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3 Section 1. Introduction 1.1 Background In the AASHTO A Policy on Geometric Design of Highways and Streets, commonly referred to as the Green Book, design speed is defined as a selected speed used to determine the various geometric design features of the roadway (AASHTO, 2018). In principle, design speed is used to determine all pertinent highway features to obtain a balanced design. The design speed concept has been the centerpiece of the geometric design process since introduced in 1936 (Barnett, 1936). Over an extended length of highway, it is desirable to select a uniform design speed. Selection of a design speed should be based upon the anticipated operating speed, topography, adjacent land use, modal mix, and functional classification of the highway (AASHTO, 2018). Where high- speed facilities meet low-speed facilities, drivers are expected to reduce their speeds suitable to the roadway environment they are entering. Ideally, this change in speed is achieved over an extended distance, but in the case of an interchange ramp, this is not possible. Conceptually, designing extended sections of highway based on the design speed process is relatively simple; however, when applied to interchange ramps, application of the design speed process and selection of an appropriate ramp design speed is a complex process. According to AASHTO policy, the ramp design speed should approximate the low-volume running speed on the intersecting highways; however, this is not always practical. Green Book Table 10-1 (Table 1) provides guidance for ramp design speeds based upon a range of percentages (i.e., upper, middle, and lower) of the higher-speed highway design speed. Further guidance is provided for selecting a ramp design speed from the table based on various conditions and ramp types. For example, design speeds between the middle and upper ranges of the highway speeds should be used for direct and semidirect connections, and upper-range values of design speeds generally are not attainable on loop ramps. Table 2 summarizes preferred and practical minimum ramp design speeds as recommended in the Institute of Transportation Engineers (ITE) Freeway and Interchange Geometric Design Handbook (Leisch, 2005). Table 1. Guide Values for Ramp Design Speed as Related to Highway Design Speed (AASHTO, 2018) U.S. Customary Highway design speed (mph) 30 35 40 45 50 55 60 65 70 75 80 Ramp design speed (mph) Upper range (85%) 25 30 35 40 45 50 50 55 60 65 70 Middle range (70%) 20 25 30 30 35 40 45 45 50 55 60 Lower range (50%) 15 20 20 25 25 30 30 30 35 40 45 Corresponding minimum radius (ft) See (Green Book) Table 3-7
4 Table 2. Design Speed Guidelines for Ramps (Leisch, 2005) Ramp Type Preferred Practical Minimum Right turning Upper range Middle range Loops 30-35 mph 25 mph Semidirect Middle-upper range 30 mph Direct Middle-upper range 40 mph The design values in Green Book Table 10-1 are applicable to the sharpest, or controlling, curve on the ramp, usually on the ramp proper, and do not pertain to the ramp terminals. The selection of a ramp design speed affects many aspects and elements of a ramp, including right-of-way requirements, horizontal and vertical alignments, speed-change lane lengths, ramp type, superelevation, and stopping sight distance, in addition to project costs and potential environmental and social impacts. Directly or indirectly, physical and economic constraints and anticipated operating speeds on the ramp should be considered when selecting the ramp design speed. The primary components of a ramp include the freeway mainline ramp terminal (i.e., an acceleration or deceleration lane), the ramp proper (i.e., the turning roadway), and the crossroad ramp terminal, and they must be considered together during the design of a ramp (Figure 1). The actual boundaries or transitions of where one component ends (e.g., freeway mainline ramp terminal) and the adjacent component begins (e.g., ramp proper) are not clearly defined in all cases. Figure 1. Primary Components of an Interchange Ramp Freeway CrossroadCrossroad ramp terminal Ramp proper Freeway mainline ramp terminal
5 The rationale of AASHTOâs current policy on ramp design speed is therefore in need of review. Guidance in the current Green Book is complex and may lead to insufficient or poor ramp designs. At the very least, the current guidance for selecting ramp design speed needs to be reviewed to determine if it can be improved and/or is consistent with the overall design speed process and concept. 