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87 Section 5. Summary of Design Guidelines This section summarizes guidelines for selecting appropriate ramp design speeds. A companion document to this final report presents the enhanced guidelines in more detail (Torbic et al., 2021). 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 highway, area type (i.e., urban or rural), ramp configuration, and site constraints (including physical, environmental, and social). Table 29 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. For the specified design condition, it is recommended that the ramp design speed for a given ramp be within the range of guide values. The lower guide values are generally associated with constrained conditions, and the higher guide values are associated with unconstrained conditions. The range of guide values in Table 29 were developed to be relatively 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 as drivers travel from one section of the ramp to the next, 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
88 Table 29. Range of Guide Values for Ramp Design Speed as Related to Highway Design Speed, Interchange Type, Ramp Configuration, and Contextual Considerations Context Ramp configuration Highway design speed (mph) 30 35 40 45 50 55 60 65 70 75 80 85 Ramp design speed (mph) System Interchanges Rural Direct connections - - - - 40-45 40-50 40-55 45-60 45-65 50-70 50-75 50-80 Semidirect connections - - - - 30-40 30-45 35-45 35-50 40-55 40-60 40-65 45-65 Loop ramps - - - - 25-30 25-30 25-35 25-35 30-35 30-40 35-45 35-45 Urban Direct connections - - - - 35-40 35-40 35-50 40-50 40-55 40-60 - - Semidirect connections - - - - 30-35 30-40 30-45 30-45 35-50 35-55 - - Loop ramps - - - - 20-25 20-30 25-30 25-30 25-35 25-40 - - Service Interchanges Rural: Primary or major arterial Direct connections - - - 35-40 35-45 40-45 40-45 40-50 45-50 45-55 50-60 50-65 Diagonal ramps1 20-25 20-25 25-30 25-35 30-40 30-45 30-50 30-55 35-55 40-60 45-65 45-70 Loop ramps 15-20 20-25 20-25 20-25 20-25 25-30 25-30 25-30 30-35 30-40 35-45 35-45 Minor arterial or collector Diagonal ramps1 20 20-25 25 25-30 25-35 30-40 30-45 30-50 35-50 40-55 45-60 45-65 Loop ramps 15-20 15-20 15-20 15-25 20-25 20-30 20-30 25-30 25-35 30-40 30-45 30-45 Urban: Primary or major arterial Direct connections - - - 30-35 35-40 35-40 35-40 40-45 40-50 40-55 - - Diagonal ramps1 15-20 20-25 25-30 25-30 30-35 30-40 30-45 30-45 35-50 40-55 - - Loop ramps 15-20 15-20 15-20 20-25 20-25 20-30 25-30 25-30 25-35 25-40 - - Minor arterial or collector Diagonal ramps1 15 20 20-25 20-30 25-35 25-40 25-45 30-45 30-50 35-55 - - Loop ramps 15-20 15-20 15-20 15-25 15-25 20-30 20-30 20-30 20-35 25-40 - - 1 Where the horizontal alignment of a diagonal entrance ramp is relatively straight and has little impact on vehicle speeds, the ramp design speed is dependent on the operational characteristics of the freeway mainline ramp terminal. For this situation, it is recommended that the ramp design speed be within 15 to 20 mph of the highway design speed. Where the horizontal alignment of a diagonal exit ramp is relatively straight and has little impact on vehicle speeds, the ramp design speed is dependent on the operational characteristics of the crossroad ramp terminal. The recommended ramp design speed for this situation is 15 or 20 mph, as it is expected that the crossroad ramp terminal at all diagonal ramps will either be stop-, yield-, or signal-controlled.
89 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 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 travel along the respective curve and tangent. If the design speed of an individual curve or tangent (or multiple curves and tangents) along the ramp proper is not consistent with the speeds that drivers are likely to be traveling along the respective curve and tangent, 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 (e.g., raising or potentially decreasing) 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 (e.g., due to physical site constraints, environmental or social impacts, economic constraints, etc.), 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 freeway and ramp proper. 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 (presented here as Figures 23 and 24) 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.
