cost. This intersection type works with either STOP signs or signal controls, depending on traffic volumes.
“Of all the types of alternative intersections, the RCUT is one of the more flexible, effective solutions and often generally a less expensive way to mitigate roadway safety problems on rural expressways,” says Jim Rosenow, design flexibility engineer at the Minnesota Department of Transportation (MnDOT).
Current methods for traffic analyses do not yet provide a valid quality-of-service comparison between RCUTs and standard intersections. Control delay, or the amount of delay attributable to traffic control devices, is the standard performance measure for intersections. Researchers at FHWA now are conducting traffic simulation studies to quantify both control delay and geometric delay (that which is due to a road’s geometry). The goal is to provide a valid comparison of interchange capacity, performance, and quality of service.
At rural expressways, longer travel distances to the U-turns for through and left-turning traffic from the minor road will require longer overall travel times for the RCUT than at conventional intersections. Nevertheless, at locations with high traffic volumes, a well-designed RCUT should reduce total travel time in comparison to a conventional two-way, STOP-controlled intersection because the delay at the STOP sign for right turns only will be reduced significantly compared with direct through and left-turning traffic.
Extensively used at locations in Maryland and North Carolina, the RCUT has a proven track record of safety benefits, attributable to reduced vehicle-to-vehicle conflict points. In fact, studies have shown that the RCUT can reduce or nearly eliminate high-speed, right-angle crashes (between vehicles on perpendicular approaches), which are the most severe type and the most likely to cause death or life-changing injuries. Safety studies in North Carolina, for example, have shown crash reductions of 50 percent after implementing RCUT intersections.
FHWA also has analyzed the safety of RCUT intersections, specifically a selection of those built in Maryland, versus a control group of intersections. That study, Field Evaluation of a Restricted Crossing U-Turn Intersection (FHWA-HRT-11-067), shows a 30 percent decrease in crashes at Maryland’s RCUT intersections.
Case Study: Minnesota
Following a series of workshops on intersection design, MnDOT adopted RCUTs as a solution to reduce high-speed, right-angle crashes. Along with drunk driving and not wearing seatbelts, right-angle crashes are among the top causes of highway fatalities, especially at two-stage crossing locations on divided highways. (In a two-stage movement, a driver first proceeds across one direction of traffic to the median opening and then waits there for the opportunity to cross or merge with traffic traveling in the other direction.) Fatal and serious injury crashes occurring on high-speed rural expressways have been a particularly vexing safety problem, especially because many of these expressways were built to increase mobility and improve safety.
In 2010, MnDOT installed the State’s first RCUT at the intersection of Business 71 and County Road 24 and 23rd Street in Willmar, MN, a location with a high crash rate and multiple fatalities. Since then, five MnDOT district offices have implemented RCUTs in their regions, and this alternative intersection design is now supported in the State’s highway safety improvement program.
“Minnesota’s Strategic Highway Safety Plan emphasizes lower cost, systemic treatments that can be implemented widely across all roads,” says Sue Groth, a State traffic engineer at MnDOT. “However, one of the few places where there is a concentration of severe crashes is at intersections on high-speed expressways. Reduced conflict intersections, or RCIs, as MnDOT has named them, provide an effective way to address this safety problem.”
The traditional strategy at MnDOT and other DOTs to correct safety problems had been to install traffic signals. But traffic signals can decrease mainline traffic flow significantly and lead to an increase in rear-end crashes, which is what happened at one intersection in Belle Plaine, MN. The initial fix to curb right-angle crashes at the intersection of U.S. 169 and County Road 3 was installation of a traffic signal. After 3 years, however, the signal had barely made a dent in the crash rate. So, in 2009, MnDOT installed an RCUT, which has nearly eliminated crashes.
In 2012, MnDOT’s Metro District, in partnership with FHWA, Carver County, and the city of Cologne, implemented an RCUT on the U.S. 212 expressway in southwestern Minneapolis. U.S. 212 had been constructed to improve travel times in Carver County, but as speeds increased on the new four-lane facility, the roadway became the site of a growing number of right-angle crashes and five fatalities in 10 years at the intersection between U.S. 212 and State Highway 284 in Cologne.
For the aforementioned intersection at U.S. 212 in Cologne, local businesses and land developers preferred the conventional solution of installing a traffic signal. However, after Will Stein, safety engineer at the FHWA Minnesota Division, described the benefits of installing an RCUT, county and city engineers eventually agreed to go with the alternative intersection design. Begun in early 2012, the RCUT was designed, built, and opened to traffic by October 2012. The cost was $1.45 million, and crews completed construction with only short-term lane closures.
