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Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2020. Alternative Intersection Design and Selection. Washington, DC: The National Academies Press. doi: 10.17226/25812.
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2020. Alternative Intersection Design and Selection. Washington, DC: The National Academies Press. doi: 10.17226/25812.
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2020. Alternative Intersection Design and Selection. Washington, DC: The National Academies Press. doi: 10.17226/25812.
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2020. Alternative Intersection Design and Selection. Washington, DC: The National Academies Press. doi: 10.17226/25812.
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2020. Alternative Intersection Design and Selection. Washington, DC: The National Academies Press. doi: 10.17226/25812.
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2020. Alternative Intersection Design and Selection. Washington, DC: The National Academies Press. doi: 10.17226/25812.
Suggested Citation:"Chapter 1 - Introduction." National Academies of Sciences, Engineering, and Medicine. 2020. Alternative Intersection Design and Selection. Washington, DC: The National Academies Press. doi: 10.17226/25812.
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5 Introduction Background As traffic demand increases, the use of alternative intersections (including at-grade inter- sections located at ramp terminals of service interchanges) such as roundabouts, diverging diamond interchanges (DDIs), superstreets (also known as J-turns, restricted crossing U-turns, or RCUTs, reduced conflict intersections, or RCIs, reduced conflict U-turns, and synchronized streets), median U-turns (MUTs), and continuous flow intersections (CFIs) (also known as displaced left turns, or DLTs) is becoming more prevalent in the United States. Alternative intersections reduce the number of major conflict points by re-routing certain traffic move- ments, notably left turns, from direct through and turning movements at multi-legged junc- tions of roads under stop, yield, or unsignalized control. This strategic reduction in the number of conflict points leads to the realization of operational and safety benefits for alternative intersections as compared to traditional intersection designs under certain traffic and site condi- tions, often at a significant cost savings compared to other more traditional alternatives, such as adding through or turn lanes or replacing the intersection with an interchange. Due to the more widespread implementation of alternative intersections, there is a need for greater under- standing of the methods and processes used by state departments of transportation (DOTs) to evaluate alternative intersections and select the most appropriate intersection configuration for a given project. Objectives and Scope The objective of this synthesis project was to review and document the procedures and poli- cies applied by DOTs to evaluate and select alternative intersection projects. The scope of the synthesis included the following topics: ⢠Use of evaluation tools such as software tools, worksheets, and flowcharts; ⢠Design criteria for alternative intersections; ⢠DOT strategies and materials for public education and outreach; ⢠Constructability and maintenance of traffic (MOT) considerations; ⢠Evaluations of operational and safety benefits; ⢠Other considerations such as right-of-way requirements and accommodating bicyclists, pedestrians, and older drivers; and ⢠Challenges to implementation and strategies used by DOTs to overcome them. The inclusion of roundabouts in the decision-making process is part of the synthesis, but design criteria and operational and safety studies specific to roundabouts are not part of the scope, given the wealth of material already produced for that intersection type, including NCHRP Synthesis 488: Roundabout Practices (Pochowski et al. 2016) and NCHRP Report 672: Roundabouts: An Informational Guide, 2nd Edition (Rodegerdts et al. 2010). C H A P T E R 1
6 Alternative Intersection Design and Selection Definitions For the purposes of this synthesis, the following definitions are used: ⢠Intersection: At-grade junction of two or more streets. ⢠Alternative intersection: An intersection (including at-grade intersections located at ramp terminals of service interchanges such as diamond interchanges) where one or more traffic movements is strategically re-routed from a âtraditionalâ form to remove major conflict points. These intersections are also called nontraditional, novel, or innovative intersections. ⢠Department of Transportation: An agency from any of the 50 states or the District of Columbia that is responsible for the implementation of alternative intersections. Types of Alternative Intersections The synthesis