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Selecting Ramp Design Speeds, Volume 1: Guide (2021)

Chapter: Section 1. Introduction

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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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Suggested Citation:"Section 1. Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Selecting Ramp Design Speeds, Volume 1: Guide. Washington, DC: The National Academies Press. doi: 10.17226/26415.
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3 Section 1. Introduction 1.1 Background In the AASHTO A Policy on Geometric Design of Highways and Streets, commonly referred to as the Green Book, design speed is defined as a selected speed used to determine the various geometric design features of the roadway (AASHTO, 2018). In principle, design speed is used to determine all pertinent highway features to obtain a balanced design. The design speed concept has been the centerpiece of the geometric design process since introduced in 1936 (Barnett, 1936). Over an extended length of highway, it is desirable to select a uniform design speed. Selection of a design speed should be based upon the anticipated operating speed, topography, adjacent land use, modal mix, and functional classification of the roadway (AASHTO, 2018). Where high- speed facilities meet low-speed facilities, drivers are expected to reduce their speeds suitable to the roadway environment they are entering. This change in speed should be achieved over an extended distance; but in the case of an interchange ramp, this is not possible. Application of the design speed concept and selection of an appropriate ramp design speed as presented in the AASHTO Green Book may be overly simplistic and does not consider many factors. The design speed of a ramp is related to the speeds of the intersecting highways that the ramp connects. According to AASHTO policy, the ramp design speed should desirably approximate the low-volume running speed on the intersecting highways; however, this is often not practical. Green Book Table 10-1 (Table 1) provides guidance for ramp design speeds based upon a range of percentages (i.e., upper, middle, and lower) of the higher design speed for the two intersecting highways. Further guidance is provided in the Green Book for selecting a ramp design speed based on various conditions and ramp types. For example, ramp design speeds between the middle and upper ranges of the highway speeds should be used for direct and semidirect connections, and lower-range values of ramp design speeds are generally used for loop ramps. Table 1. Guide Values for Ramp Design Speed as Related to Highway Design Speed (AASHTO, 2018) U.S. Customary Highway design speed (mph) 30 35 40 45 50 55 60 65 70 75 80 Ramp design speed (mph) Upper range (85%) 25 30 35 40 45 50 50 55 60 65 70 Middle range (70%) 20 25 30 30 35 40 45 45 50 55 60 Lower range (50%) 15 20 20 25 25 30 30 30 35 40 45 Corresponding minimum radius (ft) See (Green Book) Table 3-7 The guide values in Green Book Table 10-1 (Table 1) are applicable to the sharpest, or controlling, curve on the ramp, usually on the ramp proper, and do not pertain to the ramp terminals. The ramp design speed affects many aspects and elements of a ramp, including right- of-way requirements, horizontal and vertical alignments, speed-change lane lengths, ramp type, superelevation, and stopping sight distance, in addition to project costs and potential environmental and social impacts. Directly or indirectly, the selection of ramp design speed should consider physical and economic constraints and anticipated operating speeds on the ramp.

4 The primary components of a ramp at a service interchange include the freeway mainline ramp terminal (i.e., an acceleration or deceleration lane), the ramp proper, and the crossroad ramp terminal; and they must be considered together with the two intersecting roadways during the design of a ramp (see Figure 1). At a system interchange, the primary components of a ramp include two freeway mainline ramp terminals (i.e., one exit and one entrance) and the ramp proper (see Figure 2). Figure 1. Primary Components of a Ramp at a Service Interchange Freeway CrossroadCrossroad ramp terminal Ramp proper Freeway mainline ramp terminal

