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12 Literature Review This chapter presents the results of the literature review for alternative intersection design and selection. The chapter is organized into the following sections: guidance, policies, screen- ing and analysis tools, operational and safety evaluations, considerations for bicyclists and pedestrians, constructability, and public perception. Sources compiled for the literature review include guides, research reports, journal articles, policy documents, and Department of Trans- portation (DOT) policies, manuals, and standards. Guidance for Alternative Intersections Guidance for many types of alternative intersections is available through various publications. These publications include national guidance from FHWA and NCHRP as well as guidance documents developed by DOTs. An overview of guidance developed for alternative intersections is provided in the following sections. National Guidance for Alternative Intersections Many of the national manuals contain some information regarding alternative intersections. Highway Capacity Manual: A Guide for Multimodal Mobility Analysis, 6th ed., provides method- ologies for the analysis of roundabouts, roundabout interchanges, superstreets, median U-turns (MUTs), continuous flow intersections (CFIs), single point diamond interchanges (SPDIs), and diverging diamond interchanges (DDIs) (TRB 2016). The AASHTO Green Book contains some discussion of roundabouts, superstreets, MUTS, CFIs, SPDIs, DDIs, and jughandles, including advantages, disadvantages, and basic design considerations (AASHTO 2018). The Manual on Uniform Traffic Control Devices (MUTCD) (FHWA 2009) provides signal warrants and guidance regarding the use of traffic control devices that practitioners can adapt for appli- cation to alternative intersections, but only discusses roundabouts and jughandles specifically. The Highway Safety Manual (AASHTO 2010) provides quantitative crash estimation models for various segment and intersection types, but does not specifically address alternative inter- sections. Crash prediction models for roundabouts that were developed in an NCHRP study were incorporated into the HSM (Ferguson et al. 2018). The CMF Clearinghouse, which contains Crash Modification Factors (CMFs) that can be used to estimate crash reductions associated with countermeasures, contains some CMF values for alternative intersections (FHWA 2019a). For example, 19 CMF values are listed for conversion of existing at-grade intersections or diamond interchanges to DDIs. Guidance for alternative intersections at the national level is also available through several other publications. The Alternative Intersections/Interchanges: Informational Report (AIIR) includes chapters on CFIs, DDIs, MUTs, quadrant roadway intersections (QRIs), and super- streets as well as some discussion of other alternative intersection types such as jughandles, C H A P T E R 2
Literature Review 13 Continuous Green-Ts (CGTs), and SPDIs (Hughes et al. 2010). The report covers various topics at a basic level such as signalization, signing and marking, operational and safety performance, construction costs, construction sequencing, accommodations for pedestrians and bicyclists, considerations for transit users, and other considerations. The second edition of the NCHRP informational guide for roundabouts was released in 2010 (Rodegerdts et al. 2010). The guide covers planning, design, analysis, traffic control devices, lighting, construction, and maintenance. In addition, an NCHRP synthesis of state roundabout practices was completed in 2016 (Pochowski et al. 2016). A future needs study identified from the synthesis included the following: more guidance for roundabout planning, design, and eval- uation; finding ways to reduce roundabout installation costs; and determining effective methods for public outreach for roundabouts. Work on the third edition of a roundabouts guide through NCHRP was under way as of this writing. In 2014, the FHWA released separate informational guides for CFIs, DDIs, MUTs, and super- streets (Schroeder et al. 2014; Steyn et al. 2014; Hummer et al. 2014a; and Hummer et al. 2014b). A second edition of the DDI informational guide was being developed with an anticipated release date of late 2020. The informational guides cover many different topics such as planning, policy, operations, safety, design, signals, signing, marking, lighting, construction, and maintenance. Lists of known installations in the United States at the time of publication are provided in the appendices of the informational guides. A synthesis of MUTs, including some discussion of safety and operational benefits, is also available (Jagannathan 2007a). Resources available for other intersection types include an NCHRP report on SPDIs (Messer et al. 1991) and a synthesis of CGTs (CTC and Associates 2018). The FHWA YouTube Channel includes various videos on alternative intersections, including case studies (FHWA n.d.). Agency Guidance for Alternative Intersections Various DOTs have developed guidance and standards for different types of alternative inter- sections as shown in Appendix D. The guidance is usually provided as part of a design manual, a separate manual or technical document, or standard drawings. Guidance for roundabouts is the most common, followed