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Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report (2021)

Chapter: Appendix A. Suggested Text for Future Edition of AASHTO Green Book

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Suggested Citation:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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:"Appendix A. Suggested Text for Future Edition of AASHTO Green Book." 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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102 Appendix A. Suggested Text for Future Edition of AASHTO Green Book This appendix provides potential changes recommended for consideration to the next edition of the AASHTO Green Book, based upon the findings and conclusions of this research. The recommendations are based upon a review of the 2018 edition of the Green Book. Recommended modifications are specified for selected sections in Chapter 10 as follows, using track changes. Chapter 10 (Beginning on Page 10-102 of 2018 Green Book) 10.9.6 Ramps 10.9.6.1 Types and Examples The term “ramp” includes all types, arrangements, and sizes of turning roadways that connect two or more legs at an interchange. The components of a ramp are a terminal at each leg and a connecting road. The geometry of the connecting road usually involves some curvature and a grade. Generally, the horizontal and vertical alignment of ramps is based on lower design speeds than the intersecting highways, but in some cases it may be equal. Figure 10-60 illustrates several types of ramps and their characteristic shapes. Various configurations are used; however, each can be broadly classified as one of the types shown. Each ramp generally is a one-way roadway. Diagonal ramps (Figure 10-60A) are almost always one way but usually have both a left- and right-turning movement at the terminal on the minor intersecting road. A diagonal ramp may be largely tangent or wishbone in shape with a reverse curve. Diamond interchanges generally have four diagonal ramps. A loop ramp may have single turning movements (left or right) or double turning movements (left and right) at either or both ends. Figure 10-60B shows the case where there are only single turns made at both ends of the ramp. With this loop pattern, a left-turning movement is made without an at-grade crossing of the opposing through traffic. Instead, drivers making a left-turn travel beyond the highway separation, turn to the right through approximately 270 degrees to enter the other highway. A loop ramp usually involves more indirect travel distance than any other type of ramp. With a semidirect connection (Figure 10-60C), the driver exits to the right first, heading away from the intended direction, gradually reversing, and passing around other interchange ramps before entering the other road. This semidirect connection may also be used for right turns, but there is little reason for its use if the conventional diagonal can be provided. A descriptive term frequently associated with this type of ramp is “jug-handle,” the obvious plan shape. Travel distance on this ramp is less than that for a comparable loop and more than that for a direct connection. Figure 10-60D is termed an outer connection, while Figure 10-60E is referred to as a direct connection.

103 Figure 10-60. General Types of Ramps The different ramp patterns of an interchange (i.e., the different types of interchange configurations) are comprised of various combinations of these types of ramps. For example, the trumpet configuration has one loop, one semidirectional ramp, and two right-turn directional or diagonal ramps.

104 10.9.6.2 General Ramp Design Considerations 10.9.6.2.1 Ramp Design Speed The ramp design speed is a selected speed used to determine the various geometric design features of a ramp. The 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). The amount of speed transition to be accommodated along a ramp is dictated by the design speeds of the intersecting freeways at a system interchange and the design speed of the freeway and operations near the crossroad ramp terminal at a service interchange. The combination of the type of intersecting highways, area type, ramp configuration, and site constraints are the primary determinant elements and contextual considerations that should factor into the selection of an appropriate ramp design speed. Based on these key elements and contextual considerations, Table 10-1 provides a range of guide values for ramp design speeds for system and service interchanges. For the specified design condition, it is recommended that the ramp design speed for a given ramp be within the range of guide values. By providing a range of guide values for ramp design speeds, Table 10-1 provides flexibility in determining the contextual and design considerations that apply to a given ramp. Conceptually, contextual considerations can be viewed from a qualitative perspective and classified as either creating constrained or unconstrained conditions. For example, where the physical site characteristics for a given ramp may be considered typical or less restrictive than normal for a given area, the site may be classified as unconstrained. On the other hand, where physical site characteristics exist beyond what may be considered as typical or normal for a given area, the site may be classified as constrained. Similarly, sites may be classified as unconstrained or constrained as they pertain to environmental and social impacts and economic factors. In general, the lower guide values in Table 10-1 are associated with constrained conditions, and the higher guide values are associated with unconstrained conditions. With a system interchange, vehicles exit a high-speed facility and enter another high-speed facility, so it is desirable to provide a ramp between the two facilities that enables vehicles to maintain a speed as close as practical to freeway speeds, minimizing the need for deceleration or acceleration along the ramp. For situations where the design speeds of both intersecting freeways of a ramp are equivalent, the highest possible ramp design speed for the connecting ramp would be the design speed of the intersecting freeways. However, in most cases it is not practical for the ramp design speed to be equivalent to the design speed of the two intersecting freeways; therefore, in those cases, the ramp design speed may be less than the intersecting freeways’ design speed. For the situation where two intersecting freeways do not have equivalent design speeds, the ramp design speed may be a speed between the two highway design speeds of the intersecting freeways; however, where this is not practical, the ramp design speed may be less than the lower design speed of the two intersecting freeways. With a service interchange, vehicles either accelerate from a near-stop condition at the crossroad ramp terminal to a higher speed on the freeway (e.g., in the case of entrance ramps) or decelerate from a higher speed on the freeway to a stop or near-stop condition at the crossroad ramp terminal (e.g., in the case of exit ramps). For both entrance and exit ramps, ramps need to be designed to accommodate the necessary acceleration or deceleration. In rural or unconstrained areas where the intersecting crossroad is a primary highway or major street, minimal to moderate

105 acceleration or deceleration along the ramp is expected by the driver; while if the intersecting crossroad is a minor arterial or collector, moderate acceleration or deceleration is expected by the driver along the ramp. In urban or constrained areas where the intersecting facility is a primary highway or major street, moderate to significant acceleration or deceleration along the ramp is expected by the driver; while if the intersecting road is a minor arterial or collector, significant acceleration or deceleration is expected by the driver along the ramp.

106 Table 10-1. Range of Guide Values for Ramp Design Speed as Related to Highway Design Speed U.S. Customary 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.

107 Metric Context Ramp configuration Highway design speed (km/h) 50 60 70 80 90 100 110 120 130 140 Ramp design speed (km/h) System Interchanges Rural Direct connections - - - 60-70 60-80 60-90 70-100 80-110 80-120 80-130 Semidirect connections - - - 50-60 50-70 60-70 60-80 60-100 60-110 70-110 Loop ramps - - - 40-50 40-50 40-60 40-60 50-60 50-70 50-70 Urban Direct connections - - - 60-70 60-70 60-80 60-80 60-100 - - Semidirect connections - - - 50-60 50-60 50-70 50-80 60-90 - - Loop ramps - - - 30-40 30-50 40-50 40-50 40-60 - - Service Interchanges Rural: Primary or major arterial Direct connections - - 60-70 60-70 60-70 60-70 70-80 70-90 80-100 80-110 Diagonal ramps1 30-40 30-50 40-60 50-60 50-70 50-80 50-90 70-100 70-110 70-110 Loop ramps 20-30 30-40 30-40 30-40 40-50 40-50 40-50 50-60 60-70 60-70 Minor arterial or collector Diagonal ramps1 30 30-40 40-50 40-60 50-60 50-70 50-80 60-90 70-100 70-110 Loop ramps 20-30 20-30 20-40 30-40 30-50 30-50 40-50 50-60 50-70 50-70 Urban: Primary or major arterial Direct connections - - 50-60 60-70 60-70 60-70 60-80 60-90 - - Diagonal ramps1 20-30 30-40 40-50 50-60 50-60 50-70 50-80 60-90 - - Loop ramps 20-30 20-30 30-40 30-40 30-50 40-50 40-50 40-60 - - Minor arterial or collector Diagonal ramps1 20 30-40 30-50 40-60 40-60 40-70 50-80 60-90 - - Loop ramps 20-30 20-30 20-40 20-40 30-50 30-50 30-50 40-60 - - 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 20 to 30 km/h 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 20 or 30 km/h, as it is expected that the crossroad ramp terminal at all diagonal ramps will either be stop-, yield-, or signal-controlled.

108 The application of values in Table 10-1 to various conditions and ramp types is discussed below. 10.9.6.2.2 Portion of Ramp to Which the Ramp Design Speed Is Applicable The values in Table 10-1 apply to the controlling feature on the ramp proper. 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. If the horizontal alignment of the ramp is relatively straight and has little or no influence on vehicle speeds, as with some configurations of diagonal ramps, the ramp design speed applies to the tangential section of the ramp proper and 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. The ramp design speed is typically associated with an individual segment of a ramp near the upstream or downstream end 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. The ramp terminals (i.e., freeway mainline ramp terminal and crossroad ramp terminal) do not have an associated design speed. However, the operational and design conditions of both terminals are related to and coordinated with the operational and design conditions of the adjoining sections of the freeway, ramp proper, and/or crossroad. 10.9.6.2.3 Diagonal Ramps Diagonal ramps are used for service interchanges and typically have both a left- and right-turn movement at the crossroad ramp terminal. Diagonal ramps usually consist of a high-speed freeway mainline ramp terminal, a tangent or curved alignment on the ramp proper, and stop or yield conditions at the crossroad ramp terminal. Depending on the configuration of the crossroad, a free-merge right-turn maneuver may be provided. For diagonal ramps where the horizontal alignment is curvilinear, the horizontal alignment will be the controlling feature of the ramp. For diagonal ramps where the horizontal alignment is relatively flat (i.e., where the horizontal alignment has little or no impact on the speeds of vehicles), 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. For an entrance ramp, the design speed of the tangent section of the ramp proper (i.e., the ramp design speed) should be consistent with the speed entering the freeway mainline ramp terminal. For an exit ramp, the design speed of the tangent section of the