1.2 Research Objective and Scope The objectives of this research were to (a) conduct a review of the state of practice for selecting ramp design speeds, (b) develop new guidelines for selecting appropriate ramp design speeds, and (c) recommend revisions to the AASHTO Green Book based upon the results of this research. The state of practice review was based on information gathered from published and unpublished sources, a survey of engineers, designers, safety professionals, and others; and interviews with selected engineers with practical experience designing interchange ramps. While developing new guidelines for selecting appropriate ramp design speeds, the research team considered the following: â¢ The guidelines should facilitate the selection of an appropriate ramp design speed based on a combination of contextual considerations and quantitative information. â¢ The guidelines should clearly define ramp design speed and all terms and elements related to its application, including the portion(s) of the ramp to which the design speed should be applicable. â¢ The guidelines should be based upon known safety and operational relationships of ramps over a full range of interchange forms, ramp types, and area environments (i.e., rural and urban). â¢ The guidelines should result in ramp designs consistent with driver expectations and behaviors over a range of traffic conditions and the functional classification of the two intersecting roads. â¢ Development of the guidelines should include the examination of field data and site observations supplemented with safety modeling considerations and traffic operational simulations, and speed transitions within ramps should be reviewed for performance and design policy implications. â¢ The ramp design speed guidelines should address, at a minimum: - System and service interchanges. - Entrance and exit ramps. - Different types of ramp interfaces at crossroads. - Single- and multilane ramps. - Degree of physical site constraints, environmental and social impacts, and right-of- way costs. - Effects of ramp grade. - Use of transition curves into the controlling alignment feature.
6 - Presence of ramp meters. - Ramp terminal configuration (e.g., tapered and parallel, presence of auxiliary lane). - Applicability of ramp design controls as opposed to mainline design controls for certain connections. Furthermore, the guidelines should be presented in a straightforward, easy-to-comprehend format so they are readily understood by potential users and for incorporation into the Green Book. 1.3 General Research Approach The first step towards accomplishing the research objective was to summarize the state of practice and other information related to ramp design speed selection. The research team reviewed published research studies, guidance documents, and design manuals; conducted an online survey of engineers, designers, safety professionals, planners, construction managers, operations managers, and others; and interviewed several survey respondents by telephone to explore issues covered by the survey in greater depth. Based upon the literature review, the survey results, and interviews with selected professionals, the research team identified key issues and limitations of AASHTOâs current policy on selection of an appropriate ramp design speed and developed a work plan to achieve the research objectives. The research plan consisted of developing a speed profile model to predict vehicle speeds on ramps. To do so, the research team reviewed several speed prediction models that estimate speeds of vehicles at certain locations along entrance and exit ramps. In addition, the research team collected speed data at several ramps to further refine and calibrate acceleration and deceleration/speed/distance models and curve speed prediction models so that models could be combined to predict speeds along an entire ramp. Priority was given to collecting speed data on ramps of system interchanges and ramps along freeways with posted speeds limits of 75 mph or higher. While developing the RSPM, the research team also developed an initial set of enhanced guidelines for selecting an appropriate ramp design speed. The research team also identified a few ramps to serve as case studies to apply and evaluate the enhanced guidelines for selecting an appropriate ramp design speed and refined the guidelines accordingly. The research results are presented in two companion documents. This final report presents the overall research including the literature review, survey, and interviews with selected designers; details about the speed data collected during the study; the statistical analyses to develop speed prediction models for ramps; the general conclusions from the research and future research needs; and recommended changes to the AASHTO Green Book. The companion document to this final report presents the enhanced guidelines for selecting an appropriate ramp design speed, details about the RSPM, and case studies that illustrate how to use the guidelines and RSPM to help make design decisions that are expected to result in ramp designs consistent with driver expectations and behaviors over a range of traffic conditions and the functional classification of the two intersecting roads (Torbic et al., 2021).
7 1.4 Outline of Report This report presents an overview of the work conducted in National Cooperative Highway Research Program (NCHRP) Project 15-56. The remainder of this report is organized as follows: Section 2âLiterature Review and Survey of Practice Section 3âObservational Field Study Section 4âDevelopment of RSPM Section 5âSummary of Design Guidelines Section 6âProject Summary and Future Research Needs Section 7âReferences Section 8âAbbreviations, Acronyms, Initialisms, and Symbols Appendix AâSuggested Text for Future Edition of AASHTO Green Book