by DDIs and superstreets. Limited DOT guidance is available for other alternative intersection types. Some of the DOT direction documents for alternative inter- sections are highlighted in the following sections. Roundabouts As shown in Appendix D, guidance for roundabouts is prevalent among DOTs. Some states, such as Georgia and Minnesota, include chapters on roundabouts in their design manuals (Georgia DOT 2018a, Minnesota DOT 2018). Other states, for example Kansas and Alabama, offer separate roundabout guides (Kittelson & Associates 2014, Jones and Majeed 2015). A draw- ing of typical roundabout elements from the Alabama guide is shown in Figure 10. The round- about guidance covers a variety of aspects such as planning, design, safety, operations, traffic control devices, lighting, landscaping, construction, maintenance, and access management. Many DOTs also provide roundabout standards. For example, Washington developed stan- dards for roundabout concrete curbs and pavement markings (Washington State DOT 2017, Washington State DOT 2018), while roundabout landscaping and typical sections are among Georgiaâs standard drawings (Georgia DOT 2011, Georgia DOT 2012). Diverging Diamond Interchanges A few states, including Minnesota, Missouri, and Utah, have developed diverging diamond interchange (DDI) guidance documents that cover many aspects such as basic considerations, advantages and disadvantages, design, construction, operations, safety, considerations for
14 Alternative Intersection Design and Selection bicyclists and pedestrians, pavement markings, signing, lighting, and public involvement (Minnesota DOT 2016a, Missouri DOT 2019, Utah DOT 2014). The elements of a DDI as shown in the Utah guidelines are shown in Figure 11. According to the Utah DDI guidelines, design features such as additional length between crossovers, more lanes across bridges, short auxiliary lanes, and clear overhead signage can help to improve DDI performance (Utah DOT 2014). Two concerns for DDIs as noted in the Minnesota guide are sight distance and crossover distance between the intersections (Minnesota DOT 2016a). Other states such as Indiana and Oregon provide some discussion of DDIs in the interchange chapter of their design manuals (Indiana DOT 2013, Oregon DOT 2012). Wisconsin includes guidance for signing at DDIs in its traffic manual (Wisconsin DOT 2019). Superstreet Mississippi and Minnesota developed guidance that covers many aspects of superstreets (also known as J-turns, restricted crossing U-turns, or RCUTs, reduced conflict intersections, or RCIs, reduced conflict U-turns, and synchronized streets), such as design, operations, signing, mark- ing, and considerations for bicyclists and pedestrians (Mississippi DOT 2010, Minnesota DOT 2017c). Direction regarding superstreet design can also be found in the design manuals or policy guides of Missouri, North Carolina, and Ohio (Missouri DOT 2018a, North Carolina DOT 2018a, Ohio DOT 2019). A previous layout from North Carolina for the U-turn, based on a 46-foot median, 4-foot paved shoulders, 55-mph posted speed, and WB-50 design vehicle is shown in Figure 12. The layout includes a bulb-out (or loon) designed to accommodate the WB-50 design vehicle (North Carolina DOT 2018a). However, North Carolina currently requires that the design accommodate a WB-67 or WB-62 design vehicle and is in the process of revising its standards to reflect this change in policy. Typical signing and pavement marking details for superstreets are also available (Minnesota DOT 2016b, Missouri DOT 2017, Wisconsin DOT 2019). Missouri sponsored studies that developed superstreet guidance for U-turn spacing and (Jones and Majeed 2015) Figure 10. Roundabout elements as shown in Alabamaâs roundabout guide.
Literature Review 15 (Utah DOT 2014) Figure 11. DDI elements. (North Carolina DOT 2018a) (Â©2018 North Carolina DOT) Figure 12. Previous layout for U-turn for North Carolina superstreet, based on WB-50 design vehicle.
16 Alternative Intersection Design and Selection acceleration lanes based on a micro-simulation study (Claros et al. 2017) and driving simulator study (Sun et al. 2016, Sun et al. 2017). Other Intersection Types Some limited design guidance is available for other intersection types. Utah developed guide- lines for CFIs (also known as displaced left turns, or DLTs) to help encourage their acceptance in Utah and document their design elements (Utah DOT 2013). The Utah CFI guide indicates that design consistency is a key factor in managing driver expectations and designer prefer- ences (Utah DOT 2013). Michigan provides some geometric guidance for crossovers at MUTs (Michigan DOT 2014). New Jerseyâs design manual includes a section on jughandles with design guidance and drawings of basic jughandle configurations such as the one shown in Figure 13 (New Jersey DOT 2015). In 2018, California issued a design information bulletin with guidance on SPDIs (California DOT 2018). Other states included some discussion of SPDIs in their design manuals (Oregon DOT 2012, Indiana DOT 2013, South Dakota DOT n.d.). Other Guidance for Alternative Intersections Additional guidance for alternative intersections, including implementation considerations, is available through other literature sources as described. (New Jersey DOT 2015) Figure 13. Layout for typical Type âAâ jughandle.