109 ramp proper (i.e., the ramp design speed) should be consistent with the operating conditions at the crossroad ramp terminal. Because diagonal ramps are used for service interchanges and because the controlling feature of a diagonal ramp may be either the horizontal alignment or the operational characteristics of one of the terminals, diagonal ramps may be designed with ramp design speeds as low as 15 to 20 mph (20 to 30 km/h) and potentially as high as 45 to 70 mph (70 to 110 km/h)]. Diagonal ramps are unique compared to other ramp configurations in that the controlling feature of a diagonal ramp may be either the horizontal alignment or the operational characteristics of one of the terminals. With the other ramp configurations, the horizontal alignment of the ramp will likely be the controlling feature. 10.9.6.2.4 Loop Ramps Loop ramps may be used in both system and service interchanges. The design speed of a loop ramp is limited by the radius of the controlling curve. For important movements on highways with design speeds of 50 mph [80 km/h] and higher, the radii of controlling curves on loop ramps generally range from 150 to 250 ft [50 to 75 m] with corresponding ramp design speeds of about 25 to 30 mph [40 to 50 km/h]. Ramp design speeds above 30 mph [50 km/h] for loops involve large land areas that are rarely available in urban areas. The long loop ramps needed for higher design speeds are costly and require left-turning drivers to travel a considerable extra distance, but if less restrictive conditions exist, the loop ramp design speed and the radius may be increased. For minor movements on highways with design speeds of less than 50 mph [80 km/h], ramp design speeds for loop ramps may be as low as 15 to 20 mph [20 to 30 km/h]. 10.9.6.2.5 Two-Lane Loop Ramps With development and additional traffic on freeways, the need for two-lane loop ramps has increased. The two-lane loop configuration should not be immediately preceded or followed by a loop ramp. The radius of the inner edge of the traveled way of the loop ramp normally should not be less than 180 to 200 ft [55 to 60 m]. For additional design details, see the ITE Freeway and Interchange Geometric Design Handbook (15). 10.9.6.2.6 Semidirect Connections Semidirect connection ramps are commonly used at system interchanges and are intended to encourage high-speed, free-flow merging with freeway traffic. As such, high ramp design speeds are preferred but may not be practical due to the curvilinear alignment. Ramp design speeds for semidirect connections typically range from 30 to 40 mph [50 to 60 km/h]. A design speed less than 30 mph [50 km/h] should not be used. Generally, for short single-lane ramps, a design speed greater than 50 mph [80 km/h] is not practical. The horizontal alignment is commonly the controlling feature of semidirect connection ramps. For semidirect connection ramps, the sharpest curve on the ramp proper that significantly affects vehicle speed is considered the controlling curve. 10.9.6.2.7 Direct Connections Direct connection ramps are commonly used at system interchanges. As such, they are intended to encourage high-speed, free-flow merging with freeway traffic and may be designed with ramp design speeds ranging from 35 to 80 mph [60 to 130 km/h]. Direct connection ramps are also

110 used at service interchanges, and in these situations the ramp design speeds may range from 30 to 65 mph [50 to 100 km/h]. For both system and service interchanges, the horizontal alignment is the controlling feature of direct connection ramps. The sharpest curve on the ramp proper that significantly affects vehicle speed is considered the controlling curve. 10.9.6.2.8 Increasing and Decreasing Design Speeds on the Ramp Proper The highway with the greater design speed should be the control in selecting the ramp design speed for the ramp as a whole. At the same time, each individual section of the ramp proper (i.e., tangent and curve) has its own design speed. The design speed for 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 [20 to 30 km/h]. For entrance ramps, this means that the design speeds of the contiguous segments along the ramp proper may increase in a stepwise manner; and for exit ramps the design speeds of the contiguous segments along the ramp proper may 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. 10.9.6.2.9 At-Grade Terminals Where a ramp joins a major crossroad, frontage road, or street, forming an intersection at grade, Table 10-1 is not applicable to that portion of the ramp near the intersection because a stop sign or signal control is normally employed. This terminal design should be predicated on near- minimum turning conditions, as given in Section 9.6. In urban areas, where the land adjacent to the interchange is developed commercially, provisions for pedestrian and bicycle movements through the interchange area should also be considered. Where the ramp joins a frontage road, the ramp design speed may vary with the portion of the ramp closer to the lower-speed frontage road being designed for the lower speed of that frontage road. 10.9.6.2.10 Curvature The design guidelines for turning roadways at interchanges are discussed in Section 3.3.7. They apply directly to the design of ramp curves. Theoretically, when designing a horizontal curve based on a point mass and the basic curve equation, it is assumed that a vehicle travels at a constant speed. However, for an entrance ramp, it is assumed that vehicle speed will increase slightly over the length of the curve. Similarly, along an exit ramp, it is assumed that vehicle speeds will decrease slightly over the length of the curve. For the design of a horizontal curve on an interchange ramp, rather than designing a curve based on a constant speed, the speed at the MC may be used to design the curve rather than the speeds at the beginning or end of the curve. Compound or spiral curve transitions are desirable to: (1) obtain the desired alignment of ramps, (2) provide for a comfortable transition between the design speeds of the through and turning roadways, and (3) fit the natural paths of vehicles. Caution should be exercised in the use of

111 compound curvature to prevent unexpected and abrupt speed adjustments. Additional design information regarding the use of compound curves is presented in Section 3.3.7. The general shape of a ramp evolves from the type of ramp selected, as previously described and shown in Figure 10-60. The specific shape, or curvature, of a ramp may be influenced by such factors as traffic pattern, traffic volume, design speed, topography, culture, intersection angle, and type of ramp terminal. Several ramp shapes may be used for the loop and outer connection of a directional interchange, as shown in Figure 10-61A. Except for its terminals, the loop may be a circular arc or some other symmetrical or asymmetrical curve that is formed with spiral transitions. The asymmetrical arrangement may fit where the intersecting roads are not of the same importance and the ramp terminals are designed for different speeds, so that the ramp in part functions as a speed-change area. Similar shapes may be dictated by ROW controls, profile and sight distance conditions, and terminal location. The freeway terminal should normally be placed in advance of the structure. The most desirable alignment for an outer connection is on a continuous curve (line A). This arrangement, however, may involve extensive ROW. Another desirable arrangement has a central tangent and terminal curves (lines B-B and C-C). Where the loop is more important than the outer connection, reverse alignment on the outer connection may be used to reduce the area of ROW, as shown by line D-D. Any combination of lines B, C, and D may be used for a practical shape.

112 Figure 10-61. Ramp Shapes In Figure 10-61A, the loop and the outer connection are separated, as is generally desirable. However, where the movements are minor and economy is desired, a portion of the two ramps may be combined into a single two-way roadway. Where this design is used, a barrier should separate the traffic in two directions. This design is generally discouraged. Diagonal ramps may assume a variety of shapes, depending on the pattern of turning traffic and ROW limitations. As shown in Figure 10-61B, the ramp may be a diagonal tangent with connecting curves (solid line). To favor a right-turning movement, the ramp may be on a continuous curve to the right with a spur to the left for left turns. On restricted ROW along the major highway, it may be appropriate to use reverse alignment with a portion of the ramp being parallel to the through roadway. Another variation of diagonal ramps, usually called “slip ramps,” connects with a parallel frontage road, as shown in Figure 10-61C. Where this design is used, it is desirable to have one- way frontage roads. Ramps to two-way frontage roads introduce the possibility of wrong-way entry onto the through lanes. If two-way frontage roads are used, special attention should be given in the design and signing of ramps to discourage the possibility of wrong-way entry.

113 The shape of a semidirect connection (Figure 10-61D) is influenced by the location of the terminals with respect to the structures, the extent to which the structure is widened, and the curve radii needed to maintain a desired turning speed for an important left-turning movement. The angular position or the curvature may be dictated somewhat by the relative design speeds of the intersection legs and by the proximity of other roadways. 10.9.6.2.11 Sight Distance Sight distance along a ramp should be at least as great as the design stopping sight distance. Sight distance for passing is not needed. There should be a clear view of the entire exit terminal, including the exit nose and a section of the ramp roadway beyond the gore. The sight distance on a freeway preceding the approach nose of an exit ramp should exceed the minimum stopping sight distance for the through traffic design speed, desirably by 25 percent or more. Decision sight distance, as discussed in Section 3.2.3, is desired where practical. There should be a clear view of the entire exit terminal, including the exit nose. See Sections 3.2.2 and 3.3.12 for ranges in design values for stopping sight distance on horizontal and vertical curves for open road conditions and turning roadways. 10.9.6.2.12 Grade and Profile Design The profile of a typical ramp usually consists of a central portion on an appreciable grade, coupled with terminal vertical curves and connections to the profiles of the intersection legs. The following references to ramp gradient pertain largely to the central portion of the ramp profile. Profiles at the terminals largely are determined by through-road profiles and are seldom tangent grades. Ramp grades should be as flat as practical to minimize the driving effort needed in maneuvering from one road to another. Most ramps are curved, and steep ramp grades in combination with curves hamper traffic flow. The slowing down of vehicles on an ascending ramp is not as serious as on a through road, provided the speed is not decreased sufficiently to result in a peak-hour backup onto the through road. Most diamond ramps are only 400 to 1,200 ft [120 to 360 m] long, and the short central portion with the steepest gradient has only moderate operational effect. Accordingly, gradients on ramps may be steeper than those on the intersecting highways. For any one ramp, the gradient to be used is dependent on a number of factors unique to that site and quadrant. The flatter the gradient on a ramp, the longer it will be, but the effect of gradient on ramp length is not substantial. The conditions and designs at ramp terminals frequently have an effect equal to the effect of the gradient. For example, when the ramp profile is opposite in direction to that of the through highway, a fairly long vertical curve is needed because of the large algebraic difference in grade; this adds considerably to the length of ramp. As another example, additional length may be needed to warp the ramp profile to attain superelevation or to provide drainage. In general, adequate sight distance is more important than a specific gradient control and should be favored in design. Usually, these two controls are compatible. Values in Table 10-2 provide general design criteria for ramp grades and the application of grade values is discussed below. On one-way ramps, a distinction should be made between ascending and descending gradients. For high-speed ramp designs, the values cited in the next paragraph apply. However, with proper

114 ramp terminal facilities, short upgrades of 7 to 8 percent permit good operation without unduly slowing passenger cars. Short upgrades of as much as 5 percent do not unduly interfere with truck and bus operation. On one-way downgrade ramps, gradients of up to 8 percent do not cause undesirable operation due to excessive acceleration of passenger vehicles. However, there is a greater potential for heavy trucks to increase their speeds on downgrades. Therefore, downgrades should desirably be limited to 3 or 4 percent on ramps with sharp horizontal curvature and significant heavy truck or bus traffic. In many areas, consideration of snow and ice conditions may limit the choice of gradient regardless of the direction of the grade. From the foregoing discussion, it can be seen that ramp grades are not directly related to ramp design speed; however, ramp design speed is a general indication of the quality of design being used, and the gradient for a ramp with a high ramp design speed should be flatter than for one with a low ramp design speed. Where ramp terminals are properly located and fit other design needs and where the curvature conforms to a reasonable design speed, ramps are generally long enough to attain the difference in elevation with grades that are level or, at least, not too steep. The cases in which grade is a determining factor in the length of the ramp are as follows: (1) for intersection angles of 70 degrees or less, the ramp may need to be located farther from the structure to provide a ramp of sufficient length with reasonable grade; (2) where the intersection legs are on appreciable grade, with the upper road ascending and the lower road descending from the structure, the ramp will have to attain a large difference in elevation that increases with the distance from the structure; (3) where a ramp leaves the lower road on a downgrade and meets the higher road on a downgrade, longer-than-usual vertical curves at the terminals may need a long ramp to meet grade limitations. For these reasons, alignment and grade of a ramp should be determined jointly. Table 10-2. Guidelines for Maximum Ramp Grade U.S. Customary Metric Ramp Design Speed (mph) Maximum Grade for Upgrades and Downgrades (%)1,2 Ramp Design Speed (km/h) Maximum Grade for Upgrades and Downgrades (%)1,2 45 or greater 3 – 5 70 or greater 3 – 5 35 - 40 4 – 6 60 4 – 6 25 - 30 5 – 7 40 - 50 5 – 7 15 - 20 6 – 8 20 - 30 6 – 8 Notes: (1) Where appropriate for topographic conditions, upgrades steeper than the desirable may be used. (2) One-way downgrades on ramps should be held to the same general maximums, but in special cases they may be 2% greater. 10.9.6.2.13 Vertical Curves Usually, ramp profiles assume the shape of the letter “S” with a sag vertical curve at the lower end and a crest vertical curve at the upper end. Additional vertical curves may be needed, particularly on ramps that overpass or underpass other roadways. Where a crest or sag vertical curve extends onto the ramp terminal, the length of curve should be determined by using a design speed between those on the ramp and the highway. See Section 3.4.6 for design values for open and turning roadway conditions.

