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Suggested Citation:"Summary." 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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Suggested Citation:"Summary." 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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Suggested Citation:"Summary." 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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Suggested Citation:"Summary." 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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1 As operational and safety challenges continue to mount on the transportation system in the United States, the use of alternative intersections is becoming more prevalent. An alternative intersection is any intersection (including at-grade intersections located at ramp terminals of service interchanges) where one or more traffic movements is re-routed from a “traditional” intersection form to remove major conflict points. The family of alternative intersections, sometimes referenced as non-traditional, novel, or innovative intersections, or like terms, differs by state and region. Types of alternative intersections that have been imple- mented in the United States include the following [alternative names are shown in brackets]: • Roundabout, • Superstreet [J-turn, restricted crossing U-turn (RCUT), reduced conflict intersection (RCI), reduced conflict U-turn, synchronized street], • Median U-turn (MUT) [Michigan left, thru-turn, median U-turn crossover, boulevard turnaround, boulevard left], • Continuous flow intersection (CFI) [displaced left-turn (DLT)], • Continuous green-T (CGT) intersection [Continuous-T, Turbo-T, High-T, Florida-T, Florida green-T, Seagull intersection], • Jughandle [New Jersey jughandle], • Quadrant roadway intersection (QRI) [loop intersection], • Single point diamond interchange (SPDI) [single point urban interchange (SPUI), single point interchange (SPI)], and • Diverging diamond interchange (DDI) [double crossover diamond (DCD) interchange]. The objective of this synthesis was to document the evaluation and selection processes within state departments of transportation (DOTs) for intersection projects. The synthesis scope encompassed several facets related to the intersection selection process, including evaluation tools, design criteria, public outreach, operational and safety evaluations, challenges to implementation, and other considerations such as right-of-way requirements, pedestrians, and bicyclists. Attainment of the synthesis objectives involved the following three major tasks: a literature review, survey of DOTs, and development of case examples through follow-up interviews with selected agencies. Various literature sources such as guides, research reports, journal articles, and DOT policies and manuals were reviewed and compiled. In addition, an online survey questionnaire was distributed to all 50 states and the District of Columbia. Survey responses were received from all states and the District of Columbia for a response rate of 100%. Subsequent to completion of the survey, follow-up interviews to develop case examples were conducted with the following states: Alabama, Georgia, Indiana, Minnesota, Missouri, and Texas. S U M M A R Y Alternative Intersection Design and Selection

2 Alternative Intersection Design and Selection A key finding of the synthesis was that DOTs use several different approaches and tools for the evaluation and selection of intersection types. Nine states (California, Florida, Georgia, Indiana, Minnesota, Nevada, Pennsylvania, Washington State, and Wisconsin) have implemented Intersection Control Evaluation (ICE) policies that provide procedures for screening, analysis, selection, documentation, and approval of alternative intersections. DOTs have found ICE policies to be beneficial for standardizing procedures, documenting the decision-making process, and promoting the use of alternative intersections. The specifics of the ICE process vary among states. ICE policies generally follow a two- stage process consisting of a feasibility analysis for possible solutions followed by a more detailed analysis of the potential design alternatives. Some ICE policies require certain inter- section types to be evaluated at all times, while other ICE policies provide suggestions for various intersection form alternatives. The ICE process is usually triggered by a substantive change to an existing intersection. ICE policies generally require project managers to docu- ment the evaluation process using simple forms or detailed spreadsheets, and approval from an independent reviewer is necessary in most cases. Some DOTs, while not having a formal ICE policy, consider alternative intersections when trying to solve a problem and meet the purpose and need for a particular project. Other approaches used in the intersection selection process include consulting informa- tional guides developed for alternative intersections, value engineering studies, or traffic engineering studies. The suitability of the various alternative intersection types for each project is an important consideration used by DOTs in the selection process. Particular traffic and site conditions favor some forms over others, such that a geometric form fitting in one location may be ill-fitting in another. The use of engineering judgment helps to determine which inter- section types may be most appropriate at a given location based on site and traffic charac- teristics. In addition, constructability can be an important consideration in the intersection selection process. Various screening and analysis tools, such as spreadsheets and evaluation matrices, are available to facilitate the intersection evaluation and selection process. DOTs reported that the Manual on Uniform Traffic Control Devices (MUTCD), agency standards, Highway Capacity Manual (TRB 2016), and AASHTO Green Book are the resources most frequently used in the intersection selection process, although some states such as Georgia, Maryland, and Virginia have developed their own evaluation tools. With regard to DOT experiences with alternative intersections, the survey found that roundabouts are the most widely implemented type of alternative intersection. Ninety percent of DOTs reported having at least one roundabout on their system open and operational, while two DOTs indicated that they do not have a roundabout on the state system. The implementation of DDIs appears to be increasing as almost three-quarters of DOTs indicated that they have at least one DDI under project development. Single point diamond interchanges appear to be falling out of favor with some DOTs due to several concerns, including the need for large bridges, construction and maintenance issues, and challenges with accommodating bicyclists and pedestrians. CFIs are the least implemented of the alternative intersection types listed in the survey. DOTs are generally satisfied with the operational and safety performance of the alterna- tive intersections in their jurisdiction. DDIs, roundabouts, and superstreets were rated the highest in the survey by the DOTs, in part due to their prevalence and familiarity. CFIs received the lowest average rating of the alternative intersection types listed in the survey, although the range of ratings was wide, suggesting that results may be influenced by limited