Literature Review 17 Diverging Diamond Interchanges In an evaluation of Utahâs first diverging diamond interchange (DDI), Rasband et al. (2012) indicated that consistent use of pavement markings, signing, traffic signals, and road design between DDIs may improve driver expectancy. Rasband et al. (2012) also suggested that provid- ing auxiliary lanes with advanced guide signing upstream of the crossover intersections may help mitigate possible issues with lane utilization. Based on an assessment of 12 design elements at eight operational DDIs in the United States, Chlewicki (2013) noted that possible sight distance issues are an important concern during DDI design due to the presence of median barriers, signs, and other elements that may limit sight distance. Other Intersection Types Based on experiences with several superstreets in North Carolina and Maryland, Hummer and Jagannathan (2008) found that bulb-outs (loons) at superstreets can help trucks make the U-turn, but they noted that additional signing and enforcement may be needed to prevent drivers from parking in the bulb-out. In addition, Hummer and Jagannathan (2008) indicated that the cost of the signal for signalized superstreets may be higher than anticipated. In a case study of North Carolinaâs first QRI, Reese et al. (2015) suggested that the use of overhead signs and advanced in-street pavement markings can help to improve wayfinding and prevent drivers from turning left at the main intersection. Intersection Control Evaluation Policies Several DOTs have developed Intersection Control Evaluation (ICE) policies to facilitate the process of selecting the appropriate intersection type at a given project. ICE policies are performance-based frameworks used to objectively evaluate intersection control types. As described in a presentation by FHWA, the benefits of implementing an ICE policy can include providing documentation of the decision-making process, encouraging consistency regarding the use of alternative intersections, and the ability to make a decision with justification based on the analysis of safety, operations, cost, environmental considerations, stakeholder feedback, and other factors (Shaw 2017). An ICE Primer that provides information on ICE procedures, resources, and tools is available from FHWA (FHWA 2018). As of May 2019, nine states currently have ICE policies as shown in Figure 14. The ICE policies prescribe procedures for screening, analysis, selection, documentation, and approvals. Refer- ences to the ICE policies of DOTs are provided in Table 1. Minnesota and Wisconsin were the first two states to develop ICE policies in 2007 and 2008, respectively. At that time, ICE policies were designed to mainly include roundabouts as part of evaluating an intersection. The evalua- tions of other alternative intersections and service interchanges were added later to ICE policies as the safety and operational advantages of these designs became more apparent. ICE policies generally include two stages. The first stage includes an initial screening to deter- mine the feasibility of different intersection types for a given location. In the second stage, fea- sible intersection types undergo more in-depth analysis for operations, safety, costs, and other considerations to select the most suitable intersection type for a given project. Some ICE policies divide the process into more stages. For example, Florida has a third stage that exam- ines further detailed evaluations when necessary. Washington State has a five-step process: (1) background and project needs, (2) feasibility, (3) operational and safety performance analysis, (4) alternatives evaluation, and (5) selection. ICE process specifics vary from state to state. Some ICE policies require certain intersection types to be evaluated at all times, while other ICE policies provide possible suggestions for various intersection form alternatives. For example, Indiana has a two-stage process during
18 Alternative Intersection Design and Selection which engineers follow a flowchart in both stages to arrive at the best intersection alternative. The Stage 1 and Stage 2 decision trees from Indianaâs ICE policy are shown in Figure 15 and Figure 16, taken from Indianaâs Intersection Decision Guide (IDG) (Bowen et al. 2014). Floridaâs ICE policy includes a decision assistance curve from Gyawali (2014) as shown in Figure 17. Washington requires that a roundabout always be considered at the Background and Project Needs stage (Washington State DOT 2019). Wisconsin has developed a flowchart that describes its ICE process along with a table that provides guidance on when each intersection type should be considered (Wisconsin DOT 2018). Example flowcharts from ICE policies are provided in Appendix E. DOTs with ICE policies require the ICE process when there is a substantive change to an existing intersection. These changes can be due to safety improvement, congestion mitigation, corridor improvements that include intersections, enhancements for nonmotorized users, and land development changes near an intersection. An ICE policy often has a trigger and/or an exemption that helps determine whether the full ICE process needs to be conducted. A variety of tools can be used throughout the ICE process. Tools that are used nationally include the Highway Safety Manual (AASHTO 2010), Safety Performance for Intersection Control Evaluation (SPICE) (Jenior, Butsick, Haas, and Ray 2018a), Highway Capacity Manual (TRB 2016), (Map created with mapchart.net Â©) Figure 14. Map showing states that have implemented ICE policies. State Year Implemented Reference to Most Recent Policy California 2013 Policy Directive 13-02 (California DOT 2013) Florida 2017 ICE Manual (Florida DOT 2017) Georgia 2017 ICE Policy Document (Georgia DOT 2017) Indiana 2014 Indiana Decision Guide (IDG) (Bowen et al. 2014) Minnesota 2007 ICE Manual (Minnesota DOT 2017a) Nevada 2018 Traffic Operations Process Memorandum 2018-01 (Inda 2018) Pennsylvania 2017 Publication 10X (DM1-X), Appendix AI (Pennsylvania DOT 2017) Washington 2015 Design Manual, Chapter 1300 (Washington State DOT 2019) Wisconsin 2008 Facilities Development Manual, Chapter 11, Section 25 (Wisconsin DOT 2018) Table 1. ICE policies for DOTs.