115 10.9.6.2.14 Superelevation and Cross Slope The following guidelines should be used for cross-slope design on ramps: 1. Superelevation rates, as related to curvature and design speed on ramps, are given in Tables 3-8 through 3-12. 2. The cross slope on portions of ramps on tangent normally should be sloped one way at a practical rate ranging from 1.5 to 2 percent for high-type pavements. 3. In general, the rate of change in cross slope in the superelevation runoff section should be based on the maximum relative gradients (∆) presented in Equation 3-23. The values listed in this table are applicable to single-lane rotation. The adjustment factors bw listed in Table 3-15 allow for slight increases in the effective gradient for wider rotated widths. The superelevation development is started or ended along the auxiliary lane of the ramp terminal. Alternate profile lines for both edges should be studied so that all profiles match the control points and that no unsightly bumps and dips are inadvertently developed. Spline profiles are very useful in developing smooth lane/shoulder edges. 4. Another important control in developing superelevation along the ramp terminal is that of the crossover crown line at the edge of the through-traffic lane. The maximum algebraic difference in cross slope between the auxiliary lane and the adjacent through lane is shown in Table 9-18. At the ramp terminal the design of the cross slope should accommodate all roadway users. 5. Three segments of a ramp should be analyzed to determine superelevation rates that would be compatible with the design speed and the configuration of the ramp. The exit terminal, the ramp proper, and the entrance terminal should be studied in combination to ascertain the appropriate design speed and superelevation rates. The guidelines in Item 5 can vary by the type of ramp configuration used. Three ramp configurations are described in the following paragraphs. The diamond ramp usually consists of a high-speed exit terminal, tangent or curved alignment on the ramp proper, and stop or yield conditions at the entrance terminal. Deceleration to the first controlling curve speed should occur on the auxiliary lane of the exit terminal and continued deceleration to stop or yield conditions should occur on the ramp proper. As a result, superelevation rate and radii used should reflect a decreasing sequence of design speeds for the exit terminal, ramp proper, and entrance terminal. The loop ramp consists of a moderate-speed exit terminal connecting to a slow-speed ramp proper, which in turn connects to a moderate-speed acceleration lane. The curvature of the ramp proper may be a simple curve or a combination of curves, and such curvature determined by the design speed and superelevation rate used. Superelevation should be gradually developed into and out of the curves for the ramp proper, as detailed later in this discussion. Direct and semidirect ramps generally are designed with a high-speed exit, a moderate- or high- speed ramp proper, and a high-speed entrance. As a result, the design speed and superelevation rates used are comparable to open-road conditions. The method of developing superelevation at free-flow ramp terminals is illustrated in Figure 10-62. Figure 10-62A shows a tapered exit from a tangent section with the first ramp curve falling beyond the design deceleration length. The normal cross slope is projected onto the auxiliary lane, and no superelevation is needed until the first ramp proper curve is reached.

116 Figure 10-62. Development of Superelevation at Free-Flow Ramp Terminals

117 Figure 10-62B shows a parallel-type exit from a tangent section that leads into a flat exiting curve. At point b, the normal cross slope of the through roadway is projected onto the auxiliary lane. At point c, the cross slope can be gradually changed to start the development of superelevation for the exiting curve. At point d, two breaks in the crossover crown line may be conducive to developing a full superelevation in the vicinity of the physical nose. Figures 10-62C and 10-62D show ramp terminals on which the superelevation of the through roadway would be projected onto the auxiliary lane. Figure 10-62E shows a parallel entrance terminal on the high side of a curve. Wherever practical, a tangent section between the ramp and the main line should be provided to accommodate the superelevation transition. At point e, the superelevation on the ramp begins to decrease and is gradually decreased through the tangent section to point d. At point d, the cross slope is gradually rotated to eventually meet the superelevation rate of the main line at point c. Figure 10-62F shows a parallel exit from a tangent section with sharp curvature developing in advance of the physical nose. This design is typical for cloverleaf loops. Part of the cross-slope transition can be accomplished over the length of the parallel lane with about half of the total superelevation being developed at point b. Full superelevation of the ramp proper is reached beyond the physical nose. Care should be exercised to see that the rate of change in cross slope in the runoff section is based on the maximum relative gradients from Equation 3-23 and that the algebraic difference in cross slope does not exceed the values presented in Table 9-18. Figure 10-63. Typical Exit Gore Area Characteristics 10.9.6.2.15 Gores The term “gore” indicates an area downstream from the shoulder intersection points as illustrated in Figure 10-63. The painted nose is a point, having no dimensional width, occurring at the separation of the roadways. The gore point is located where the pair of solid white pavement edge markings that separate the ramp from the intersecting roadway are 2 ft apart. If the markings do not extend to a point where they are 2.0 ft apart, then the gore point is found by extrapolating both markings until the extrapolated portion is 2.0 ft apart. The physical nose is a

118 point upstream from the gore, having some dimensional width that separates the roadways. The neutral area refers to the triangular area between the painted nose and the gore nose and incorporates the physical nose. The geometric layout of these is an important part of exit ramp terminal design. It is the decision point area that should be clearly seen and understood by approaching drivers. Furthermore, the separating ramp roadway not only should be clearly evident but should also have a geometric shape appropriate for the likely speeds at that point. In a series of interchanges along a freeway, the gores should be uniform and have the same appearance to drivers. The width at the gore nose is typically between 20 to 30 ft [6.0 to 9.0 m], including paved shoulders, measured between the traveled way of the main line and that of the ramp. This dimension may be increased if the ramp roadway curves away from the freeway immediately beyond the gore nose or if speeds in excess of 60 mph [100 km/h] are expected to be common. The entire triangular area, or neutral area, should be striped to delineate the proper paths on each side and to assist the driver in identifying the gore area. The MUTCD (9) may be referenced for guidance on channelization. Standard or snow-plowable raised reflective markers can be employed for additional delineation. Rumble strips may be placed in the neutral area but should not be located too close to the gore nose because such placement renders them ineffective for warning high-speed vehicles. In all cases, supplemental devices of this type should be placed to provide the driver with ample advance warning to make timely corrections in the vehicle’s path. The rate of crashes in gore areas is typically greater than the rate of run-off-the-road crashes at other locations. For this reason, the gore area, and the unpaved area beyond, should be kept as free of obstructions as practical to provide a clear recovery area. The unpaved area beyond the gore nose should be graded to be as nearly level with the roadways as practical so that vehicles inadvertently entering will not be overturned or abruptly stopped by steep slopes. Heavy sign supports, luminaire supports, and roadway structure supports should be kept well out of the graded gore area. In addition, yielding or breakaway supports should be employed for the exit sign, and concrete footings, where used, should be kept flush with the ground level. Unfortunately, there will be situations where placement of a major obstruction in a gore is unavoidable. Gores that occur at exit ramp terminals on elevated structures are a prime example. Also, there are occasions when locating a bridge pier in a gore cannot be avoided. Guardrails and bridge rails are designed to handle angular impacts but are not effective in handling the kind of near head-on impacts that occur at these gores. In recognition of the exposed position of fixed objects in gore areas, a considerable effort has been directed toward the development of cushioning or energy-dissipating devices for use in front of such fixed objects. At present, several types of crash cushions are being used. These devices substantially reduce the severity of fixed-object collisions. Thus, adequate space should be provided for the installation of a crash-cushion device whenever a major obstruction is present in a gore on a high- speed highway. Reference may be made to Section 4.10.4 and to the Roadside Design Guide (3) for details on the installation of crash-cushion devices.

119 Although the term “gore” generally refers to the area between a through roadway and an exit ramp, the term may also be used to refer to the similar area between a through roadway and a converging entrance ramp. At an entrance terminal, the point of convergence (beginning of all paved area) is defined as the “merging end.” In shape, layout, and extent, the triangular maneuver area at an entrance terminal is much like that at an exit. However, it points downstream and separates traffic streams already in lanes; thus, it is less of a decision area. The width at the base of the paved triangular area is narrower, however, and is usually limited to the sum of the shoulder widths on the ramp and freeway plus a narrow physical nose 4 to 8 ft [1.2 to 2.4 m] wide. Figure 10-64 illustrates typical gore designs for free-flow exit ramps. Figures 10-64A and 10-64B depict a recovery area adjacent to the outside through lane and moderate offset to the left of the ramp traveled way. Figure 10-64. Typical Gore Details

120 Figure 10-64C presents a major fork, with neither diverging roadway having priority. The offset is equal for each roadway, and striping or rumble strips are placed upstream from the physical nose. Desirably, curbs, utility poles, and sign supports should be omitted from the gore area, especially on high-speed facilities. When curbs are used, they should be low-profile, sloping designs, and the geometry of the gore area intersection points is usually curved. When curbs are not used, the geometry of the gore area intersection points can be squared or truncated. Table 10-3 gives the minimum lengths for tapers beyond the offset nose (shown as length Z in Figure 10-64). However, another alternative for providing a recovery area is the use of the paved shoulder of the through lane. Table 10-3. Minimum Length of Taper Beyond an Offset Nose U.S. Customary Metric Design Speed of Approach Highway (mph) Length of Nose Taper (Z) per Unit Width of Nose Offset Design Speed of Approach Highway (km/h) Length of Nose Taper (Z) per Unit Width of Nose Offset 30 15.0 50 15.0 35 17.5 60 20.0 40 20.0 70 22.5 45 22.5 80 25.0 50 25.0 90 27.5 55 27.5 100 30.0 60 30.0 110 35.0 65 32.5 120 40.0 70 35.0 130 45.0 75 37.5 80 40.0 Figure 10-65 shows an entrance ramp, as at a cloverleaf loop, where a reduction in the ramp lane width is appropriate to maintain a single-lane entrance. Another option is to begin the reduction in the ramp lane width at the end of the ramp curvature. Figure 10-65. Traveled Way Narrowing on Entrance Ramps Figure 10-66 presents a photograph of a single-lane exit. The striping, pavement reflectors, delineators, and fixed-source lighting help guide the exiting motorist.