includes all types of alternative intersections meeting the definition in the pre- vious section. The more common alternative intersection types are briefly described in the following sections. Although various DOTs have different names for many of these intersection types, only one name for each alternative intersection type is used for consistency throughout the synthesis. However, information regarding other names for types of alternative intersections is provided in the following paragraphs. In addition to the alternative intersections described here, there are many other types of less frequently used alternative intersections. Roundabout A roundabout is an alternative intersection where vehicles travel in a counterclockwise direc- tion around a central island. Although traffic circles have been in use in the United States since the early 1900s, the modern roundabout had its inception in the 1960s in the United Kingdom (Rodegerdts et al. 2010). The key features of the modern roundabout are the rule for entering traffic to yield to circulating traffic and the use of channelization and smaller radii to encourage lower entry and circulating speeds. Roundabouts can be characterized as mini-roundabouts (lower diameter of inscribed circle), single-lane roundabouts (at most one lane per approach), and multi-lane roundabouts (approaches can have more than one lane) (Rodegerdts et al. 2010). As with practically all alternative intersections, roundabouts displace left turns to effect better operation. They also displace through-movements. An example roundabout is shown in Figure 1. Superstreet Superstreets are also known as J-turns (when unsignalized), restricted crossing U-turns, reduced conflict intersections, reduced conflict U-turns, and synchronized streets. Superstreets reduce conflicts by redirecting the minor street through and left-turn movements. Through and left- turning minor street vehicles turn right, make a U-turn, and either turn right again or continue straight on the major street as seen in Figure 2. Left-turning vehicles traveling on the major street navigate the intersection like a conventional intersection, if the median is left open. In a variation of this intersection type, the median is closed, and vehicles turning left from the major street are redirected to the U-turn opening. Superstreets are most common in North Carolina. Median U-Turn MUTs are also known as Michigan lefts, thru-turns, median U-turn crossovers, boulevard turnarounds, and boulevard lefts. Figure 3 shows the navigation of a MUT. This type of inter- section is prevalent in Michigan. This design removes all left-turn conflicts by redirecting vehicles on both the major and minor streets to make a U-turn to turn left, whereas through movements from the mainline and cross road remain unaffected.
Introduction 7 Figure 1. Roundabout. (Brown et al. 2015) (Imagery ©2014 Google, Map data ©2014 Google) Figure 2. Superstreet. (Brown et al. 2015) (Imagery ©2014 Google, Map data ©2014 Google) Figure 3. Median U-Turn. (Adapted from Brown et al. 2015) (Imagery ©2014 Google, Map data ©2014 Google)
8 Alternative Intersection Design and Selection Continuous Flow Intersection CFIs (Figure 4) are also known as displaced left turns (DLTs) and crossover displaced left turns. They reduce conflict points at the main intersection by directing left-turning vehicles to crossover at a location upstream of the main intersection. Thus, left-turning vehicles do not encounter opposing traffic at the main intersection. Essentially, the traditional intersection is broken down into smaller movements to allow more traffic to flow continuously, because left turns and opposing through-movements occur simultaneously at the main intersection. The CFI has an interchange variation called a continuous flow interchange. Utah currently has the most CFIs in the United States. Continuous Green-T Continuous green-T (CGT) intersections, also known as Continuous-Ts, Turbo-Ts, High-Ts, Florida-Ts, Florida green-Ts, or Seagull intersections, are usually implemented at intersections with three legs, although there may be a fourth leg with a minor movement such as a driveway. In this signal-controlled design, the traffic moving in the direction on the top side of the âTâ passes through the intersection without stopping while the other direction is typically signal- ized. Vehicles turning left onto the major street use a channelized receiving lane to merge onto the major street as seen in Figure 5. CGTs improve efficiency by allowing free flow of the major street through movement in one direction. Figure 4. Continuous flow intersection. (Brown et al. 2015) (Imagery ©2014 Google, Map data ©2014 Google) (Imagery ©2019 Google, Imagery ©2019 Commonwealth of Virginia, Maxar Technologies, U.S. Geological Survey, USDA Farm Service Agency, Map data ©2019 Google) Figure 5. Continuous green-T.