Literature Review 19 (Bowen et al. 2014) Figure 15. Indiana decision tree for Stage 1: Initial Feasibility Screening.
20 Alternative Intersection Design and Selection (Bowen et al. 2014) Figure 16. Indiana decision tree for Stage 2: Secondary, Expanded Performance Assessment.
Literature Review 21 (Gyawali 2014) Major Approach Volume (vph) M in or A pp ro ac h Vo lu m e (v ph ) Figure 17. Decision assistance curve from Floridaâs ICE policy. Capacity Analysis for Planning of Junctions (CAP-X) (Jenior, Haas, Butsick, and Ray 2018b), NCHRP Web-Only Document 220: Estimating the Life-Cycle Cost of Intersection Designs Tool developed in NCHRP Project 3-110 (Rodegerdts et al. 2016), and various traffic simulation models. States are also developing their own specific tools to help with evaluating intersections and are using these tools in the process of intersection selection, whether or not through a formal ICE process. Some of these tools are described in more detail in the subsequent sections. ICE policies generally require project managers to document the evaluation process. The type of documentation varies from simple forms to detailed spreadsheets. Example forms are shown in Appendix E. In most cases, the approval process is from someone independent of the project. For example, Georgia organizes projects into three different categories based on the improve- ment type, and each category has a different approval process (Georgia DOT 2017). Screening and Analysis Tools There are many tools available to facilitate the screening and analysis of alternative inter- sections. These tools allow for the comparison of design alternatives based on operations, safety, life-cycle cost analysis, and other factors. A summary of available tools for alternative intersections is provided in Appendix F. Some of these tools are described in the following sections. General Screening and Analysis Tools To support screening of alternative intersections, FHWA sponsored the development of the CAP-X Tool for operational analyses and the SPICE Tool for safety analyses. CAP-X
22 Alternative Intersection Design and Selection provides volume-to-capacity ratio as output based on critical lane analysis and an evaluation of pedestrian and bicycle accommodations for different types of intersections and service interchanges (Jenior, Haas, Butsick, and Ray 2018b). The SPICE Tool uses the HSM meth- odology, along with CMFs from the CMF Clearinghouse, to estimate the predicted crash frequencies and severities for different intersection types (Figure 18) (Jenior, Butsick, Haas, and Ray 2018a). Other available screening and analysis tools perform life-cycle cost analyses and provide eval- uation matrices or other guidance. The Life-Cycle Cost Estimating Tool provides the capability to compare alternatives based on a variety of factors such as operational and safety impacts, initial construction costs, maintenance and operations costs, and emissions (Rodegerdts et al. 2016). Evaluation matrices developed in a study by Abou-Senna et al. (2015) include informa- tion regarding the suitability of various intersection types based on the following factors: area type, roadway conditions, right-of-way, pedestrian-bicycle interactions, wayfinding, signaliza- tion, cost-to-benefits ratio, and operational advantages and disadvantages. Performance curves for diamond interchange types for different scenarios were developed in a micro-simulation study (Tarko et al. 2017a, Tarko et al. 2017b), and an example curve from the study is shown in Figure 19. Agency Developed Screening and Analysis Tools Some DOTs use their own customized software tools that allow screening of different intersection designs. Virginia utilizes the VDOT Junction Screening Tool (VJuST) that helps (Jenior, Butsick, Haas, and Ray 2018a) Figure 18. Sample output from the SPICE tool.