121 Figure 10-66. Gore Area, Single-Lane Exit Source: Arizona DOT Figure 10-67 shows a gore at a major fork between two freeways. The small angle of divergence results in the long, gradual split with a clear recovery area. Overhead signs are provided. Figure 10-67. Gore Area, Major Fork Source: Georgia DOT

122 Whereas Figure 10-68 shows a gore at a two-lane exit from a freeway, Figure 10-69 shows a typical gore and ramp terminal for a ramp entering a freeway. Figure 10-68. Gore Area, Two-Lane Exit Source: Virginia DOT Figure 10-69. Entrance Terminal Source: Virginia DOT

123 10.9.6.3 Ramp Traveled-Way Widths 10.9.6.3.1 Width and Cross Section Ramp traveled-way widths are governed by the type of operation, curvature, and volume and type of traffic. It should be noted that the roadway width for a turning roadway includes the traveled-way width plus the shoulder width or equivalent offset outside the edges of the traveled way. Section 3.3.11 on “Widths for Turning Roadways at Intersections” may be referenced for additional discussion on the treatments at the edge of the traveled way. Design widths of ramp traveled ways for various conditions are given in Table 3-27. Values are shown for three general design traffic conditions, as follows: • Traffic Condition A—predominantly P vehicles, but some consideration for SU trucks • Traffic Condition B—sufficient SU vehicles to govern design, but some consideration for semitrailer vehicles • Traffic Condition C—sufficient buses and combination trucks to govern design Traffic conditions A, B, and C are described in broad terms because design traffic volume data for each type of vehicle are not available to define these traffic conditions with precision in relation to traveled-way width. In general, traffic condition A has a small volume of trucks or only an occasional large truck, traffic condition B has a moderate volume of trucks (in the range of 5 to 10 percent of the total traffic), and traffic condition C has more and larger trucks. 10.9.6.3.2 Shoulder Widths and Lateral Offset Design values for shoulders and lateral offsets on the ramps are as follows: • Where paved shoulders are provided on ramps, they should have a uniform width for the full length of ramp. A paved shoulder width of 2 to 4 ft [0.6 to 1.2 m] is desirable for the left-side shoulder. A paved shoulder width of 6 to 10 ft [2.4 to 3.0 m] is desirable for the right-side shoulder. For one-way operation, the combined left- and right-side shoulders should be 10 to 14 ft [3.0 to 4.3 m]. • The left and right shoulder widths may be reversed if needed to provide additional sight distance. • The ramp traveled-way widths from Table 3-27 for Case II and Case III should be modified when paved shoulders are provided on the ramp. The ramp traveled-way width for Case II should be reduced by the total width of both right and left shoulders. However, in no case should the ramp traveled-way width be less than needed for Case I. For example, with condition C and a 400-ft [125-m] radius, the Case II ramp traveled-way width without shoulders is 21 ft [6.4 m]. If a 2-ft [0.6-m] left shoulder and an 8-ft [2.4-m] right shoulder are provided, the minimum ramp traveled-way width should be 15 ft [4.8 m]. • Directional ramps with a ramp design speed over 40 mph [60 km/h] should have a paved right shoulder width of 8 to 10 ft [2.4 to 3.0 m] and a paved left shoulder width of 1 to 6 ft [0.3 to 1.8 m]. • For freeway ramp terminals where the ramp shoulder is narrower than the freeway shoulder, the paved shoulder width of the through lane should be carried into the exit terminal. It should

124 also begin within the entrance terminal, with the transition to the narrower ramp shoulder accomplished gradually on the ramp end of the terminal. Abrupt changes should be avoided. • Ramps should have a lateral offset on the right outside of the edge of the traveled way of at least 6 ft [1.8 m], and preferably 8 to 10 ft [2.4 to 3.0 m], and a lateral offset on the left of at least 4 ft [1.2 m] beyond the edge of traveled way. These lateral offsets may be shifted between the right and left sides of the ramp in some cases to provide the needed horizontal sightline offset. • Where ramps pass under structures, the total roadway width should be carried through the structure. Desirably, structural supports should be located beyond the clear zone. As a minimum, structural supports should be at least 4 ft [1.2 m] beyond the edge of paved shoulder. The AASHTO Roadside Design Guide (3) provides guidance on clear zone and the use of roadside barriers. • Ramps on overpasses should have the full approach roadway width carried over the structure. • Edge lines or some type of color or texture differentiation between the traveled way and shoulder is desirable. 10.9.6.3.3 Shoulders and Curbs Shoulders should be provided on ramps and ramp terminals in interchange areas to provide a space that is clear of the traveled way for emergency stopping, to minimize the effect of breakdowns, and to aid drivers who may be confused. Ramps at interchanges should be designed without curbs. Curbs should be considered only to facilitate particularly difficult drainage situations, such as in urban areas where restrictive ROW favors enclosed drainage. In some cases, curbs are used at the ramp terminals but are omitted along the central ramp portions. Where curbs are not used, full-depth paving should be provided on shoulders because of the frequent use of shoulders for turning movements. On low-speed facilities, curbs may be placed at the edge of roadway. Vertical curbs are seldom used in conjunction with shoulders, except where pedestrian protection is needed. Where curbs are used on high-speed facilities, sloping curbs should be placed at the outer edge of the shoulder. Because of fewer restrictions and more liberal designs in rural areas, the need for curbs seldom arises. See Section 4.4 for a full discussion of shoulder cross section elements. 10.9.6.4 Ramp Terminals The terminal of a ramp is that portion adjacent to the through traveled way, including SCLs, tapers, and islands. Ramp terminals may be the at-grade type, as at the crossroad terminal of diamond or partial cloverleaf interchanges, or the free-flow type where ramp traffic merges with or diverges from high-speed through traffic at flat angles. Design elements for the at-grade type are discussed in Chapter 9, and those for the free-flow type are discussed in the following sections. Terminals are further classified as either single or multilane, according to the number of lanes on the ramp at the terminal, and as either a taper or parallel type, according to the configuration of the speed-change lane.

125 10.9.6.4.1 Left-Side Entrances and Exits Left-side entrances and exits are contrary to driver expectancy when intermixed with right-side entrances and exits and should be avoided, where practical. Left-side entrances and exits for managed lane facilities are also contrary to driver expectancy. See Section 8.4.8 for additional information on managed lanes and transit facilities. Left-side ramp terminals break up the uniformity of interchange patterns and generally create uncertain operation on through roadways. Left-side entrances and exits are considered satisfactory for CD roads; however, their use on high-speed, free-flow ramp terminals is not recommended. Because left-side entrances and exits are contrary to driver expectancy, special attention should be given to acceleration/deceleration lengths, signing, and the provision for decision sight distance preceding the approach nose of the exit ramp in order to alert the driver that an unusual situation exists. There should be a clear view of the whole of the exit terminal. If it is not practical to provide decision sight distance because of horizontal or vertical curvature or if relocation of decision points is not practical, additional traffic control devices for advance warning of the conditions should be considered. 10.9.6.4.2 Terminal Location and Sight Distance Where diamond ramps and partial cloverleaf arrangements intersect the crossroad at grade, an at- grade intersection is formed. Desirably, this intersection should be located an adequate distance from the separation structure to provide adequate sight distance for all approaches. Sight distance criteria are detailed in Section 3.2. Drivers prefer and expect to exit in advance of the separation structure. The use of CD roads and single exits on partial cloverleafs and other types of interchange configurations automatically positions the mainline exit in advance of the separation structure. Designs that result in an exit concealed behind a crest vertical curve should be avoided, especially on high-speed facilities. Desirably, high-speed entrance ramp terminals should be located on descending grades to aid truck acceleration. Adequate sight distance at entrance terminals should be available so that merging traffic on the ramp can adjust speed to merge into gaps on the main facility. Loop ramps that are located beyond the structure, as in the conventional cloverleaf or in certain arrangements of partial cloverleafs, usually need a parallel deceleration lane. The actual exit from the auxiliary lane is difficult for drivers to locate even when sight distance is not restricted by a vertical curve. Placing the exit in advance of the structure via a single exit alleviates this concern. See “Two-Exit versus Single-Exit Interchange Design” in Section 10.9.5. 10.9.6.4.3 Ramp Terminal Design Profiles of ramp terminals should be designed in association with horizontal curves to avoid sight restrictions that will adversely affect operations. At an exit into a ramp on a descending grade, a horizontal curve ahead should not appear suddenly to a driver. Instead, the initial crest vertical curve should be made longer and sight distance over it should be increased so that the location and direction of the horizontal curve are apparent to the driver sufficiently in advance to provide time for the driver to respond appropriately. At an entrance terminal from a ramp on an

126 ascending grade, the portion of the ramp intended for acceleration and the ramp terminal should closely parallel the through-lane profile to permit entering drivers to have a clear view of the through road ahead, to the side, and to the rear. It is desirable that profiles of highway ramp terminals be designed with a platform on the ramp side of the approach nose or merging end. This platform should be at least 200 ft [60 m] in length and should have a profile that does not greatly differ from that of the adjacent through-traffic lane. A platform area should also be provided at the at-grade terminal of a ramp. The length of this platform should be determined from the type of traffic control and the capacity at the terminal. For further discussion, see Section 9.4.3. 10.9.6.4.4 Traffic Control On major highways, ramps are arranged to facilitate all turning movements by merging or diverging maneuvers. On minor highways, some of the left-turning movements often are made at grade. The left-turning movements leaving the crossing highway preferably should have median left-turn lanes. For low-volume crossroads, the left-turning movements from ramps normally should be controlled by stop signs. The right-turning movements from ramps into multilane crossroads should be provided with an acceleration lane or generous taper, or should be controlled by stop or yield signs. Ramps approaching stop signs should be nearly perpendicular to the crossroad and be nearly level for storage of several vehicles. Ramp terminals at cross streets can also be controlled by roundabouts. Traffic signal controls may be needed at ramp terminals on the minor road where there is sufficient volume of through and turning traffic. In such cases, the intersections formed at the terminals should be designed and operated in the same manner as any other traffic-signal- controlled intersection at grade. Signal controls should be avoided on express-type highways and confined to the minor highways on which other intersections are at grade and some of which are signalized. In or near urban areas, signal control is especially appropriate at ramp terminals on streets that cross over or under an expressway. Here the turning movements usually are sizable, and the cost of ROW and improvements is high. As a result, appreciable savings may be realized by the use of diamond ramps with high-type terminals on the expressway and signalized terminals on the cross streets. Warrants for the installation of traffic signals that can be applied to diamond ramp terminals are given in Part 4 of the MUTCD (9). 10.9.6.4.5 Distance Between a Free-Flow Terminal and Structure The terminal of a ramp should not be near the grade-separation structure. If it is not practical to place the exit terminal in advance of the structure, the exiting terminal on the far side of the structure should be well removed to provide drivers leaving the through lanes some distance after passing the structure to see the exit and begin the exit maneuver. Decision sight distance should be provided, where practical. The distance between the structure and the approach nose at the ramp terminal should be sufficient for exiting drivers to leave the through lanes without undue hindrance to through traffic. Such distance also aids drivers who enter from a ramp terminal on the far side of the structure so they have a clear view well back on the through road behind or to the left. Such drivers may be able to see back along the road beyond the limits of the structure,