The cost benefit of RCUT applications compared to traditional solutions merits underscoring. In the case of the Business 71/County Road 24 location, the original solution called for building a standard intersection with installation of traffic signals to minimize the crashes. The estimated cost of this solution, which required roadway realignment, was $4 million, plus ongoing costs to the tune of $8,000 per year to operate the large signal system. By contrast, the cost for the RCUT was $750,000, and design and construction were completed in less than 5 months. Construction of the traditional intersection with necessary mainline realignment and widening would have taken a full year and involved significant complications due to work zone staging.
Quadrant Roadway Intersections
Much like displaced left-turn intersections, the quadrant roadway (QR) intersection can relieve congestion at high-volume suburban intersections and may be used in place of grade-separated interchanges. This intersection form can provide up to 30 percent more capacity than a traditional intersection through the use of a connecting roadway added to a standard intersection in one quadrant to carry all left-turning traffic.
With a QR intersection, cycle lengths and phases are minimized at all signals, and traffic managers can employ a much more efficient signal timing plan. Whereas a large, traditional intersection might use a single signal with four to eight phases, the main intersection in a QR intersection always employs an efficient two-phase signal.
The QR intersection is a particularly flexible solution and may take a variety of geometric designs with minimal impact on overall operations. A DOT can adjust the length and alignment of the quadrant roadway and design the route to go around developed parcels of land and sensitive environmental features. According to FHWA’s study, this flexibility can reduce the cost for right-of-way significantly and may even allow for use of existing streets or frontage roads as the quadrant roadway.
FHWA researchers performed numerous traffic simulations during development of their Alternative Intersections/Interchanges: Informational Report, and those simulations revealed a variety of operational benefits associated with the QR intersection. They tested multiple geometric configurations with five different traffic patterns. The geometric configurations varied in the numbers of through lanes and turn lanes, and the traffic patterns included a range of through volumes and left-turn percentages.
Simulation results showed that the QR intersections performed comparably to the conventional intersections for moderate and balanced through volumes on the major road. However, the QR intersections had higher throughput and lower travel times compared to the conventional intersections for scenarios with heavy through traffic and moderate left-turn volumes on the major road and heavy through and left-turn volumes on the minor road. For such scenarios, the increase in throughput ranged from 5 to 20 percent, with a 50 to 200 percent savings in travel times.
Case Study: Fairfield, OH
After FHWA published its report on alternative intersection and interchange designs, one of the first applications of the QR intersection was completed in 2012 in Fairfield, OH, at the intersection of SR–4 and the SR–4 bypass. The Ohio Department of Transportation (ODOT) and city officials had built the bypass as an at-grade arterial, with a plan to upgrade it to a freeway in the future. But freeway conversion never came to be, and the arterial had been carrying a heavy traffic load and was approaching failure in its level of service.
With the goal of achieving a satisfactory level of service on the SR–4 bypass, ODOT and city officials considered a range of possible intersection types, including traditional and alternative configurations. Alternative solutions included a set of T-intersections, partial quadrant intersections, a displaced left-turn intersection, and a full quadrant intersection. Engineering studies confirmed that the full QR intersection would minimize delay and travel time.
The city constructed the QR intersection at a cost of $3 million and opened it to traffic just 18 months later on January 11, 2012. The cost to build a typical diamond interchange would have been more than $10 million, and the project would have taken up to 3 years.
“By using the quadrant intersection design at this location, the city was able to minimize right-of-way acquisition, in part, because the diversion road was able to be constructed within the limits of the existing right-of-way,” says Dave Butch, public works director with the city of Fairfield. “The functionality and design of the intersection has met the city’s overall expectations for such a high-volume intersection and corridor by reducing delays and improving traffic flow.”
Workshops and Training
Getting the word out about the variety of alternative intersection and interchange designs has been a key focus for FHWA since publication of the 2010 study. Workshops hosted by the Resource Center and division offices have been critical to sharing insights on how these alternative configurations work and highlighting successful installations around the country. The workshops have included interactive, hands-on design exercises with State DOT and local agency personnel to apply knowledge of alternative intersections to existing transportation problems. In collaboration with the National Highway Institute (NHI), the Resource Center also developed and conducts the NHI course 380109 Alternative Intersections and Interchanges.
In addition, the Resource Center and FHWA division offices have helped State and local partners review preliminary design proposals and operational analyses for alternative intersections and interchanges. To support public outreach, FHWA staff also has created high-resolution video animations showing how alternative designs work.
Further still, FHWA spearheaded development of a software tool called Capacity Analysis for Planning of Junctions (CAP-X, available at http://tsi.cecs.ucf.edu/index.php/cap-x/release-info), which designers can use to evaluate selected types of innovative junction designs -- eight intersections, five interchanges, and three roundabouts--using peak traffic volumes. Because deterministic methods do not yet exist to analyze alternative intersection designs, project teams need to use microsimulation programs to evaluate performance and refine project designs.