Literature Review 23 practitioners determine the intersection and interchange types that are most suitable for a given location (Virginia DOT 2018). VJuST provides results for both congestion and safety (based on conflict points). Georgia created a spreadsheet tool to implement its ICE policy (Georgia DOT 2018b). The Georgia ICE tool performs analyses for both Stage 1 (Initial Screening) and Stage 2 (Alternative Selection) and incorporates operational, safety, and environmental impacts along with stake- holder concerns to generate a final score for each feasible alternative (Figure 20). Pennsylvania also developed a spreadsheet tool for its ICE Policy (Pennsylvania DOT n.d.). The Pennsylvania ICE tool is based on the Life-Cycle Cost Estimating Tool created by Rodegerdts et al. (2016). Maryland uses the Maryland Unconventional Intersection/Interchange Design Analysis Tool and Maryland Intersection and Interchange Design and Capacity Analysis Program for its alter- native intersection analyses (Applied Technology and Traffic Analysis Program 2019, Maryland DOT State Highway Administration 2017). In lieu of software, some DOTs provide a general framework for the alternative intersection selec- tion process. For example, Indiana includes decision trees for Stage 1 (Initial, Feasibility Screen- ing) and Stage 2 (Secondary, Expanded Performance Assessment) in its IDG (Bowen et al. 2014). Operational and Safety Evaluations Many studies to evaluate the operational or safety impacts of alternative intersections have been completed. A summary of the operational studies is provided in Appendix G, and safety studies are summarized in Appendix H. Micro-simulation is the most common tool used in the operational studies, although some studies apply other methods such as linear and nonlinear (Tarko et al. 2017b) (Â©2017 Purdue University) (RA = roundabout interchange, DDI = diverging diamond interchange, TDI = tight diamond interchange, DI = diamond interchange, and SPI = single point interchange) Figure 19. Example performance curve for different diamond interchange types.
24 Alternative Intersection Design and Selection programming and field analysis. Safety studies are generally based on the analysis of data from one or more states. Some of these operational and safety studies are highlighted in the following sections. Continuous Flow Intersection The availability of evaluation studies for continuous flow intersections (CFIs) (also known as displaced left turns, or DLTs) in the United States is limited. Many of the existing operational studies relate to the development of models or procedures. Several studies developed optimiza- tion models for CFI elements such as phasing, distance to crossovers, and signal timing (Bai and Li 2017, Yang and Cheng 2017, and Zhao et al. 2016). Coates et al. (2014) proposed a signal control procedure that accounts for pedestrian wait time and queue length. Yang et al. (2013) developed models for CFI geometry for use at the planning level. A deterministic model of CFI operations built by Carroll and Lahusen (2013) found that left-turn length was the most important factor for CFI performance. A comparison of a CFI and parallel-flow intersection found comparable operational performance for the two designs (Dhatrak et al. 2010). A limited number of safety studies have found that the safety performance of CFIs is comparable with other intersection types, although a CFI has fewer conflict points than a conventional inter- section (Figure 21) (Utah DOT 2013, Qi et al. 2018). Continuous Green-T Evaluations of continuous green-Ts (CGTs) have focused on safety with mixed results. A study using sites from Florida and South Carolina found that the CGT reduced total crashes by 4% and fatal/injury crashes by 15% (Donnell et al. 2016). An FHWA case study of two rural CGTs in Colorado, including the one in Figure 22, showed a 60% reduction in total crashes and a 70% reduction in injury crashes (FHWA 2010). While these evaluations found safety ben- efits for CGTs, another evaluation found that the conversion of a CGT back to a conventional T-intersection resulted in crash reductions of 46% for total crashes and 56% for fatal/injury crashes (Lee et al. 2019). (Georgia DOT 2018b) Figure 20. Example summary of environmental/stakeholder data and final results from the Georgia ICE Tool.
Literature Review 25 (Utah DOT 2013) Figure 21. CFI conflict points. (Imagery Â©2019 Google, Imagery Â©2019 Maxar Technologies, USDA Farm Service Agency, Map data Â©2019 Google) Figure 22. CGT at US-50 and SR-141 in Grand Junction, Colorado.