127 but as a general rule, the entering driver’s view is obstructed by the crest of the profile at an overpass and by the columns, abutments, and approach walls at an underpass. The conditions for determining the distance between a structure and the far side approach nose are similar to those discussed for speed-change lanes. A minimum distance between the structure and an exit nose of about the same length as a speed-change taper is suggested. Decision sight distances are desirable but are not rigid controls for ramp design. Topographic or ROW controls may govern the overall shape of the ramp. While a long separation distance between a structure and an exit ramp terminal is desirable to achieve efficient operations and low crash frequencies, this distance can be too long for certain ramp arrangements such as cloverleaf loop ramps. Unusually large ROW needs as well as increased travel time and length on the loops may result. Where only one loop is needed and it falls on the far side of the structure, a SCL should be developed on the near side of the structure and carried across the structure if sight distance is limited. The separation distance between a structure and a ramp terminal does not need to be as long for ramp terminals on the near side of a structure as for those beyond the structure. Both the view of the terminal ahead for drivers approaching on the through road and the view back along the road for drivers on an entrance ramp are not affected by the structure. Where an entrance ramp curve on the near side of the structure needs an acceleration lane, the ramp terminal should be located to provide sufficient length for it between the terminal and the structure, or the acceleration lane may be continued through or over the structure. Where ramp terminals on the far side of a structure are located close to it, the horizontal sight line may be limited by the abutment or parapet; available sight distance should, therefore, be checked. 10.9.6.4.6 Distance Between Successive Ramp Terminals On urban freeways, two or more ramp terminals are often located in close succession. To provide sufficient weaving length and adequate space for signing, a reasonable distance should be provided between successive ramp terminals. Spacing between successive outer ramp terminals is dependent on the classification of the interchanges involved, the function of the ramp pairs (entrance or exit), and weaving potential. The five possible ramp-pair combinations are: (1) an entrance followed by an entrance (EN-EN), (2) an exit followed by an exit (EX-EX), (3) an exit followed by an entrance (EX-EN), (4) an entrance followed by an exit (EN-EX) (weaving), and (5) turning roadways. Figure 10-70 presents recommended minimum ramp terminal spacing for the various ramp-pair combinations as they are applicable to interchange classifications. These recommended minimum ramp terminal spacing values represent a reasonable starting point during planning and early design. The recommendations presented in Figure 10-70 are based on operational experience and the need for flexibility and adequate signing. They should be checked in accordance with the procedure outlined in the Highway Capacity Manual (HCM) (24). Also, the procedure for measuring the length of the weaving section is given in the HCM. The distances labeled L in the Figure 10-70 are measured between the painted noses (see Figure 10- 63). Figure 10-71 shows the definition of the ramp spacing dimension. A minimum distance of

128 300 ft [90 m] is recommended between the end of the taper (as shown in Figure 10-72) for the first entrance ramp and the painted nose for the succeeding entrance ramp. Recent research focused on the EN-EX and EN-EN ramp pair combinations at relatively simple, single-lane service type interchanges. Geometry, traffic operations, safety, and signing were considered in providing a performance based approach to interchange ramp spacing. The ramp terminal spacing needs may be greater for conditions involving multilane ramps such as freeway-to-freeway connections. Ramp spacing distances and the potential geometric feasibility of specific ramp spacing dimensions were provided for simple service type interchanges for each of the ramp pair combinations outlined in Figure 10-70 (18). EN-EN or EX-EX EX-EN Turning Roadways EN-EX (Weaving) Full Freeway CDR Full Freeway CDR System Interchange Service Interchange System to Service Interchange Service to Service Interchange Full Freeway CDR Full Freeway CDR Minimum Lengths Measured between Successive Ramp Terminals 1000 ft (300 m) 800 ft (240 m) 500 ft (150 m) 400 ft (120 m) 800 ft (240 m) 600 ft (180 m) 2000 ft (600 m) 1600 ft (480 m) 1600 ft (480 m) 1000 ft (300 m) Notes: EN—Entrance CDR—Collector-distributor road EX—Exit Figure 10-70. Recommended Minimum Ramp Terminal Spacing Figure 10-71. Ramp Spacing Dimension

129 Where an entrance ramp is followed by an exit ramp, the absolute minimum distance between the successive noses is governed by weaving considerations. The spacing policy for EN-EX ramp combinations is not applicable to cloverleaf loop ramps. For these interchanges, the distance between EN-EX ramp noses is primarily dependent on loop ramp radii and roadway and median widths. A recovery lane beyond the nose of the loop ramp exit is desirable. When the distance between the successive noses is less than 1,500 ft [450 m], the SCLs should be connected to provide an auxiliary lane. This auxiliary lane improves traffic operation over relatively short sections of the freeway route and is not considered an addition to the basic number of lanes. See “Auxiliary Lanes” of Section 10.9.5 for alternate methods of dropping these lanes. There are two typical scenarios of an EX-EN combination. The shortest dimension (400 to 500 ft [120 to 150 m]) would be that of an exit followed by the entrance for a “buttonhook” or “gull wing” design where the freeway ramps are serving a local street parallel to the freeway versus a local street crossing the freeway as an over or underpass. The second scenario would be when an exit ramp and subsequent entrance ramp are servicing grade separated ramps (ramp braids). Due to the vertical and horizontal relationships of this configuration, the spacing will generally be greater than the minimum values in Figure 10- 70 reflecting a condition where both ramp profiles are changing. 10.9.6.4.7 Speed-Change Lanes Drivers leaving a highway at an interchange are required to reduce speed as they exit onto a ramp. Drivers entering a highway from a turning roadway accelerate until the desired highway speed is reached. Because the change in speed is usually substantial, provision should be made for acceleration and deceleration to be accomplished on auxiliary lanes to minimize interference with through traffic and to reduce crash potential. Such an auxiliary lane, including tapered areas, may be referred to as a SCL. The terms “SCL,” “deceleration lane,” or “acceleration lane” as used herein apply broadly to the added lane that joins the traveled way of the highway to the turning roadway and do not necessarily imply a definite lane of uniform width. This additional lane is a part of the elongated ramp terminal area. A SCL should have sufficient length to enable a driver to make the appropriate change in speed between the highway and the turning roadway. Moreover, in the case of an acceleration lane, there should be additional length to permit adjustments in speeds of both through and entering vehicles so that the entering driver can position the vehicle opposite a gap in the through-traffic stream and then maneuver into the stream before the acceleration lane ends. This latter consideration influences both the configuration and length of an acceleration lane. Two general forms of SCLs are: (1) the taper type and (2) the parallel type. The taper type provides a direct entry or exit at a flat angle, whereas the parallel type has an added lane for changing speed. Either type, when properly designed, will operate satisfactorily. However, the parallel type is still favored in certain areas, and some agencies use the taper type for exits and the parallel type for entrances. Furthermore, taper-type entrances have been found to encourage merge speeds that are closer to freeway speeds than parallel type entrances (21); however, where

130 there are mainline volumes that meet or exceed capacity, parallel type entrances allow additional flexibility to drivers in selecting a merge location. See Section 9.7 for discussion of SCLs applicable to at-grade intersections. 10.9.6.5 Single-Lane Free-Flow Terminals, Entrances 10.9.6.5.1 Taper-Type Entrance When properly designed, the taper-type entrance usually operates smoothly at all volumes up to and including the design capacity of merging areas. By relatively minor speed adjustment, the entering driver can see and use an available gap in the through-traffic stream. A typical single- lane, taper-type entrance terminal is shown in Figure 10-72A. The entrance is merged into the freeway with a long, uniform taper. Operational studies show a desirable rate of taper of approximately 50:1 to 70:1 (longitudinal to lateral) between the outer edge of the acceleration lane and the edge of the through-traffic lane. The gap acceptance length, Lg, is also a consideration in the design of taper-type entrances, as illustrated in Figure 10-72A.

131 Notes: 1. LAcc Length is the recommended acceleration length as shown in Table 10-4 or as adjusted by Table 10-5. 2. Point A controls speed on the ramp. LAcc Length should not start back on the curvature of the ramp unless the radius equals 1,000 ft [300 m] or more. 3. LGap Acpt is the recommended gap acceptance length. LGap Acpt should be a minimum of 300 to 500 ft [90 to 150 m]. 4. The value of LAcc Length or LGap Acpt, whichever produces the greater distance downstream from where the gore point equals 2 ft [0.6 m], is suggested for use in the design of the ramp distance. Figure 10-72. Typical Single-Lane Entrance Ramps The geometrics of the ramp proper should be such that motorists may attain a merge speed that is within 5 mph [10 km/h] of the operating speed of the freeway by the time they reach the point where the left edge of the ramp joins the traveled way of the freeway. For consistency of