5 Figure 2. Primary Components of a Ramp at a System Interchange The design guidelines presented herein provide further detail for selecting an appropriate ramp design speed than presented in the 2018 Green Book, to address several overarching challenges that may lead to confusion or inconsistent interpretation of existing AASHTO guidance for selecting an appropriate ramp design speed. In particular, the design guidelines presented herein addresses the following issues: • The Green Book does not provide a specific definition for ramp design speed: The Green Book provides general guidance on selecting appropriate values for ramp design speed, describes the portion of the ramp to which the ramp design speed applies, and indicates which aspects and elements of the ramp that may be affected by the ramp design speed; however, the Green Book does not provide a specific definition for ramp design speed. • Green Book Table 10-1 does not provide sufficient detail for selecting ramp design speeds for specific ramp types and configurations: Green Book Table 10-1 is the centerpiece of AASHTO’s guidance on selecting an appropriate ramp design speed for the design of all ramp types (i.e., entrance, exit, and freeway-to-freeway ramps) and ramp configurations (e.g., diagonal, loop, semidirect, direct). Rather than Green Book Table 10-1 addressing entrance and exit ramps separately and different ramp Freeway mainline ramp terminal Freeway mainline ramp terminal Freeway Freeway Freeway mainline ramp terminal Freeway mainline ramp terminal

6 configurations separately, Green Book Table 10-1 provides upper-, middle-, and lower- range values for ramp design speeds. AASHTO policy then provides further guidance regarding desired, practical, and/or minimum ramp design speeds corresponding to the upper-, middle-, and lower-speed ranges for different ramp configurations. However, it would be desirable to revise Green Book Table 10-1 to address different ramp types and configurations separately. • AASHTO policy provides little guidance on how the selected ramp design speed is to be used to develop a balanced design among all components of a ramp: AASHTO policy states that the ramp terminals are to be properly transitioned and provided with speed-change facilities adequate for the speed of the highway being considered. For freeway mainline ramp terminals, Green Book Tables 10-4 and 10-6 provide the minimum acceleration and deceleration lengths for entrance and exit ramps, respectively. The minimum acceleration and deceleration lengths are determined based on the highway design speed and initial speeds at the beginning of the acceleration length and final speeds at the end of the deceleration length. Green Book Tables 10-4 and 10-6 do not specify a corresponding design speed for acceleration or deceleration lengths, nor is there guidance tying the conditions specified in Green Book Tables 10-4 and 10-6 for acceleration and deceleration lengths to the ramp design speed values in Green Book Table 10-1 for the controlling curve. Beyond AASHTO policy stating that the ramp terminals are to be properly transitioned and provided with speed-change facilities adequate for the speed of the highway being considered, there is no guidance on how this is to be achieved. The other type of ramp terminal is the crossroad ramp terminal. For exit ramps, the Green Book provides some guidance regarding how the functional area of an intersection relates to speed transitions approaching a crossroad ramp terminal. For entrance ramps, there is no corresponding guidance in the Green Book. Guidance in Green Book Chapter 10 implies the use of the vehicle performance curves for acceleration and deceleration distances for passenger cars for coordinating the design of the ramp terminals with the controlling curve on the ramp proper. However, use of the vehicle performance curves provided in Chapter 2 of the Green Book is not explicitly stated or recommended in Chapter 10 as it relates to the use of Green Book Tables 10-1, 10-4, and 10-6. Also, associated with the need to provide transitions between the controlling curve and the ramp terminals, there may be connecting portions of the ramp that are tangent or have a radius larger or smaller than the controlling curve that have to be addressed in the design of the entire ramp. Little guidance is provided within the Green Book on how the ramp terminals and adjoining sections are to be properly transitioned to connect with the controlling curve over the entire length of the ramp for a balanced ramp design that meets driver expectations. • There is no consensus on the portion of the ramp to which ramp design speed applies: Even though the Green Book specifically states that the guide values for ramp design speed in Green Book Table 10-1 apply to the sharpest, or controlling, ramp curve, usually on the ramp proper, and the ramp design speeds do not pertain to the ramp terminals, there is no consensus amongst practitioners as to what portion of the ramp the ramp design speed is applicable. Some practitioners believe that ramp design speed