26 Alternative Intersection Design and Selection Diverging Diamond Interchange Several operational studies of diverging diamond interchanges (DDIs) have been done, mostly based on micro-simulation. One of the early operational studies found that the DDI performed better than a conventional diamond interchange (CDI) for higher volumes but provided comparable performance to CDI at lower volumes (Bared et al. 2006). Another study used critical lane volumes (CLVs) and found that the DDI outperformed a CDI in two-thirds of the instances, while the DDI was still more cost-effective than a CDI (Chlewicki 2011). An evaluation using both CLVs and micro-simulation found that the DDI performed better than a CDI when left-turn proportions were greater than 50% (Guin et al. 2018). A study by Almoshaogeh et al. (2018) developed recommendations for cases that warrant consideration of a DDI (typically left-turn capacity between 500 and 750 vehicles per hour, per lane). An evalua- tion of the first DDI (Figure 23) found that the DDI improved traffic flow and reduced traffic delay and queuing for left-turning vehicles (Chilukuri et al. 2011). An empirical evaluation of DDIs using before and after field data also found that the DDI generally performed better than the CDI (Yeom and Cunningham 2017). A DDI discrete event simulation model was developed by Anderson et al. (2012), who suggested that the DDI is often a good solution but not always the best solution. An operational comparison of a DDI and SPDI found that the DDI performed better for volume imbalance conditions greater than or equal to 30% to 70% (Afshar et al. 2009). Several studies have also presented operational tools or guidance for DDIs, including the following: â¢ Guidance on crossover speeds, location, and cycle length (Chlewicki 2010); â¢ Lessons learned on eight operational DDIs (Chlewicki 2013); â¢ Guidance regarding the use of phasing schemes (Cunningham et al. 2016a); â¢ Lane utilization models (Yeom et al. 2017); â¢ CLV methodology for DDI evaluation (Maji et al. 2013); and â¢ Method to estimate DDI approach capacity based on analytical model (Pang et al. 2016). Other efforts have investigated corridor considerations for DDIs. One assessment of corridor operations near a DDI found that reducing the number of phases at downstream adjacent signals increased capacity, but created concerns about cost and disruption to users (Cunningham et al. (Imagery Â©2019 Google, Imagery Â©2019 Maxar Technologies, USDA Farm Service Agency, Map data Â©2019 Google) Figure 23. DDI at I-44 and SR-13 in Springfield, Missouri.
Literature Review 27 2016b). Zhang and Kronprasert (2015) suggested that use of Relaxed Bowtie, QRI, or super- streets at intersections adjacent to the DDI could help to improve capacity at nearby signals. Another study found that the presence of a DDI did not affect safety on adjacent intersections and speed-change lanes (Claros et al. 2017a). As shown in Table 2, multiple studies have shown crash reductions for DDIs, especially for fatal and injury crashes. Jughandle A micro-simulation study of three cases, including a forward/reverse jughandle as shown in Figure 24, found that the jughandle reduced average intersection delay and increased capacity when traffic conditions were nearly saturated (Jagannathan 2007b). Under low-to-medium traffic conditions, the operational performance of the jughandle was comparable to the conventional intersection. A safety evaluation using New Jersey data found that the jughan- dle resulted in lower crash rates for PDO, fatal/injury, and head-on crashes (Jagannathan et al. 2006). Median U-Turn A brochure from Michigan (Figure 25) indicates that median U-turns (MUTs) increase capacity by 20% to 50% (Michigan DOT n.d.). Michiganâs brochure also mentions that MUTs can reduce crash severity by 30% to 60% (Michigan DOT n.d.). A safety evaluation by Rista et al. (2018) found that crash reductions were achieved with the MUT. Another safety evaluation of MUTs in Michigan found significant crash reductions for fatal/injury crashes at unsignalized MUTs, although there were more PDO crashes at higher volumes (Kay et al. 2019). The safety results for signalized MUTs from the study by Kay et al. (2019) were inconclusive. Quadrant Roadway Intersection Safety and operational evaluations of QRIs have been limited. A study by Reid (2000) used micro-simulation to evaluate two levels of the following traffic characteristics: level of service (LOS) intersection volumes, arterial and cross-street volume splits, arterial directional splits, Crash Reductions State Reference Total Fatal/Injury Injury Project Level Minnesota Walls et al. 2018 52%â58% - - Missouri Claros et al. 2015 41%â48% 59%â63% - Multiple Hummer et al. 2016 33% 41% - Multiple Nye et al. 2019 37% 54% - Utah Song and Lloyd 2018 31%â36% 56%â57% - Site Specific Missouri Claros et al. 2017b 37.5% 55% - Utah Song and Lloyd 2018 26%â50% 42%â68% - Note: Hyphens indicate that crash reductions were not reported for that particular crash severity and state. Table 2. Crash reductions for DDIs.