132 application, this point of convergence of the left edge of the ramp and the right edge of the through lane may be assumed to occur where the right edge of the ramp traveled way is 12 ft [3.6 m] from the right edge of the through lane of the freeway. While it is desirable for motorists to merge onto the freeway at speeds near the operating speed of the freeway, some motorists may choose to enter the freeway at speeds below the operating speed of the freeway without using the full length of the SCL. Taper-type entrances have been shown to encourage motorists to merge closer to freeway speeds (21). The acceleration length is the distance needed for acceleration upon exiting the controlling feature of the ramp proper or the final section of the ramp proper upstream of the gore point to the point of convergence with the freeway. The acceleration length begins at Point A and ends where the gap acceptance length ends and the taper begins. Point A represents the location where the ramp proper ends and the freeway mainline ramp terminal begins. Conceptually, the acceleration length begins at the end of the controlling curve or feature of 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 design purposes, it may be assumed that horizontal curves with radii less than 1,000 ft will significantly affect vehicle speeds. If the radius of the final curve of the ramp is greater than 1,000 ft and drivers on the ramp have a clear view of traffic in the right lane of the freeway, the beginning of the acceleration length may be located upstream of the gore point (i.e., along the final curve of the ramp proper). Table 10-4 shows minimum values of acceleration lengths for entrance terminals. Figure 10-72 illustrates the minimum lengths for gap acceptance. Referring to Figure 10-72, the larger value of the acceleration length (LAcc Length) or the gap acceptance length (LGap Acpt) is suggested for use in the design of the ramp entrance. Where the minimum values for lane width 16 ft [4.8 m]) and taper rate (50:1) are used with high traffic volumes, taper lengths longer than the larger of LAcc Length or LGap Acpt may be needed to avoid inferior operation and to reduce abrupt moves when merging into the mainline traffic stream. Where grades are present on ramps, speed-change lengths should be adjusted in accordance with Table 10-5. The design values in Table 10-4 are conservative estimates based on free-merge conditions (i.e., free-flow conditions) for passenger cars. Additionally, if trucks constitute a substantial percentage of the traffic volume to be selected as the design vehicle, acceleration lane lengths designed to better accommodate heavier design vehicles can be derived using Figures 3-16 and 3-18 in Chapter 3. 10.9.6.5.2 Parallel-Type Entrances The parallel-type entrance provides an added lane of sufficient length to enable a vehicle to accelerate to near-freeway speed prior to merging. A taper is provided at the end of the added lane. The process of entering the freeway is similar to a lane change to the left. The driver is able to use the side-view and rear-view mirrors to monitor surrounding traffic. A typical design of a parallel-type entrance is shown in Figure 10-72B. Desirably, a curve with a radius of 1,000 ft [300 m] or more and a length of at least 200 ft [60 m] should be provided in advance of the added lane. If this curve has a short radius, motorists tend to drive directly onto

133 the freeway without using much of the gap acceptance length. This behavior results in undesirable merging operations. The taper at the downstream end of a parallel-type acceleration lane should be a suitable length to guide the vehicle gradually onto the through lane of the freeway. A taper length of approximately 300 ft [90 m] is suitable for design speeds up to 70 mph [110 km/h]. The minimum acceleration lengths for entrance terminals are given in Table 10-4, and the adjustments for grades are given in Table 10-5. The acceleration length is the distance needed for acceleration upon exiting the controlling feature of the ramp proper or the final section of the ramp proper upstream of the gore point to the point of convergence with the freeway. Conceptually, the acceleration length begins at the end of the controlling curve or feature of the ramp proper (Point A) and ends where the gap acceptance length ends and the taper begins. For an entrance ramp, the last curve encountered along the ramp proper that significantly affects vehicle speed is considered the controlling curve. It may be assumed that horizontal curves with radii less than 1,000 ft will significantly affect vehicle speeds. If the radius of the final curve of the ramp is greater than 1,000 ft and drivers on the ramp have an unobstructed view of traffic in the right lane of the freeway, the beginning of the acceleration length may be located upstream of the gore point (i.e., along the final curve of the ramp proper). The advantages in efficient traffic operations and low crash frequencies of long SCLs provided by parallel type entrances are well recognized. A long SCL provides more time for the merging vehicles to find an opening in the through-traffic stream. An acceleration length of at least 1,200 ft [360 m] plus the taper is desirable wherever it is anticipated that the ramp and freeway will frequently carry traffic volumes approximately equal to the design capacity of the merging area.

134 Table 10-4. Minimum Acceleration Lengths for Entrance Terminals with Flat Grades of Less Than Three Percent U.S Customary 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) (mph) Stop Condition 15 20 25 30 35 40 45 50 55 60 65 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) (mph) 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 85 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. VDS Hwy = design speed of highway (mph) VAcc Length(f) = merge speed (i.e., the speed at which a driver merges with through traffic at the end of the acceleration length (mph) VDS = design speed of controlling feature on the ramp proper (or the final section of the ramp proper upstream of the gore point) (mph) VAcc Length(i) = average running speed exiting the controlling feature on the ramp proper (or the final section of the ramp proper upstream of the gore point) (mph) LAcc Length = acceleration length (ft)

135 Metric Highway Acceleration Length, LAcc Length (m), for Design Speed (VDS) of Controlling Feature on the Ramp Proper (or the Final Section of the Ramp Proper Upstream of the Gore Point) (km/h) Stop Condition 20 30 40 50 60 70 80 90 100 Design Speed (VDS Hwy) (km/h) Merge Speed (VAcc Length(f)) (km/h) 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) (km/h) 0 20 28 35 42 51 63 70 77 85 50 37 60 50 30 — — — — — 60 45 95 80 65 45 — — — — 70 53 150 130 110 90 65 — — — 80 60 200 180 165 145 115 65 — — 90 67 260 245 225 205 175 125 35 — 100 74 345 325 305 285 255 205 110 40 110 81 430 410 390 370 340 290 200 125 42 120 88 545 530 515 490 460 410 325 245 170 92 130 92 610 580 550 530 520 500 375 300 220 152 140 96 680 630 605 590 570 550 415 330 286 215 Note: Uniform 50:1 to 70:1 tapers are recommended where lengths of acceleration lanes exceed 400 m. VDS Hwy = design speed of highway (km/h) VAcc Length(f) = merge speed (i.e., the speed at which a driver merges with through traffic at the end of the acceleration length (km/h) VDS = design speed of controlling feature on the ramp proper (or the final section of the ramp proper upstream of the gore point) (km/h) VAcc Length(i) = average running speed exiting the controlling feature on the ramp proper (or the final section of the ramp proper upstream of the gore point) (km/h) LAcc Length = acceleration length (m)

136 Table 10-5. SCL Adjustment Factors as a Function of Grade U.S. Customary Design Speed of Highway (VDS Hwy) (mph) Deceleration Length Ratio of Length on Grade to Length on Level for Design Speed (VDS) of Controlling Feature on the Ramp Proper (or the Final Section of the Ramp Proper Upstream of the Gore Point) (mph)a All Speeds 3 to 4% upgrade 0.9 3 to 4% downgrade 1.2 All Speeds 5 to 6% upgrade 0.8 5 to 6% downgrade 1.35 Design Speed of Highway (VDS Hwy) (mph) Acceleration Length Ratio of Length on Grade to Length on Level for Design Speed (VDS) of Controlling Feature on the Ramp Proper (or the Final Section of the Ramp Proper Upstream of the Gore Point) (mph)a 20 30 40 50 All Speeds 3 to 4% Upgrade 3 to 4% Downgrade 40 1.3 1.3 — — 0.7 45 1.3 1.35 — — 0.675 50 1.3 1.4 1.4 — 0.65 55 1.35 1.45 1.45 — 0.625 60 1.4 1.5 1.5 1.6 0.6 65 1.45 1.55 1.6 1.7 0.6 70 1.5 1.6 1.7 1.8 0.6 75 1.6 1.7 1.8 2.0 0.6 80 1.7 1.8 2.0 2.1 0.6 85 1.8 1.9 2.1 2.2 0.6 5 to 6% Upgrade 5 to 6% Downgrade 40 1.5 1.5 — — 0.6 45 1.5 1.6 — — 0.575 50 1.5 1.7 1.9 — 0.55 55 1.6 1.8 2.05 — 0.525 60 1.7 1.9 2.2 2.5 0.5 65 1.85 2.05 2.4 2.75 0.5 70 2.0 2.2 2.6 3.0 0.5 75 2.15 2.35 2.8 3.25 0.5 80 2.3 2.5 3 3.5 0.5 85 2.45 2.65 3.2 3.75 0.5 a Ratio from this table multiplied by the length in Table 10-4 or Table 10-6 gives length of SCL on grade.

137 Table 10-5. SCL Adjustment Factors as a Function of Grade (Continued) Metric Design Speed of Highway (km/h) Deceleration Lanes Ratio of Length on Grade to Length on Level for Design Speed of Turning Curve (km/h)a All Speeds 3 to 4% upgrade 0.9 3 to 4% downgrade 1.2 All Speeds 5 to 6% upgrade 0.8 5 to 6% downgrade 1.35 Design Speed of Highway (km/h) Acceleration Lanes Ratio of Length on Grade to Length on Level for Design Speed of Turning Curve (km/h)a 40 50 60 70 80 All Speeds 3 to 4% Upgrade 3 to 4% Downgrade 60 1.3 1.4 1.4 — — 0.7 70 1.3 1.4 1.4 1.5 — 0.65 80 1.4 1.5 1.5 1.5 1.6 0.65 90 1.4 1.5 1.5 1.5 1.6 0.6 100 1.5 1.6 1.7 1.7 1.8 0.6 110 1.5 1.6 1.7 1.7 1.8 0.6 120 1.5 1.6 1.7 1.7 1.8 0.6 130 1.6 1.7 1.8 1.8 1.8 0.6 140 1.7 1.7 1.8 1.8 1.8 0.6 5 to 6% Upgrade 5 to 6% Downgrade 60 1.5 1.5 — — — 0.6 70 1.5 1.6 1.7 — — 0.6 80 1.5 1.7 1.9 1.8 — 0.55 90 1.6 1.8 2.0 2.1 2.2 0.55 100 1.7 1.9 2.2 2.4 2.5 0.5 110 2.0 2.2 2.6 2.8 3.0 0.5 120 2.15 2.35 2.8 3.2 3.5 0.5 130 2.3 2.5 3.0 3.2 3.5 0.5 140 2.45 2.65 3.2 3.2 3.5 0.5 a Ratio from this table multiplied by the length in Table 10-4 or Table 10-6 gives length of SCL on grade. 10.9.6.6 Single-Lane Free-Flow Terminals, Exits The design criteria provided for minimum deceleration lengths at exit terminals assume that there is no deceleration on the mainline prior to exiting. Although it is common that drivers start decelerating prior to leaving the mainline, the designer should not rely on this in the design of the exit ramp. 10.9.6.6.1 Taper-Type Exits The taper-type exit fits the direct path preferred by most drivers, permitting them to follow an easy path within the diverging area. The taper-type exit terminal beginning with an outer edge alignment break usually provides a clear indication of the point of departure from the through lane and has generally been found to operate smoothly on high-volume freeways. The divergence angle is usually between 2 and 5 degrees. Studies of this type of terminal show that most vehicles leave the through lane at relatively high speeds, thereby reducing the potential for rear-end collisions as a result of deceleration on the through lane. The speed change can be achieved off the traveled way as the exiting vehicle moves along the taper onto the ramp proper. Figure 10-73A shows a typical design for a taper- type exit.