Summary 3 experience with them. Evaluation studies identified through the literature search and survey generally show that alternative intersections improve both operations and safety, although the availability of evaluation studies for CFIs is limited. As some DOTs gain more experi- ence with alternative intersections, they are moving toward corridor implementations of alternative intersections in addition to assessing isolated intersections. Design criteria and standards for alternative intersections were reviewed as part of this synthesis. The literature review found that guidance for roundabouts, in the form of design manual chapters or standalone roundabout guides, is more prevalent than guidance for other alternative intersection types. DOT guidance documents, such as design manual sec- tions, standalone manuals, technical documents, or standard drawings, are more widely available for DDIs and superstreets than for other intersection types. DOTs have found that driver expectancy and human factors are important considerations as the design is developed. In addition, DOT experience indicates that lane assignments (with comple- menting “wayfinding signs” and pavement markings) are important, and their effective use can help drivers navigate through the intersection. Guidance regarding considerations for bicyclists and pedestrians at alternative intersections is under development with an anti- cipated completion date of September 2019. Some DOTs expressed a need for additional design guidance pertaining to alternative intersections. Regarding staffing for alternative intersections, approximately two-thirds of DOTs indi- cated that they currently have champions for alternative intersections or are in the process of identifying them. Some DOTs also have special in-house engineers or consultants review projects for certain types of alternative intersections, especially roundabouts (63% of DOTs) and DDIs (47% of DOTs). For example, the Georgia Department of Transportation pro- vides support for alternative intersections through its Roundabout and Alternative Inter- section Design team that performs design reviews and analyses for alternative intersections. Almost half of the DOTs responded that they provide training for employees or consultants to evaluate or design intersection types. Roundabout training is the most common type of training provided by DOTs, and four DOTs mentioned the use of FHWA training resources. The survey and follow-up interviews helped to identify challenges to the implementation of alternative intersections and strategies to overcome those issues. Barriers to implementa- tion as identified by DOTs include public and internal resistance, finding design guidance, land use constraints, and access management considerations. Overcoming public resistance and educating the public about alternative intersections are major concerns for DOTs. Much of the public resistance encountered revolves around changes in traffic movements or unfamiliarity with the design. Public resistance can vary among projects based on intersec- tion type and whether the project was initiated at the local or state level. DOTs often see that public and internal resistance declines as benefits become apparent and greater familiarity with the designs is achieved. Strategies used by DOTs to help overcome public resistance include project websites, meeting proactively with stakeholders, and creating project advi- sory committees. Visualizations at the drivers’ level have been an effective way to educate the public about alternative intersections. DOTs have found that increased internal and external awareness of alternative intersections can help to promote their use, and DOTs have benefited from learning about the experiences of other states through direct peer contact, published research, and FHWA assistance. This synthesis identifies some gaps in existing knowledge and suggests future study to address those gaps. Suggestions for future study include • The development of more advanced design guides for various types of alternative inter- sections to address the need for additional practitioner guidance for the selection and design of alternative intersections. These design guides include design-level elements such

4 Alternative Intersection Design and Selection as geometrics (e.g., offset of displaced movements from the central intersection), signing, pavement markings, lighting, and signal cabinets as well as information regarding traffic volumes and physical site conditions suitable for a given intersection type. • The development of operational and safety evaluation studies and enhanced practitioner guidance for CFIs. • The development of a handbook on public education and outreach that describes effective strategies to provide additional support for the implementation of alternative inter- sections by improving public awareness. • The development of briefing materials suitable for executives at public agencies to educate decision-makers regarding the implementation of alternative intersections. • The creation of a national clearinghouse that includes data on alternative intersection installations to facilitate the sharing of knowledge among DOTs.

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State departments of transportation often encounter public resistance to alternative intersections, with 86% of respondents in a new survey of state DOTs agreeing or strongly agreeing that public resistance hinders their implementation. Public resistance can vary among projects based on intersection type and whether the project was initiated at the local or state level.

The TRB National Cooperative Highway Research Program's NCHRP Synthesis 550: Alternative Intersection Design and Selection documents the evaluation and selection processes within state departments of transportation (DOTs) for intersection projects.

Roundabouts are the most widely implemented type of alternative intersection. Ninety percent of state DOTs that responded to the synthesis survey reported having at least one roundabout in their jurisdiction open and operational. Roundabouts also had the highest reported number of facilities in project development as 88% of respondents indicated there was at least one roundabout under development at their DOT.

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