28 Alternative Intersection Design and Selection (Jagannathan 2007b) Figure 24. Geometry for micro-simulation modeling of forward/reverse jughandle. (Michigan DOT n.d.) Figure 25. MUT diagram from Michigan brochure. and turning movement percentages. The study found that the QRI resulted in an average of 22% lower system travel time compared with a conventional intersection. The QRI performed better at higher volumes. A spot safety evaluation of the QRI at US-21 and SR-73 in North Carolina (Figure 26) found that total crashes increased by 31% and the total severity index increased by 1% in the first 3 years following construction of the QRI (Simpson 2017). This QRI does have several additional elements compared to a typical QRI, such as a superstreet ele- ment, four-legged intersection on the quadrant roadway intersections, and an adjacent inter- section closely spaced.
Literature Review 29 Roundabout As described in NCHRP Synthesis 488 on roundabout practices (Pochowski et al. 2016), several DOTs have calibrated the HCM (2016) parameters or studied operational characteristics of roundabouts. Many states have documented crash reductions associated with roundabout intersections, as shown in Table 3. A study of roundabout terminals at interchanges found crash reductions for single-lane roundabout terminals, but crash increases for dual-lane roundabout terminals (Claros et al. 2018). Single Point Diamond Interchange (SPDI) The availability of operational and safety evaluations for SPDIs is limited. Jones and Selinger (2003) compared the operational performance of an SPDI and tight diamond interchange (TDI) using micro-simulation analysis of 30 test cases. The results showed that SPDIs provided better performance with higher travel speeds and fewer stops and phase failures. The TDI typically reached capacity at a point when the SPDI was still operating at average conditions. A field study of SPDIs found that smaller designs performed better and that the use of continuous frontage (Imagery Â©2019 Google, Imagery Â©2019 Maxar Technologies, Orbis Inc, U.S. Geological Survey, Map data Â©2019 Google) Figure 26. QRI at US-21 and SR-73 in Huntersville, North Carolina.
30 Alternative Intersection Design and Selection roads appeared to inhibit performance (Dorothy et al. 1997). A recent micro-simulation study found that a TDI performed better operationally than an SPDI with frontage roads that allowed ramp/frontage road-through movements (Yue et al. 2018). The study evaluated seven scenario groups based on volumes. A study by Lee et al. (2002) compared the operational and safety performance of SPDIs with frontage roads and TDIs based on Arizona interchanges such as the one shown in Figure 27. The operational assessment found that the TDI generally performed better, especially with greater distance between frontage roads. The safety evaluation found that there was no significant difference between the crash rates at the SPDI and TDI (Lee et al. 2002). Another study found that there was no significant difference in total crashes between an SPDI and TDI, but the SPDI experienced fewer injury crashes than the TDI (Bared et al. 2005). Superstreet Several operational evaluations using micro-simulation concluded that the superstreet (also known as J-turns, restricted crossing U-turns, or RCUTs, reduced conflict intersections, or RCIs, reduced conflict U-turns, and synchronized streets) performed better than a conventional intersection (Kim et al. 2007, Hummer et al. 2010, Haley et al. 2011, Taha and Abdelfatah 2015, Hallmark et al. 2016, Morello and Sangster 2018). A field evaluation of an unsignalized super- street in Missouri (Figure 28) found that the wait time on the minor approach at an unsignalized superstreet intersection was half the wait time at a conventional intersection (Edara et al. 2015). Two field studies concluded that the travel time increased by one minute for vehicles crossing or turning left from the minor road due to the need for vehicles to travel downstream and make a U-turn (Inman and Haas 2012, Edara et al. 2015). Many states have documented crash reductions associated with unsignalized superstreets, as shown in Table 4. Considerations for Bicyclists and Pedestrians Resources that discuss considerations for bicyclists and pedestrians at alternative inter- sections are available. A qualitative study of methods to improve pedestrian safety at several types of alternative intersections assessed several operational and geometric elements such as phase combinations, coordination of signal timing, additional locations for crosswalks, and changes to geometry (Chlewicki 2017). A guide to help practitioners include bicycle and Crash Reduction State Reference Total Fatal/Injury Injury Georgia Gbologah et al. 2019 37%â48% 51%â60% - Maine Maine DOT n.d. 48% - 69% Minnesota Leuer 2017 and Leuer 2018 - 80% - North Carolina North Carolina DOTn.d.a 46% 75% - Pennsylvania Pennsylvania DOT 2018 47% - - Note: Hyphens indicate that crash reductions were not reported for that particular crash severity and state. Table 3. Crash reductions for roundabouts.