90 Figure 23. Acceleration of Passenger Cars, Level Condition (AASHTO, 2018) Figure 24. Deceleration Distances for Passenger Vehicles Approaching Intersections (AASHTO, 2018)
91 Green Book Tables 10-4 and 10-6, presented here as Tables 30 and 31 respectively, have been adapted slightly based on this research. The column headings were adapted to be more consistent with the enhanced design guidelines. In addition, the guide values were expanded to include additional design conditions for higher ramp design speeds and higher highway (or freeway) design speeds. Table 30. Minimum Acceleration Lengths for Entrance Terminals with Flat Grades of Less Than 3 Percent (adapted from AASHTO, 2018) Highway Acceleration Length, LAcc Length (ft), for Design Speed (VDS) of Controlling Feature on the Ramp Proper (or the Final Section of the Ramp Proper Upstream of the Gore Point) Stop Condition 15 20 25 30 35 40 45 50 55 1 601 651 Design Speed VDS Hwy (mph) Merge Speed VAcc Length(f) (mph) Average Running Speed (i.e., Initial Speed) Exiting the Controlling Feature on the Ramp Proper (or the Final Section of the Ramp Proper Upstream of the Gore Point), VAcc Length(i) 0 14 18 22 26 30 36 40 44 48 52 56 30 23 180 140 35 27 280 220 160 40 31 360 300 270 210 120 45 35 560 490 440 380 280 160 50 39 720 660 610 550 450 350 130 55 43 960 900 810 780 670 550 320 150 60 47 1200 1140 1100 1020 910 800 550 420 180 65 50 1410 1350 1310 1220 1120 1000 770 600 370 135 70 53 1620 1560 1520 1420 1350 1230 1000 820 580 350 135 75 55 1790 1730 1630 1580 1510 1420 1160 1040 780 550 300 80 57 2000 1900 1800 1750 1680 1600 1340 1240 980 725 500 200 851 59 2230 2090 1985 1930 1865 1790 1525 1455 1250 940 700 500 NOTE: Uniform 50:1 to 70:1 tapers are recommended where acceleration lengths exceed 1,300 ft. 1 Acceleration lengths estimated based on extrapolation. Appendix A presents recommended changes to the next edition of the Green Book, based upon the findings and conclusions of this research.
92 Table 31. Minimum Deceleration Lengths for Exit Terminals with Flat Grades of Two Percent or Less (adapted from AASHTO, 2018) Highway Design Speed, VDS Hwy (mph) Diverge Speed, VOS Hwy VDec Lenth (i) (mph) Deceleration Length, LDec Length (ft), for Design Speed (VDS) of Controlling Feature on the Ramp Proper (or the Initial Section of the Ramp Proper Downstream of the Gore Point) Stop Condition 15 20 25 30 35 40 45 50 Average Running Speed at the End of the Deceleration Length, VDec Length (f) (mph), Entering the Controlling Feature on the Ramp Proper (or the Initial Section of the Ramp Proper Downstream of the Gore Point) 0 14 18 22 26 30 36 40 44 30 28 235 200 170 140 - - - - - 35 32 280 250 210 185 150 - - - - 40 36 320 295 265 235 185 155 - - - 45 40 385 350 325 295 250 220 - - - 50 44 435 405 385 355 315 285 225 175 - 55 48 480 455 440 410 380 350 285 235 - 60 52 530 500 480 460 430 405 350 300 240 65 55 570 540 520 500 470 440 390 340 280 70 58 615 590 570 550 520 490 440 390 340 75 61 660 635 620 600 575 535 490 440 390 80 64 705 680 665 645 620 580 535 490 440 851 67 750 725 710 690 665 625 580 540 485 1 Deceleration lengths estimated based on extrapolation.