138 Minimum deceleration lengths for various combinations of design speeds for the highway and for the ramp roadway are given in Table 10-6. Grade adjustments are given in Table 10-5. The deceleration length is the distance needed for deceleration after clearing the through-traffic lane of the freeway and before reaching the first location that significantly affects vehicle speed on the ramp proper. The deceleration length is governed by the speed of traffic on the through lane of the freeway and the speed to be attained on the ramp. Deceleration may end in a complete stop, as at a crossroad terminal for a diamond ramp, or may be governed by the operating speed of the curvature of the ramp proper. The deceleration length begins where the width of the auxiliary lane increases to 12 ft [3.6 m] or greater (i.e., the end of taper) and ends at Point D. Point D represents the location where the freeway mainline ramp terminal ends and the ramp proper begins. Point D cannot be located upstream of the gore point but may be located downstream of the gore point. To determine the boundary between the freeway mainline ramp terminal and the ramp proper, the ramp beyond the gore point should be divided into individual tangent and curve sections. If the horizontal alignment of the ramp is curvilinear, the beginning of the first curve encountered at the gore point or downstream of the gore point that significantly affects vehicle speed is the boundary between the freeway mainline ramp terminal and the ramp proper. It may be assumed that any curve radius less than or equal to 1,000 ft [300 m] will significantly affect vehicle speed. If the first curve that significantly affects vehicle speed begins at the gore point, then the boundary between the freeway mainline ramp terminal and the ramp proper is at the gore point. If the beginning of the first curve that significantly affects vehicle speed is downstream of the gore point, then the boundary between the freeway mainline ramp terminal and the ramp proper is downstream of the gore point. If the horizontal alignment of the ramp is relatively straight and has little or no influence on vehicle speeds, then conceptually the acceleration length extends to the beginning of the functional area of the crossroad ramp terminal. The divergence zone length is associated with the freeway mainline ramp terminal of an exit ramp (and diverging maneuvers of a system interchange) and allows for deceleration after clearing the through-traffic lane of the freeway up to the gore point. The divergence zone length begins where the width of the auxiliary lane increases to 12 ft [3.6 m] or greater (i.e., the end of taper) and ends at the gore point. The divergence zone length is always less than or equal to the deceleration length. For a taper-type exit ramp, the divergence zone may be relatively short. Although it is not desirable for vehicles to decelerate on the freeway mainline prior to moving into a deceleration lane, recent research (21) has found that this does occur. Because the values in Table 10-6 for minimum deceleration length on exit ramps do not account for deceleration in the through lanes, these design values provide a conservative estimate for design. It is still prudent for the designer to assume that all deceleration takes place in the SCL when determining the minimum deceleration length. The taper-type exit terminal design can be used advantageously in developing the desired long, narrow, triangular emergency maneuver area just upstream from the exit nose located at a proper offset from both the through lane and separate ramp lane. The taper configuration also works

139 well in the length-width superelevation adjustments to obtain a ramp cross slope different from that of the through lane. The width of the recovery area or the distance between the inner edges of the diverging lanes at the ramp nose is usually 20 to 30 ft [6.0 to 9.0 m]. This entire area should be paved to provide a maneuver and recovery area, but the desired travel path for the ramp roadway should be clearly delineated by pavement markings. 10.9.6.6.2 Parallel-Type Exits A parallel-type exit terminal usually begins with a taper, followed by an added lane that is parallel to the traveled way. A typical parallel-type exit terminal is shown in Figure 10-73C. This type of terminal provides an inviting exit area, because the foreshortened view of the taper and the added width are very apparent. A parallel-type exit operates best when drivers choose to exit the through lane sufficiently in advance of the exit nose to permit deceleration to occur on the added lane (deceleration lane) and allows them to follow a path similar to that encouraged by a taper design. Drivers who do not exit the through lane sufficiently in advance of the exit nose will likely utilize a more abrupt reverse-curve maneuver, which is somewhat unnatural and can sometimes result in the driver slowing in the through lane. In locations where both the main line and ramp carry high volumes of traffic, the deceleration lane provided by the parallel-type exit provides storage for vehicles that would otherwise undesirably queue up on the through lane or on a shoulder, if available.

140 Figure 10-73. Exit Ramps—Single Lane

141 Table 10-6. Minimum Deceleration Lengths for Exit Terminals with Flat Grades of Less Than Three Percent U.S. Customary 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) (mph) 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 85 67 750 725 710 690 665 625 580 540 485 VDS Hwy = design speed of highway (mph) VOS Hwy = average operating speed on highway (i.e., diverge speed) (VDec Lenth (i)) (mph) VDS = design speed of controlling feature on ramp (mph) VDec Length (f) = average running speed entering the controlling feature on the ramp (or the initial section of the ramp proprer downstream of the gore point) (mph) LDec Length = deceleration lane length (ft) Metric Highway Design Speed (VDS Hwy) (km/h) Diverge Speed (VOS Hwy) (VDec Lenth (i)) (km/h) 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) (km/h) Stop Condition 20 30 40 50 60 70 80 Average Running Speed at the End of the Deceleration Length, VDec Length (f) (km/h), Entering the Controlling Feature on the Ramp Proper (or the Initial Section of the Ramp Proper Downstream of the Gore Point) 0 20 28 35 42 51 63 70 50 47 75 70 60 45 — — — — 60 55 95 90 80 65 55 — — — 70 63 110 105 95 85 70 55 — — 80 70 130 125 115 100 90 80 55 — 90 77 145 140 135 120 110 100 75 60 100 85 170 165 155 145 135 120 100 85 110 91 180 180 170 160 150 140 120 105 120 98 200 195 185 175 170 155 140 120 130 103 215 210 205 195 185 170 155 135 140 108 230 220 215 210 205 185 165 150 VDS Hwy = design speed of highway (km/h) VOS Hwy = average operating speed on highway (i.e., diverge speed) (VDec Lenth (i)) (km/h) VDS = design speed of controlling feature on ramp (km/h) VDec Length (f) = average running speed entering the controlling feature on the ramp (or the initial section of the ramp proprer downstream of the gore point) (km/h) LDec Length = deceleration length (m)

142 Minimum deceleration lengths for exit ramps are given in Table 10-6, and adjustments for grades are given in Table 10-5. For a parallel-type exit ramp, the deceleration length begins where the width of the auxiliary lane increases to 12 ft [3.6 m] or greater (i.e., the end of taper) and ends at the beginning of the first curve or feature on the ramp proper that significantly affects vehicle speeds (i.e., Point D). It may be assumed that any curve radius less than or equal to 1,000 ft [300 m] will significantly affect vehicle speed. Where the ramp proper is curved, it is desirable to provide a transition downstream of the gore point. A compound curve may be used with the initial curve desirably having a long radius of about 1,000 ft [300 m] or more. A transition or a long- radius curve is also desirable if the deceleration lane connects with a relatively straight ramp. Where the horizontal alignment of the ramp is relatively straight and has little or no influence on vehicle speeds, conceptually the deceleration length extends to the beginning of the functional area of the crossroad ramp terminal. Providing deceleration lengths longer than the minimum values listed in Table 10-6 may promote more casual deceleration by exiting drivers, particularly under uncongested or lightly congested conditions. This is not necessarily a negative result, but it may change the operational characteristics of the ramp, as those drivers may maintain higher speeds further into the speed- change lane and possibly into the ramp proper. The taper portion of a parallel-type deceleration lane should have a taper of approximately 15:1 to 25:1 [longitudinal:transverse]. A long taper indicates the general path to be followed and reduces the unused portion of the deceleration lane. However, a long taper tends to entice the through driver into the deceleration lane. A short taper produces a better “target” to the approaching driver, giving a positive indication of the added lane ahead. 10.9.6.6.3 Free-Flow Terminals on Curves The previous discussion was based on highways with a tangent alignment. Because the curvature on most freeways is slight, there is usually no need to make any appreciable adjustments at ramp terminals on curves. However, where the curves on a freeway are relatively sharp and there are exits and entrances located on these curves, some adjustments in design may be desirable to avoid operational difficulties. On freeways having design speeds of 60 mph [100 km/h] or more, the curves are sufficiently gentle so that either the parallel type or the taper type of speed-change lane is suitable. With the parallel type, the design is about the same as that on tangent and the added lane is usually on the same curvature as the main line. With the taper type, the dimensions applicable to terminals located on tangent alignment are also suitable for use on curves. A method for developing the alignment of tapered speed-change lanes on curves is illustrated in Figure 10-74. On curved sections, the ramp is tapered at the same rate relative to the through-traffic lanes as on tangent sections.

143 Wherever a part of a tapered speed-change lane falls on curved alignment, it is desirable that the entire length be within the limits of the curve. Where the taper is introduced on tangent alignment just upstream from the beginning of the curve, the outer edge of the taper will appear as a kink at the point of curvature. Figure 10-74a. Layout of Taper-Type Terminals on Curves (U.S. Customary)

144 Figure 10-74b. Layout of Taper-Type Terminals on Curves (Metric)

145 At ramp terminals on relatively sharp curves, such as those that may occur on freeways having a design speed of 50 mph [80 km/h], the parallel type of speed-change lane has an advantage over the taper type. At exits the parallel type is less likely to confuse through traffic, and at entrances this type will usually result in smoother merging operations. Parallel-type speed-change lanes at ramp terminals on curves are illustrated in Figure 10-75. Figure 10-75. Parallel-Type Ramp Terminals on Curves Entrances on curved sections of highway generally operate better than exits. Figures 10-75A and 10-75B show entrances with the highway curving to the left and right, respectively. It is important that the approach curve on the ramp has a very long radius as it joins the acceleration lane. This aligns the entering vehicle with the acceleration lane and lessens the chances of

