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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26414.
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Suggested Citation:"Summary." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report. Washington, DC: The National Academies Press. doi: 10.17226/26414.
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1 Summary The design speed concept has been the centerpiece of American Association of State Highway and Transportation Officials (AASHTO’s) geometric design policy since the 1930s. Designing extended sections of highway based on the design speed process is relatively straightforward. However, when applied to interchange ramps where high-speed facilities meet low-speed facilities and drivers are expected to accelerate or decelerate over short distances, application of the design speed process is more complex. 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. This final report presents the overall research findings including the literature review, survey, and interviews with selected designers; details about the speed data collected during the study; statistical analyses to develop speed prediction models for ramps; project summary and future research needs; and recommended changes to the AASHTO Green Book. A companion document to this final report presents enhanced guidelines for selecting an appropriate ramp design speed, details about the ramp speed profile model, and case studies that illustrate how to use the guidelines and ramp speed file model 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. Based upon the literature review, the survey results, and interviews with selected professionals, it was assessed that the design guidelines presented in the AASHTO Green Book related to application of the design speed process for ramps and the appropriate selection of a ramp design speed have several overarching challenges that lead to confusion and inconsistent interpretation of the guidelines. In particular, the Green Book does not specifically define ramp design speed. Green Book Table 10-1, which is the centerpiece of AASHTO’s guidance on selecting an appropriate ramp design speed for the design of all ramp types, does not provide sufficient detail for selecting ramp design speeds for specific ramp types and configurations. Little guidance is provided on how the selected ramp design speed is to be used to develop a balanced design among all components of a ramp, nor is there consensus on the portion of the ramp to which the ramp design speed applies or whether the ramp design speed should be a single value or vary along the ramp. Therefore, development of enhanced design guidelines is necessary for selecting appropriate ramp design speeds to enable practitioners to design ramps in a consistent manner based on the selected ramp design speed, accounting for sequential speed transitions from one component or section to the next, consistent with performance capabilities of vehicles and driver expectations. An observational field study was conducted to collect vehicle speeds at interchange ramps. The purpose of the study was to gather supplemental speed data along interchange ramps that could be used in combination with speed data collected along interchange ramps during previous research to further develop and refine speed prediction models for use in a spreadsheet-based ramp speed profile model (RSPM) tool that designers and traffic engineers can use to estimate vehicle speeds along interchange ramps. A database was assembled to develop ramp speed prediction models that could be incorporated into the spreadsheet-based RSPM tool. Separate

2 statistical analyses were conducted to develop ramp speed prediction models for curves and tangents. While developing the RSPM tool, 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 enhanced guidelines are summarized in this final report but are presented in more detail in the companion document. The enhanced guidelines define ramp design speed and related terminology and discuss in detail the most relevant elements that affect selecting an appropriate ramp design speed. A new table developed as part of the guidelines provides a range of guide values for ramp design speeds for system and service interchanges, and the guide values are presented as a range of values given the context, ramp configuration, and type of interchange. The guidelines also address how to apply design controls and criteria so that individual components and features of ramps are designed in a consistent manner, accounting for sequential speed transitions from one component or section to the next, and several case studies are presented to demonstrate how to evaluate the consistency of a ramp design based on the selected ramp design speed. These case studies demonstrate use of the RSPM tool and how it can be used by designers and traffic engineers to conduct operational analyses and to help make design decisions. The enhanced guidelines are intended to facilitate the selection of an appropriate ramp design speed based on a combination of contextual considerations and quantitative information and 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. The enhanced guidelines (1) are not expected to introduce unforeseen safety or operational issues, create the need for more design exceptions, nor increase or decrease construction costs; (2) may help to improve safety and operational issues related to ramp design by reducing the number of ramps designed and built in an inconsistent manner due to confusion or inconsistent interpretation of existing AASHTO policy; (3) may benefit an agency’s overall program budget by reducing the number of ramps that need to be reconstructed at a later date because they were originally designed and built in an inconsistent manner; and (4) are not expected to create any specific tort liability concerns related to ramp design.

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Designing extended sections of highway based on the design speed process is relatively straightforward. However, when applied to interchange ramps where high-speed facilities meet low-speed facilities and drivers are expected to accelerate or decelerate over short distances, application of the design speed process is more complex.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 313: Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report provides enhanced design guidelines for selecting appropriate ramp design speeds in a consistent manner, accounting for sequential speed transitions from one component or section to the next, consistent with performance capabilities of vehicles and driver expectations.

Supplemental to the document are NCHRP Web-Only Document 313: Selecting Ramp Design Speeds, Volume 1: Guide and Ramp Speed Profile Model figures.

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