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93 Section 6. Project Summary and Future Research Needs The design speed concept has been the centerpiece of AASHTOâs 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 design guidance presented in the 2018 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 guidance. In particular, the Green Book does not specifically define ramp design speed. Green Book Table 10-1, which is the centerpiece of AASHTOâs 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, enhanced design guidelines are necessary for selecting appropriate ramp design speeds so ramps are designed 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. The enhanced guidelines developed as part of this project [summarized here and presented in more detail in the companion document (Torbic et al., 2021) to this final report] address the overarching limitations of AASHTOâs current policy related to selection and application of ramp design speeds. 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 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. In addition to the guidelines, a design tool was developed that can be used to estimate vehicle speeds along interchange ramps to help designers and traffic engineers conduct operational analyses and 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. In addition, the new table (i.e., Table 29) of guide values for ramp design speeds was developed to be relatively consistent with guidance presented in the 2018 edition of the Green Book. In the Green Book further guidance is provided beyond the design table (i.e., Green Book Table 10-1) for selecting a ramp design speed based on various conditions and ramp types. For example, text in the Green Book indicates that ramp design speeds for semidirect connections are typically 30 to
94 40 mph, and ramp design speeds less than 30 mph should not be used. Similarly, text in the Green Book indicates that the minimum ramp design speed for direct connections preferably should be 40 mph. For a majority of the conditions and guidance presented in the Green Book related to appropriate ramp design speeds for specific ramp configurations, the range of guide values for ramp design speeds in Table 29 are consistent with such guidance. In some cases, the range of guide values in Table 29 extend slightly beyond the range of values suggested in the Green Book. In some cases, this was done intentionally. A table of guide values should not be too prescriptive that it limits flexibility and inhibits designers from exercising engineering judgment. In other cases, extending beyond the range of values suggested in the Green Book was necessary to address the fact that many states are posting freeway speed limits as high as 75, 80, and 85 mph, and guidelines for selecting ramp design speeds to address such conditions is necessary. Because the range of guide values for ramp design speeds in Table 29 are relatively consistent with guidance presented in the Green Book, the enhanced guidelines are not expected to introduce unforeseen safety or operational issues, create the need for more design exceptions, or increase or decrease construction costs. The enhanced guidelines should result in the design of ramps that would be similar to ramps designed using guidance from the 2018 edition of the Green Book as intended. The enhanced guidelines simply provide more clarity on the primary elements that factor into selecting an appropriate ramp design speed and how the ramp design speed is used to coordinate the design of adjacent sections of the ramp to accommodate sequential speed transition from one component or section on the ramp to next. The enhanced guidelines 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. Ramps designed in an inconsistent manner may not sufficiently accommodate sequential speed transitions from one component to the next. As such, driver expectations may be violated, resulting in abrupt changes in speed and/or creating safety and operational concerns. The enhanced guidelines may also benefit an agencyâs 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. The enhanced guidelines are not expected to create any specific tort liability concerns related to ramp design. The enhanced guidelines developed through this research are relatively consistent with guidance presented in the Green Book and simply provide more clarity on selecting an appropriate ramp design speed and how the ramp design speed is used to coordinate the design of adjacent sections of the ramp. There is no indication that guidance presented in the Green Book has created any tort liability concerns related to the selection and application of ramp design speed, so there is not an apparent need for the enhanced guidelines to alleviate tort liability concerns based on use of current design guidance. (Note, adherence to standards or design guidance does not in itself provide tort immunity.) Terms such as âmustâ, âshallâ, or ârequiredâ that could create obvious tort liability concerns are not used in the enhanced guidelines; and in general, if engineering decisions are made sensibly and are well documented, then no tort liability concerns should arise. Thus, the enhanced guidelines are not expected to create any specific tort liability concerns related to ramp design.
95 Finally, throughout this project, several research needs were identified, namely: â¢ Green Book Table 10-4, which provides guide values for minimum acceleration lengths, was adapted (see Table 30). In addition to modifying the caption headings, the design values were expanded to include additional design conditions for higher ramp design speeds and higher highway (or freeway) design speeds. These new design values were estimated through numeric extrapolation of the minimum acceleration lengths for the lower-speed design conditions and to be consistent with the trends and acceleration rates of the design values in the original table. They were not developed based on field data or vehicle dynamics models that estimate vehicle performance. Similarly, Green Book Table 10-6, which provides guide values for minimum deceleration lengths, was adapted (see Table 31) for use with higher design speed facilities. Additional research should be conducted to verify and/or modify these design values for higher-speed facilities. â¢ A first version of a spreadsheet tool was developed as part of this research to estimate vehicle speeds along interchange ramps to help designers and traffic engineers conduct operational analyses and make design decisions. Additional research should be conducted to address limitations of this design tool and expand its capabilities.