7 should be applicable to other ramp components, and not simply the controlling curve on the ramp proper. • There is no consensus on whether the ramp design speed should be a single value or should vary along the ramp: Again, the Green Book states that the guide values in Green Book Table 10-1 are applicable to the sharpest, or controlling, curve on the ramp, usually on the ramp proper, and do not pertain to the ramp terminals; but as speeds may vary along a ramp, it is not directly evident whether a single ramp design speed should be used or the ramp design speed should vary along the ramp. As a result, there is confusion or inconsistent interpretation by practitioners of whether the ramp design speed should be a single value or vary along the ramp. 1.2 Objective and Scope of Guidelines This document provides guidelines for selecting appropriate design speeds for ramps. The term “ramp” includes all types, arrangements, and sizes of turning roadways that connect two or more legs at an interchange. The guidelines presented herein facilitate the selection of an appropriate ramp design speed based on a combination of contextual considerations and quantitative information. The guidelines define ramp design speed and all terms and elements related to its application. The guidelines are based on known safety and operational relationships of ramps over a range of interchange forms, ramp types, and environments (i.e., rural and urban) and are expected to result in ramp designs consistent with driver expectations and behaviors over a range of traffic conditions and the functional classification of the two intersecting roads. The guidelines address the following topics related to selecting an appropriate ramp design speed: • System and service interchanges. • Ramp configuration. • Entrance and exit ramps. • Freeway mainline ramp terminal configuration (i.e., taper vs. parallel). • Type of crossroad ramp terminal (i.e., stop control, signal control, free-flow). • Ramp grade. • Type of horizontal curvature (i.e., simple, spiral, compound, and reverse curves). • Number of lanes (i.e., single vs. multilane ramps). • Crossover crown line. • Superelevation and superelevation transition. • Ramp metering. • Collector-distributor (C-D) and frontage roads. • Physical site constraints, environmental and social impacts, and right-of-way costs. The focus of this document is on providing assistance on selecting appropriate ramp design speeds for ramps at service interchanges that connect freeways to lesser facilities and ramps at system interchanges. In other words, this guide addresses selecting ramp design speeds for intersecting facilities, where at least one of the intersecting facilities is a freeway. The design speeds applied in the design of early freeways in urban areas were generally 50 to 60 mph, but today 60 mph is usually the minimum design speed for urban freeways and freeways in mountainous areas; while design speeds of freeways in rural areas are usually 70 to 80 mph or

8 higher (Leisch, 2005). Thus, this guide addresses selecting ramp design speeds for ramps where at least one of the interconnecting facilities has a design speed of at least 50 mph or higher. Guide values for ramp design speeds that connect nonfreeway, arterial roadways to lesser facilities are provided in this document, but are merely an extrapolation of the guide values for interchange ramps where at least one of the intersecting facilities is a freeway. 1.3 Concept and Definition of Ramp Design Speed The definition of ramp design speed is derived from the definition for design speed, from which the term originates. Recall that design speed is defined as a selected speed used to determine the various geometric design features of the roadway, and the selected design speed should be logical with respect to the anticipated operating speed, topography, adjacent land use, modal mix, and functional classification of the roadway (AASHTO, 2018). Similarly, ramp design speed is defined herein as a selected speed used to determine the various geometric design features of a ramp. 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 is based on the operational characteristics of the freeway mainline ramp terminal for an entrance ramp and the operational characteristics of the crossroad ramp terminal for an exit ramp and applies to the tangential section of the ramp proper. The ramp design speed 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. This definition of ramp design speed includes several key concepts: 1. Ramp design speed is used to determine the various geometric features of the ramp, which means that various geometric features of the ramp are related to the ramp design speed, but are not necessarily designed with the same design speed. For example, if the ramp proper consists of a compound curve with two curves with separate design speeds (e.g., 25 mph and 35 mph), only one of the design speeds will be designated as the ramp design speed for the entire ramp. As another example, if the freeway mainline ramp terminal of an exit ramp (i.e., a deceleration lane) connects to the controlling curve on the ramp proper, the freeway mainline ramp terminal should be designed such that operating