146 motorists entering directly onto the through lanes. The taper at the end of the acceleration lane should be long, preferably about 300 ft [90 m] in length. When reverse-curve alignment occurs between the ramp and speed-change lane, an intervening tangent should be used to aid in superelevation transition. An exit may be particularly troublesome where the highway curves to the left (Figure 10-75C) because traffic on the outside lane tends to follow the ramp. Exits on left-turning curves should be avoided, if practical. Caution should be used in positioning a taper-type deceleration lane on the outside of a left-turning mainline curve. The design should provide a definite break in the right edge of the traveled way to provide a visual cue to the through driver to avoid being inadvertently led off the through roadway. To make the deceleration lane more apparent to approaching motorists, the taper should be shorter, preferably no more than 100 ft [30 m] in length. The deceleration lane should begin either upstream or downstream from the PC. It should not begin right at the PC, as the deceleration lane appears to be an extension of the tangent, and motorists are more likely to be confused. The ramp proper should begin with a section of tangent or a long-radius curve to permit a long and gradual reversing of the superelevation. An alternate design, which will usually avoid operational concerns, is to locate the exit terminal a considerable distance upstream from the PC. In this design, a separate and parallel ramp roadway is provided to connect with the ramp proper. With the highway curving to the right and the exit located on the right (Figure 10-75D), there is a tendency for vehicles to exit inadvertently. Again, the taper should be short to provide additional “target” value for the deceleration lane. With this configuration, the superelevation of the deceleration lane is readily achieved by continuing the rate from the traveled way and generally increasing it to the appropriate rate for the ramp curve. 10.9.6.6.4 Multilane Free-Flow Terminals Multilane terminals are appropriate where traffic is too great for single-lane operation. Other considerations that may call for multilane terminals are through-route continuity, queuing on long ramps, lane-balance, and design flexibility. The most common multilane terminals consist of two-lane entrances and exits at freeways. Other multilane terminals are sometimes termed “major forks” and “branch connections.” The latter terms denote a separating and joining of two major routes. 10.9.6.6.5 Two-Lane Entrances Two-lane entrances are warranted for two situations: either as branch connections or because of capacity needs for the on-ramp. To satisfy lane-balance needs, at least one additional lane should be provided downstream. This addition may be a basic lane, if needed for capacity, or an auxiliary lane that may be reduced 2,500 to 3,000 ft [750 to 900 m] downstream from the entrance or at the next interchange. In some instances, two additional lanes may be needed because of capacity considerations. If the two-lane entrance is preceded by a two-lane exit, there is probably no need to increase the basic number of lanes on the freeway from a capacity standpoint. In this case, the added lane that results from the two-lane entrance is considered an auxiliary lane, and it may be reduced

147 approximately 2,500 ft [750 m] or more downstream from the entrance. Details of lane reductions are presented in Section 10.9.5.11. Figure 10-76 illustrates simple two-lane entrance terminals where a lane has been added to the freeway. The number of lanes on the freeway has little or no effect on terminal design. Figure 10-76A presents a taper-type entrance and Figure 10-76B shows a parallel-type entrance. Intermixing of the two designs is not recommended within a system route or an urban-area system. The basic form or layout of a two-lane taper-type entrance, as shown in Figure 10-76A, is the same as a single-lane taper, as described earlier in this section, with a second lane added to the right or outer side and continued as an added or auxiliary lane on the freeway. Table 10-4 shows minimum acceleration lengths for entrance ramps. The gap acceptance length is also a consideration as illustrated in Figure 10-76A. Where ramp grades are involved, the lengths should be adjusted as shown in Table 10-5. As in the case of a single-lane entrance, it is most desirable that the geometrics of the ramp proper permit motorists to attain the approximate running speed of the freeway before reaching the tapered section. With the parallel type of two-lane entrance, as shown in Figure 10-76B, the left lane of the ramp is continued onto the freeway as an added lane. The right lane of the ramp is carried as a parallel lane for at least 300 to 500 ft [90 to 150 m] and terminated by a tapered section at least 300 ft [90 m] in length. The length of the right lane should, as a minimum, be determined from the acceleration length or gap acceptance length, as shown in Figure 10-76B. Major factors in determining the appropriate length are the traffic volume on the ramp and the traffic volume on the freeway. When the volume of the two-lane ramp, either the taper type or parallel type, exceeds the capacity of a through lane as specified in the HCM (24), it is suggested that the value for Lg (Figure 10-76) be in the range of 900 to 2,000 ft [300 to 665 m] to allow sufficient time and distance for vehicles in the left ramp lane to move into the mainline lanes. This opens space and provides time for vehicles in the right ramp lane to move into the left ramp lane. Following the termination of the left ramp lane, an additional distance in the range of 900 to 2,000 ft [300 to 665 m] should be provided, plus a taper before terminating the right ramp lane. Although both the taper type and the parallel type of two-lane entrances will operate efficiently when properly designed, some designers prefer the parallel type. This is based on the premise that the taper type involves an “inside merge” with traffic traveling on both sides of the merging lanes. If either vehicle involved with the merging movement abandons the merge, traffic in the adjacent lanes could prevent the merging vehicles from escaping to the adjacent lanes. By contrast, the parallel type allows the merging vehicle to escape to the right shoulder without any interference. Where the predominant two-lane entrances in a particular state or locality are of the parallel type and, therefore, drivers are accustomed to that type of entrance, a taper-type entrance would violate driver expectancy, and vice versa. Thus, a particular type of entrance terminal is sometimes criticized as being unsatisfactory when in fact the difficulty may be lack of

148 uniformity. Either form of two-lane entrance is satisfactory if used exclusively within an area or a region, but they should not be intermixed along a given route. Notes: 1. LAcc Length is the acceleration length as shown in Table 10-4 or as adjusted by Table 10-5. 2. Point A controls speed on the ramp. LAcc Length should not start back on the curvature of the ramp unless the radius equals 1000 ft [300 m] or more. 3. LGap Acpt is the gap acceptance length. LGap Acpt should be a minimum of 300 to 500 ft [90 to 150 m], with suggested LGap Acpt values up to 2,000 ft [600 m] for high-volume conditions. 4. The value of LAcc Length or LGap Acpt, whichever produces the greater distance downstream from the gore point, is suggested for use in the design of the ramp entrance. Figure 10-76. Typical Two-Lane Entrance Ramps

149 10.9.6.6.6 Two-Lane Exits Where the traffic volume leaving the freeway at an exit terminal exceeds the design capacity of a single lane, a two-lane exit terminal should be provided. To satisfy lane-balance needs and not reduce the basic number of through lanes, it is usually appropriate to add an auxiliary lane upstream from the exit. A distance of approximately 1,500 ft [450 m] is recommended to develop the full capacity of a two-lane exit. As with single-lane exits, attention should be given to obtaining the appropriate deceleration distance between the exit and first horizontal curve on the ramp. Typical designs for two-lane exit terminals are shown in Figure 10-77; the taper is illustrated in Figure 10-77A and the parallel type in Figure 10-77B. Figure 10-77. Two-Lane Exit Terminals In cases where the basic number of lanes is to be reduced beyond a two-lane exit, the basic number of lanes should be carried beyond the exit before the lane reduction of the outer lane. This design provides a recovery area for any through vehicles that remain in that lane. This was discussed in Section 10.9.5.11. With the parallel type of two-lane exit, as shown in Figure 10-77B, the operation is different from the taper type in that traffic in the outer through lane of the freeway must change lanes to exit. In fact, an exiting motorist is required to move two lanes to the right to use the right lane of the ramp. Thus, considerable lane changing is needed in order for the exit to operate efficiently. This entire operation takes place over a substantial length of highway, which is dependent in part on the total traffic volume on the freeway and especially on the volume using the exit ramp. The

150 total length from the beginning of the first taper to the point where the ramp traveled way departs from the right-hand through lane of the freeway should range from 2,500 ft [750 m] for turning volumes of 1,500 veh/h or less upward to 3,500 ft [1,000 m] for turning volumes of 3,000 veh/h. 10.9.6.6.7 Two-Lane Terminals on Curved Alignment The design of ramp terminals where the freeway is on curved alignment is discussed under single-lane terminals. The same principles of design, in which offsets from the edge of roadway are used, may be used in the layout of two-lane terminals. 10.9.6.6.8 Major Forks and Branch Connections A major fork is defined as the bifurcation of a directional roadway of a terminating freeway route into two directional multilane ramps that connect to another freeway, or of a freeway route into two separate freeway routes of about equal importance. When designing major forks, the nose should be placed in direct alignment with the centerline of one of the interior lanes, as illustrated in Figures 10-78A, 10-78B, or 10-78C, where the horizontal alignments of the two departing roadways are in curves. This interior lane is continued as a full-width lane, both left and right of the gore. Thus, the width of this interior lane will be at least 24 ft [7.2 m] at the painted nose (prolongation of pavement edge stripes) and preferably not over 28 ft [8.4 m]. The length over which the widening from 12 to 24 ft [3.6 to 7.2 m] takes place should be within the range of 1,000 or 1,800 ft [300 or 540 m]. However, in the case where at least one of the approaches is on a tangent alignment and continues on a tangent, a true optional interior lane cannot be physically developed. As such, the principles of the two-lane exit facility should be used as shown in Figure 10-78D, depending on the nose width, with suggested lengths up to 2,000 ft [600 m] in high-volume conditions. When the approach to a major fork has four or more lanes, the development of an interior option lane may present signing complications. Adding an auxiliary lane and having dedicated lanes in advance of the split (no option lane) may allow for simpler signing to improve driver guidance into their intended lanes.

151 Figure 10-78. Major Forks In the case of a two-lane roadway separating into two, two-lane routes, there is no interior lane. In such cases, it is advisable to widen the approach roadway to three lanes, thus creating an interior lane. The lane is added on the side of the fork that serves the lesser traffic volume. In Figure 10-78A, the right (lower) fork would be the more lightly traveled of the two. The widening from 36 ft [10.8 m] for the approach roadway to about 48 or 50 ft [14.4 or 15.0 m] at

152 the painted nose should be accomplished in a continuous sweeping curve with no reverse curvature in the alignment of the roadway edges. A branch connection is defined by (1) the beginning of a directional roadway of a freeway formed by the convergence of two directional multilane ramps from another freeway or by (2) the converging of two freeway routes to form a single freeway route. The number of lanes downstream from the point of convergence may be one lane less than the combined total on the two approach roadways. In some cases, the traffic demand may indicate that the number of lanes going away from the merging area be equal to the sum of the number of lanes on the two roadways approaching it, and a design of this type will operate efficiently. Such a design is illustrated in Figure 10-79A. Figure 10-79. Branch Connections Where a lane is to be eliminated, the most common method for accomplishing this is a lane reduction, as discussed in Section 10.9.5.11. The lane that is terminated will ordinarily be the exterior lane from the roadway serving the lowest volume per lane. However, some

153 considerations should also be given to the fact that the outer lane from the roadway entering from the right is the slow-speed lane for that roadway, whereas the opposite is true for the roadway entering from the left. If the traffic volumes per lane are about equal, it would be proper to terminate the lane on the right, as shown in Figure 10-79B. In any case, consistency within an area or region is often more important than volume per lane since the latter may change with the specific design or with traffic demand changing over time. The lane being terminated should be carried at full lane width for a distance of approximately 1,000 ft [300 m] before being tapered out. Another consideration is the possibility of a high-speed inside merge, as in Figure 10-79C. This merge should be treated as any other high-speed merging situation; see the discussion of the advantages of parallel-type entrance in Section 10.9.6.6.5.

Selecting Ramp Design Speeds, Volume 2: Conduct of Research Report Get This Book
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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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