Literature Review 31 (Imagery Â©2019 Google, Imagery Â©2019 Maxar Technologies, U.S. Geological Survey, USDA Farm Service Agency, Map data Â©2019 Google) Figure 27. SPDI at I-17 and Camelback Road in Phoenix, Arizona. (Imagery Â©2019 Google, Imagery Â©2019 Maxar Technologies, USDA Farm Service Agency, Map data Â©2019 Google) Figure 28. Superstreet at US-63 and Deer Park Road in Columbia, Missouri.
32 Alternative Intersection Design and Selection Crash Reduction State Reference Total Fatal/Injury Injury Fatal Louisiana Sun et al. 2019 29% - - 100% Maryland Inman and Haas 2012 28%â40% - - - Minnesota Leuer and Fleming 2017 - - 50% - Missouri Edara et al. 2015 31% 64% - - North Carolina North Carolina DOT n.d.b 59% 71% - - Wisconsin Porter and Nelson 2018 21% 89% - - Note: Hyphens indicate that crash reductions were not reported for that particular crash severity and state. Table 4. Crash reductions for unsignalized superstreets. pedestrian considerations at alternative intersections is under development with an anti- cipated completion date of 2020 (Kittelson & Associates, Inc. 2020). Some public outreach materials for bicyclists and pedestrians are available, such as a brochure from Pennsylvania (Figure 29) that contains general guidance for bicyclists and pedestrians at roundabouts (Pennsylvania DOT 2013). Some studies looked at bicyclist and pedestrian considerations for specific alternative intersection types. For example, an evaluation of various crossing alternatives for bicyclists (Pennsylvania DOT 2013) Figure 29. Diagram from Pennsylvania informational roundabout brochure for bicyclists and pedestrians.
Literature Review 33 and pedestrians at a superstreet recommended the use of a combination of a direct cross and midblock cross for bicyclists and a combination of a diagonal cross and midblock cross for pedestrians (Hummer et al. 2014). An advanced signal control method to reduce vehicle delay at CFIs through incorporation of pedestrian wait time and queue length was developed by Coates et al. (2014). Many of the alternative intersection guidance documents such as Utahâs DDI guidelines (Utah DOT 2014) and Missouriâs Engineering Policy Guide (EPG) section on DDIs (Missouri DOT 2019) discuss considerations for bicyclists and pedestrians at alternative intersections. Constructability Guidance for Maintenance of Traffic (MOT) for alternative intersection projects was devel- oped in the form of example phasing diagrams for both the initial construction and mainte- nance of several alternative intersection types (Brown et al. 2015, Brown et al. 2016). A phasing diagram for the final conversion to a DDI is shown in Figure 30. The diagrams were developed (Brown et al. 2015) Figure 30. Phasing diagram for final conversion to DDI.
34 Alternative Intersection Design and Selection based on a literature review, practitioner survey and interviews, and review of project plans. The FHWA Alternative Intersection Guides (Hummer et al. 2014a, Hummer et al. 2014b, Schroeder et al. 2014, Steyn et al. 2014) also include some discussion of options for construction staging on alternative intersection projects. Public Perception There have been a few studies regarding the public perception of alternative intersections. An evaluation of the public perception of West Virginiaâs first two roundabouts found that the publicâs approval rating one year after construction was approximately 25% higher than before construction (Elyard et al. 2016). An assessment of public perception of Missouriâs first DDI showed that at least 80% of respondents thought that the DDI improved operations and safety and understood how the interchange worked (Chilukuri et al. 2011). In a report describing its experience with its first DDI, Missouri noted that visualizations at the driversâ level, such as the one shown in Figure 31, may be more effective than aerial views in helping the public become more comfortable with DDIs (Missouri DOT 2010). A study using focus groups for three DDI sites found that participants generally thought the DDI was an improvement over the previous conditions but expressed concerns regarding the effects to adjacent intersections and the design of pedestrian and bicycle facilities (Jackson et al. 2014). A study of public perception of super- streets included residential, commuter, and business surveys and found that business managers perceived that the existence of access issues and driver confusion was having a negative impact on their businesses (Hummer et al. 2010). In a case study of North Carolinaâs first QRI, Reese et al. (2015) found that public reaction was generally negative when the project was announced. Soon after the project was built, the public became supportive of the QRI after experiencing improved traffic flow at the intersection. Figure 31. Screenshot from Missouri DDI drive-through animation. (Missouri DOT 2013)