Introduction 9 Jughandle Jughandles, also known as New Jersey jughandles, redirect left-turning movements at the main intersection using two types of turning movements as shown in Figure 6. In the first type of turning movement, left-turn vehicles exit to the right using a connector and then turn left to complete their desired movement. The other type of turning movement requires left-turning traffic to pass through the intersection, exit via a connector (which loops around to the right), and then transverses the intersection as part of the through movement. Jughandles are com- mon in New Jersey. Quadrant Roadway Intersection Quadrant roadway intersections (QRIs) (Figure 7), also known as loop intersections, consist of one main intersection with two secondary intersections linked by a connector road. The intention of the design is to increase efficiency by reducing the number of signal phases from four to two at the main intersection and improve safety by reducing points of potential conflict between vehicles. Left turns from all four legs are re-routed through the connector road instead of turning at the main intersection. Single Point Diamond Interchange Single point diamond interchanges (SPDIs) (Figure 8), also known as single point urban interchanges (SPUIs) or single point interchanges (SPIs), include only one signalized intersec- tion on the arterial instead of the two signalized intersections that are typically used at signalized conventional diamond interchanges (CDIs). All freeway ramps begin or end at one signalized intersection on the arterial. Right-turn movements are typically separated with yield control or traffic signalization. SPDIs can be designed as an overpass or underpass. (Imagery ©2019 Google, Imagery ©2019 Maxar Technologies, USDA Farm Service Agency, Map data ©2019 Google) Figure 6. Jughandle.
10 Alternative Intersection Design and Selection (Adapted from Brown et al. 2015) (Imagery ©2014 Google, Map data ©2014 Google) Figure 8. Single point diamond interchange. Diverging Diamond Interchange Diverging diamond interchanges (Figure 9) are placed at service interchanges and can be designed as overpasses or underpasses. They are occasionally known as double crossover dia- mond (DCD) interchanges. DDIs reduce conflicts by having traffic cross to the other side of the roadway between ramp terminals, thus removing left-turn conflicts at intersections. Right turns onto ramps occur before the traffic crosses to the opposite side of the roadway. Each crossover (Imagery ©2019 Google, Imagery ©2019 Maxar Technologies, U.S. Geological Survey, USDA Farm Service Agency, Map data ©2019 Google) Figure 7. Quadrant roadway intersection.
Introduction 11 (Brown et al. 2015) (Imagery ©2014 Google, Map data ©2014 Google) Figure 9. Diverging diamond interchange. intersection is usually signalized, although it is possible for some of the intersections to be unsig- nalized (Missouri DOT 2019). The CFI is an intersection variation of the DDI. Synthesis Methodology The synthesis approach included a literature review, survey, and case examples. The existing literature from many different sourcesâincluding guides, research reports, and DOT policies and standardsâwas reviewed and synthesized. An online survey was distributed to all 50 states and to the District of Columbia. Responses were received from all 51 agencies, leading to a 100% response rate. Follow-up interviews were conducted with representatives from six DOTs to develop case examples of agenciesâ experiences with alternative intersections. Synthesis Organization The remaining chapters of this synthesis are organized as follows: ⢠Chapter 2 describes the comprehensive literature review of studies, guidance, and policies. ⢠Chapter 3 provides information on DOT practices based on the survey results. ⢠Chapter 4 contains case examples for six DOTs. ⢠Chapter 5 presents the conclusions of the synthesis and recommendations for future study. There are also eight appendices. They are as follows: ⢠Appendix A presents the survey questionnaire. ⢠Appendix B is the list of responding agencies. ⢠Appendix C presents the individual survey responses. ⢠Appendix D provides design guidance and standards for departments of transportation. ⢠Appendix E presents example flowcharts and tables from intersection control evaluation policies. ⢠Appendix F presents screening and analysis tools for alternative intersections. ⢠Appendix G is a summary of operational studies for alternative intersections. ⢠Appendix H is a summary of safety evaluations of alternative intersections. A list of abbreviations and references for the text and appendices are at the end of the synthesis.
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