9 speeds of vehicles exiting the freeway mainline ramp terminal will be consistent with the anticipated operating speeds entering the controlling curve. 2. The freeway mainline ramp terminal, the crossroad ramp terminal, and the adjoining sections of the ramp proper should be designed to allow for appropriate speed transitions, consistent with driver behaviors and expectations and vehicle performance capabilities, for the selected ramp design speed. For practical purposes, the ramp proper should be divided into individual tangents and horizontal curves. Figures 3 and 4 illustrate this principle for entrance and exit ramps, respectively. In Figure 3, in the direction of travel, the first section of the ramp proper is a tangent (designated RP-Tangent 1) followed by a curve (designated RP-Curve 1). Design criteria for the freeway mainline ramp terminal component of the ramp are slightly different from the design controls for the ramp proper, so the freeway mainline ramp terminal portion of the ramp is not divided into tangents and curves. In this example, RP-Curve 1 would be considered the controlling curve for the ramp; as the horizontal alignment of the ramp is curvilinear, and RP- Curve 1 is the last and only curve encountered along the ramp proper that significantly affects vehicle speed. Thus, the design speed for RP-Curve 1 would be designated as the ramp design speed for the entire ramp. Figure 3. Illustration of Dividing the Ramp Proper of an Entrance Ramp into Individual Tangents and Curves In Figure 4, the first curve encountered on the ramp proper is RP-Curve 1, followed by a second curve (RP-Curve 2), and then a tangent (RP-Tangent 1). The first curve encountered along the ramp proper (RP-Curve 1) would likely significantly impact vehicle speed so it would be designated as the controlling curve for the ramp. As such, the design speed for RP-Curve 1 would be designated as the ramp design speed for the entire ramp. All other sections of the ramp would be designed consistent with the ramp design speed.

10 Figure 4. Illustration of Dividing the Ramp Proper of an Exit Ramp into Individual Tangents and Curves 3. At a service interchange, a driver is expected to either (a) increase speed sequentially transitioning from one adjoining section of the ramp to the next, in the case of an entrance ramp, or (b) decrease speed sequentially transitioning from one adjoining section of the ramp to the next, in the case of an exit ramp. From a practical standpoint, this may not be achieved in the design of every ramp depending upon the alignment. For example, depending upon the entry speed to a curve and the sharpness of the curve, it may be expected that vehicle speeds will decrease slightly or remain constant along a curve or tangent rather than accelerate along the section. Each individual section of the ramp proper (i.e., tangent and curve) has its own design speed. Depending upon the length and the number of individual sections, the change in design speed between adjoining sections should be limited to no more than 10 to 15 mph. Also, the assumed exiting speed of one section should be the assumed entering speed for the adjoining section downstream. Design speed has a direct impact on the minimum radius for a horizontal curve and may affect the design of other geometric elements on horizontal curves, as well. On tangent sections, the design speed may have only a limited effect on the roadway design, unless a critical sight distance limitation is present. 4. The concept of a design speed for an individual section of a ramp is slightly different from the traditional concept of design speed. With the traditional concept of design speed, an individual roadway segment is designed assuming that a vehicle travels at a constant speed; however, with interchange ramps, vehicles are expected to accelerate or decelerate over the length of an individual section. Theoretically, when designing a horizontal curve based on a point mass and the basic curve equation (see Equation 1), it is assumed that a vehicle travels at a constant speed. However, for an entrance ramp, it is assumed that vehicle speed will increase over the length of the curve. Similarly, for an exit ramp, it is assumed that vehicle speeds will decrease over the length of the curve. Thus, for the design of a horizontal curve on an interchange ramp, rather than designing a curve based on a constant speed, the anticipated speed at the midpoint of the curve may

