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68 C H A P T E R 5 Synthesis Objectives, Scope, and Methodology The objective of this synthesis was to document the existing state of the practice by Depart- ments of Transportation (DOTs) for alternative intersection design and selection. The synthesis scope included evaluation tools, design criteria, public outreach, operational and safety evalu- ations, challenges to implementation, and other considerations such as right-of-way require- ments, pedestrians, and bicyclists. The synthesis included any type of intersection in which select traffic movements (e.g., mainline left turns) are re-routed, including at-grade intersections located at ramp terminals of service interchanges. Types of alternative intersections covered by the synthesis include (but are not limited to) the following [alternative names are shown in brackets]: â¢ Roundabout, â¢ Superstreet [J-turn, restricted crossing U-turn (RCUT), reduced conflict intersection (RCI), reduced conflict U-turn, synchronized street], â¢ Median U-turn (MUT) [Michigan left, thru-turn, median U-turn crossover, boulevard turn- around, boulevard left,], â¢ Continuous flow intersection (CFI) [displaced left-turn (DLT)], â¢ Continuous green-T (CGT) intersection [Continuous-T, Turbo-T, High-T, Florida-T, Florida green-T, Seagull intersection], â¢ Jughandle [New Jersey jughandle], â¢ Quadrant roadway intersection (QRI) [loop intersection], â¢ Single point diamond interchange (SPDI) [single point urban interchange (SPUI), single point interchange (SPI)], and â¢ Diverging diamond interchange (DDI) [double crossover diamond (DCD) interchange]. Methods used to achieve the synthesis objectives included a literature review, survey ques- tionnaire, and follow-up interviews. Various sources such as guides, research reports, jour- nal articles, and DOT policies and manuals were reviewed and compiled. An online survey questionnaire was distributed to all 50 states and the District of Columbia. Survey responses were received from all 51 agencies for a response rate of 100%. Case examples were devel- oped through follow-up interviews with Alabama, Georgia, Indiana, Minnesota, Missouri, and Texas. Summary of Key Findings Key findings from the synthesis based on the literature review, survey results, and follow-up interviews are described in the following sections, which are organized by topic. Conclusions
Conclusions 69 Experience with Alternative Intersections â¢ Roundabouts are the most widely implemented type of alternative intersection. Ninety percent of DOTs reported having at least one roundabout in their jurisdiction open and operational. Two DOTs indicated that they currently do not have roundabouts 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. â¢ CFIs are the least implemented of the alternative intersection types listed in the survey with 11 DOTs reporting at least one open and operational CFI. â¢ DOTs are generally satisfied with the performance of the alternative intersections in their jurisdiction. DDIs, roundabouts, and superstreets were rated the highest by the DOTs. 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 experience with them. Some concerns were raised with CFIs (signalization, right-of- way requirements) and SPDIs (bridge size, construction issues, maintenance, and lane assign- ment). SPDIs appear to be falling out of favor with some DOTs. â¢ Evaluation studies have generally found that alternative intersections improve operations and safety. â¢ The availability of evaluation studies for CFIs is limited. â¢ Some DOTs are moving toward corridor implementations of alternative intersections instead of looking only at individual intersections. â¢ Many DOTs are blending elements of different geometric forms of alternative intersections into the same intersection or interchange. Methods and Tools Used for Alternative Intersection Evaluation and Selection â¢ As of May 2019, nine states had implemented Intersection Control Evaluation (ICE) poli- cies that provide procedures for screening, analysis, selection, documentation, and approval of alternative intersections. States have found ICE policies to be beneficial for standardizing procedures, documenting the decision-making process, and promoting the use of alternative intersections. â¢ The specifics of the ICE process vary between 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. The ICE process is usually triggered by a substan- tive change to an existing intersection. â¢ ICE policies generally require project managers to document the evaluation process using simple forms or detailed spreadsheets, and approval from an independent reviewer is neces- sary in most cases. â¢ Six DOTs indicated that they are either developing an Intersection Control Evaluation (ICE) policy or tool or are planning to develop one in the future. â¢ Other states, although not having a formal ICE policy, consider alternative intersections when trying to meet the purpose and need of the project. The FHWA Informational Guides (Steyn et al. 2014; Schroeder et al. 2014; Hummer et al. 2014a; Hummer et al. 2014b), value engi- neering studies, or traffic engineering studies are sometimes used during the intersection selection process. â¢ DOTs reported that the Manual on Uniform Traffic Control Devices (MUTCD), agency stan- dards, Highway Capacity Manual (TRB 2016), and AASHTO Green Book are the resources most frequently used in the intersection selection process. Some states such as Georgia, Maryland, and Virginia have developed their own evaluation tools. â¢ Constructability can be an important consideration in the intersection selection process.
70 Alternative Intersection Design and Selection â¢ 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 intersection types may be most appropriate at a given location. â¢ A common approach to the selection process involves focusing on solving the problem as indicated by the purpose and need of the project. Design Criteria and Standards â¢ Many DOTs (approximately 60%) have developed guidance for roundabouts, and to a lesser extent superstreets, either as chapters in their design manuals or standalone guides. â¢ DOT guidance documents such as design manual sections, standalone manuals, technical documents, or standard drawings are more widely available for DDIs and superstreets than for other intersection types. â¢ Additional guidance regarding considerations for bicyclists and pedestrians at alternative intersections is under development with an anticipated completion date of 2020 (Kittelson & Associates, Inc. et al. 2020). â¢ DOTs have found that driver expectancy and human factors are important considerations as the design is developed. â¢ DOT experience indicates that lane assignments (with complementing âwayfinding signsâ and pavement markings) are important, and their effective use can help drivers navigate through the intersection. Challenges to Implementation â¢ DOTs often encounter public resistance to alternative intersections, with 86% of survey respondents agreeing or strongly agreeing that public resistance hinders their implementation. Much of the public resistance revolves around changes in traffic movements or unfamiliarity with the design. Public resistance can vary among projects based on intersection type and whether the project was initiated at the local or state level. â¢ Strategies used by DOTs to help overcome public resistance include project websites, meet- ing proactively with stakeholders, and creating project advisory committees. Visualizations at the driversâ level have been an effective means of educating the public about alternative intersections. â¢ DOTs have found that increased internal and external awareness of alternative intersections can help to promote their use. â¢ Other challenges to implementation include internal resistance, finding design guidance, land use constraints, access management, driver awareness, large vehicles, and funding constraints. â¢ DOTs have benefited from learning about the experiences of other states through direct peer contact, published reports, and FHWA assistance. Suggestions for Future Study This synthesis identifies some gaps in existing knowledge and suggests future study to address those gaps. Suggestions for future study include â¢ 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 as geometrics (e.g., offset of displaced movements from the central intersection), signing,
Conclusions 71 pavement markings, lighting, and signal cabinets as well as information regarding traffic volumes and physical site conditions suitable for a given intersection type. â¢ The development of operational and safety evaluation studies and enhanced practitioner guidance for CFIs. â¢ The development of a handbook on public education and outreach that describes effective strategies to provide additional support for the implementation of alternative intersections by improving public awareness. â¢ The development of briefing materials suitable for executives at public agencies to educate decision-makers regarding the implementation of alternative intersections. â¢ The creation of a national clearinghouse that includes data on alternative intersection instal- lations to facilitate the sharing of knowledge among DOTs.