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122 Section 6. Summary of Design Guide and Future Research Needs This section summarizes guidelines for selecting appropriate ramp design speeds. The guidelines facilitate the selection of an appropriate ramp design speed based on a combination of contextual considerations and quantitative information. The guidelines 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. Ramp design speed is a selected speed used to determine the various geometric design features of a ramp. The selected ramp design speed should be a logical one with respect to the type of intersecting highways, area type (i.e., urban or rural), ramp configuration, and site constraints (including physical, environmental, and social). Table 4 provides guide values for ramp design speeds for system and service interchanges. The guide values are presented as a range of values given the context, ramp configuration, and type of interchange. The lower guide values are generally associated with constrained conditions, and the higher guide values are associated with unconstrained conditions. The guide values in Table 4 were developed to be consistent with guidance presented in the 2018 edition of the Green Book. Where the horizontal alignment of the ramp is curvilinear, the ramp design speed applies to the controlling curve on the ramp proper. For an entrance ramp, the last curve encountered along the ramp proper that significantly affects vehicle speed is considered the controlling curve. For an exit ramp, the controlling curve is the first curve encountered along the ramp proper that significantly affects vehicle speed. For direct connection and outer connection ramps, the sharpest curve on the ramp proper is considered the controlling curve. The freeway mainline ramp terminal, the crossroad ramp terminal, and the adjoining sections of the ramp proper (i.e., adjoining tangents and horizontal curves in the direction of travel) that connect to the controlling curve should be designed to allow for appropriate speed transitions, consistent with driver behavior and expectations and vehicle performance capabilities, for the selected ramp design speed. Where the horizontal alignment of the ramp is relatively straight and has little or no influence on vehicle speeds, the ramp design speed is based on the operational characteristics of the freeway mainline ramp terminal for an entrance ramp and the operational characteristics of the crossroad ramp terminal for an exit ramp and applies to the tangential section of the ramp proper. The ramp design speed is typically associated with an individual segment of a ramp near the upstream or downstream end of the ramp. For an entrance ramp, the ramp design speed is normally associated with a segment on the downstream portion of the ramp. With an exit ramp, the ramp design speed is usually associated with a segment on the upstream portion of the ramp. Each individual section of the ramp proper (i.e., tangent and curve) has its own design speed. The design speed of the controlling feature of the ramp (i.e., the ramp design speed) is selected first, and then the design speeds of the other elements or segments of the ramp proper should be determined accordingly. It is desirable (but not essential) that the design speeds of the contiguous segments along the ramp proper increase or decrease in a stepwise manner. The change in design speed between adjoining sections should be limited to no more than 10 to 15 mph. For entrance ramps, this means that the design speeds of the contiguous segments along the ramp proper
123 should typically increase in a stepwise manner; and for exit ramps the design speeds of the contiguous segments along the ramp proper should decrease in a stepwise manner. Decreasing design speeds sequentially along a ramp in a stepwise manner is more critical for exit ramps than increasing design speeds along a ramp for entrance ramps, as unexpected and abrupt changes in design speeds along an exit ramp are more likely to be associated with potential lane keeping and/or loss of control issues. The design speed of each individual curve and tangent along the ramp proper should be consistent with the speeds that drivers are likely to be traveling along the respective curve and tangent. If the design speed of an individual curve or tangent along the ramp proper is not consistent with the speeds that drivers are likely to be traveling along the respective section, consider potential options so that the design speed would be more consistent with operating speeds. For example, if the anticipated operating speed exceeds the design speed, consider changing the design speed for the respective section. If changing the design speed of an individual section means that the design speeds of the contiguous segments along the ramp proper no longer increase or decrease in a stepwise manner, consider changing the ramp design speed. If the design speed or ramp design speed cannot be changed, consider implementing speed-control measures so that the design speed or ramp design speed would be more consistent with operating speeds. The freeway mainline ramp terminal portion of a ramp does not have an associated design speed. However, the operational and design conditions of the freeway mainline ramp terminal are related to and coordinated with the operational and design conditions of the adjoining sections of the ramp proper and freeway. Each component of the ramp should be designed such that a vehicle has sufficient distance to accelerate or decelerate within the performance capabilities of the vehicle and driver expectations given the design speeds and features of the individual components of the ramp. Several design controls and criteria may be used in the design of ramps so that the individual components and features of the ramps are designed in a consistent manner, accounting for sequential speed transitions from one component or section to the next. Green Book Figures 2-33 and 2-34 may be used as a guide for determining minimum practical lengths of individual tangent and curve sections along the ramp proper, and Green Book Tables 10-4, 10-5, and 10-6 may be used to determine minimum acceleration and deceleration lengths for the freeway mainline ramp terminal portion of the ramp. Green Book Tables 10-4 and 10-6 are presented herein as Tables 6 and 7, respectively, with a few adaptions. The headings were adapted to be more consistent with the design guidelines presented herein. In addition, the guide values were expanded to include additional design conditions for higher ramp design speeds and higher highway (or freeway) design speeds. A spreadsheet-based tool was developed to estimate vehicle speeds along interchange ramps to help designers and traffic engineers conduct operational analyses and make design decisions. The spreadsheet tool is applicable to entrance and exit ramps at service interchanges including diagonal ramps, loop ramps, and outer connection ramps. The spreadsheet-based tool incorporates several speed prediction models based on vehicle speeds observed on ramps. The models consist of equations that contain calibration coefficients and terms to account for the influence of variables such as initial speed, ramp segment length (curve or tangent segment), curve radius, and others. With the capability to adjust several input parameters, the spreadsheet-
124 based tool may be used to predict vehicle speeds based upon anticipated driver behaviors or design conditions. The output from the tool may be interpreted to help design an interchange ramp in a consistent manner that meets driver expectations. Because the range of guide values for ramp design speeds in Table 4 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, nor 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 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 no 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, no tort liability concerns should arise. Thus, the enhanced guidelines are not expected to create any specific tort liability concerns related to ramp design. Based on the findings from this research, several research needs were identified: â¢ Green Book Table 10-4, which provides guide values for minimum acceleration lengths, was adapted (see Table 6). 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
125 10-6 which provides guide values for minimum deceleration lengths was adapted (see Table 7) 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. Section 4.4 documents several known limitations of this tool. Additional research should be conducted to address these limitations and expand the capabilities of this tool.