Through a pooled fund study, FHWA recently initiated a project to create deterministic analysis methods relevant for inclusion in future editions of the Highway Capacity Manual. The methods focus on DDIs and DLT, RCUT, and median U-turn intersections. The new analysis methods will fill the void between sketch planning tools, such as CAP-X and existing microsimulation programs, and will be critical to advancing the use of alternative intersections.
Together, these concerted efforts have been instrumental in moving alternative intersection and interchange designs from the research stage to deployment in the field.
Taking It to the Streets
As with any intersection modifications that change access, public resistance is to be expected, so DOTs need to demonstrate and communicate the time-saving attributes and safety benefits of these types of intersections.
Encouraging adoption of new intersection designs requires creativity, technical skill, and strong leadership. Most agencies and designers are accustomed to applying default solutions and may be reluctant to take risks on something new. That’s where FHWA’s Every Day Counts (EDC) initiative comes in, offering national leadership and technical expertise to help State and local agencies adopt proven solutions that can shorten project delivery and improve safety for all road users. Intersection and interchange geometrics are among the latest round of innovations FHWA is promoting under EDC2.
Today’s efforts build on the lessons learned over the past decade encouraging use of roundabouts as an alternative to traffic signals to achieve safety and operational benefits. Many of the tools and strategies used then also are proving effective now. For example, says Jeffrey Shaw, intersections program manager in the FHWA Office of Safety and a member of the EDC2 marketing team for intersection and interchange geometrics, “establishing a process or policy for evaluating intersection control scenarios will be an important part of mainstreaming these innovative designs.”
As a result of ongoing implementation efforts for roundabouts, at least six State DOTs -- Georgia, Minnesota, New Hampshire, New York, Washington, and Wisconsin--have implemented policies that require consideration of a roundabout as an alternative for certain intersection projects. More States, including California and Indiana, are in the process of studying or instituting such a policy change.
Adopting this type of design policy should lead engineers to consider alternative intersection and interchange designs routinely at the program and policy levels, and help institutionalize the design flexibility that allows for use of alternative configurations. In the end, putting these new designs into practice could save lives, time, and money.
Hughes, Warren, Ram Jagannathan, Dibu Sengupta, and Joe Hummer. Alternative Intersections/Interchanges: Informational Report (FHWA-HRT-09-060). FHWA, U.S. Department of Transportation, 2010. www.fhwa.dot.gov/publications/research/safety/09060.
Bared, Joe, and Don Saiko, “The Double Crossover Diamond.” Public Roads November/December 2010: 2–5. www.fhwa.dot.gov/publications/publicroads/10novdec/01.cfm.
Haas, Robert P., and Vaughan W. Inman, Field Evaluation of a Restricted Crossing U-Turn Intersection (FHWA-HRT-11-067). FHWA, U.S. Department of Transportation, 2012. www.fhwa.dot.gov/publications/research/safety/hsis/11067/index.cfm.
James McCarthy, PE, PTOE, is a traffic operations engineer with FHWA’s Minnesota Division Office in St. Paul, MN. He also splits his time working for the FHWA Resource Center, providing technical assistance to staff in other division offices. McCarthy serves on the Transportation Research Board’s Highway Capacity and Quality Service Committee and FHWA’s Traffic Analysis Tools Team. He holds a B.S. and an M.S. in civil engineering from the University of Minnesota.
Joe Bared leads the Concepts and Analysis Team in the FHWA Office of Operations Research and Development (R&D). He manages research related to analysis modeling and simulation of transportation concepts. He managed the development of the first roundabout guide in the United States and the FHWA publication Alternative Intersections/Interchanges: Informational Report (FHWA-HRT-09-060). He has a Ph.D. in transportation engineering from the University of Maryland.
Wei Zhang is program manager for the Intersection Safety R&D program in FHWA’s Office of Safety R&D. He manages research projects on alternative intersection designs, dilemma zone protection, corridor-level crash prediction, and sensor technology applications for intersection safety. Zhang holds a Ph.D. from the University of Minnesota and has been a registered professional engineer since 1995.
Mark Doctor is a safety and geometric design engineer with FHWA’s Resource Center. He promotes the use of flexible and innovative design practices on a national level and serves as a technical expert for FHWA in the areas of geometric design, freeway and interchange design, and highway intersection design. Doctor has a bachelor’s degree in civil engineering from Clemson University and a master of transportation engineering degree from the University of Florida.
For more information, visit www.fhwa.dot.gov/publications/research/safety/09060 or contact Wei Zhang at 202–493–3317 or mailto:firstname.lastname@example.org.
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