11 be used to design the curve rather than the anticipated speeds at the beginning or end of the curve. The basic curve equation is: 𝑅 = ( . ) (1) where: Rmin = minimum radius of curvature, ft. VDS Curve = design speed, mph. emax = maximum superelevation rate, percent. fmax = maximum side friction factor. For the design of a tangent section on an interchange ramp, the speed at the downstream end is considered the design speed for the tangent. 5. If the horizontal alignment of the ramp is relatively straight and has little to no influence on vehicle speeds, as with some configurations of diagonal ramps, the ramp design speed is based on the operational characteristics of the freeway mainline ramp terminal for an entrance ramp and the operational characteristics of the crossroad ramp terminal for an exit ramp and applies to the tangent section of the ramp proper. 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 assumed initial speed entering the freeway mainline ramp terminal. For an exit ramp, the design speed of the tangent section of the ramp proper (i.e., the ramp design speed) should be consistent with the operational conditions at the crossroad ramp terminal. 6. 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). This indicates that a combination of contextual issues should be considered when selecting a ramp design speed, and the selected design speed should result in the design of a ramp consistent with driver behaviors and expectations (e.g., anticipated acceleration or deceleration along the ramp), vehicle performance capabilities, and the context of the local environment. For instance, for the design of an individual tangent or curve, the assumed acceleration or deceleration expected to take place over the length of a section should not exceed driver comfort levels nor the performance capabilities of vehicles. In addition, the selected ramp design speed should be consistent within the context of the environment. For example, at a system interchange that connects two rural freeways, drivers expect to maintain a high speed through a connecting ramp so a higher ramp design speed for the ramp would be a logical choice. However, at a service interchange in a downtown urban area, drivers would likely expect moderate to significant speed adjustment along a ramp, and with limited right-of-way, a lower ramp design speed would be logical. 1.4 Related Terms and Definitions This section provides definitions and descriptions of key terms related to ramp design and selection of an appropriate ramp design speed. For some terms, existing definitions from the Green Book or other resources are provided; however, for many terms, the 2018 Green Book

12 does not provide an explicit definition, but rather the definition or meaning of the term is understood. For these terms an explicit definition of the term is provided herein. Several figures are presented as well to provide additional clarity. The terms are organized according to the following topics: • Interchange type • Primary components of interchange ramps • Speed-related terms • Ramp configurations • Characteristics of ramps • Horizontal alignment • Vertical alignment • Sight distance Interchange Type: Service interchange A service interchange connects a freeway to a lesser facility. System interchange A system interchange connects two intersecting freeway facilities. Primary Components of Interchange Ramps: Freeway mainline ramp terminal (Figure 5) The freeway mainline ramp terminal is that portion of the ramp adjacent to the through traveled way of the primary highway or freeway and can extend beyond the length of the speed-change lane as necessary. At freeway mainline ramp terminals, ramp traffic merges with or diverges from high-speed through traffic at flat angles. Freeway mainline ramp terminals are further classified as either single lane 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. The boundary between the freeway mainline ramp terminal and ramp proper can differ based on ramp configuration and horizontal and vertical alignment. Where the speed on the ramp proper is limited by horizontal curvature, the beginning or end of the horizontal curvature generally serves as the boundary between the freeway mainline ramp terminal and the ramp proper. For other situations, the boundaries between the freeway mainline ramp terminal and ramp proper are based on engineering judgement. The freeway mainline ramp terminal does not have an associated design speed.

13 Figure 5. Freeway Mainline Ramp Terminals (Entrance and Exit Ramps) Ramp proper (Figures 1 and 2) The ramp proper is the roadway that connects the two terminals of the ramp. The geometry of the connecting road usually involves some curvature and grade. Crossroad ramp terminal (Figure 1) The crossroad ramp terminal is that portion of the ramp that connects the ramp proper to the crossroad. The crossroad ramp terminal may be the at- grade type with interruption of traffic destined to or from the ramp, 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, or a combination of the two. The crossroad ramp terminal does not have an associated design speed. Speed-Related Terms: Design speed Design speed is a selected speed used to determine the various geometric design features of the roadway. The selected design speed should be a logical one with respect to the anticipated operating speed, topography, the adjacent land use, modal mix, and the functional classification of the highway (AASHTO, 2018).

14 Ramp design speed Ramp design speed is a selected speed used to determine the various geometric design features of the ramp. 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 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 is based on the operational characteristics of the freeway mainline ramp terminal for an entrance ramp and the operational characteristics of the crossroad ramp terminal for an exit ramp and applies to the tangential section of the ramp proper. The ramp design speed 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. Operating speed Operating speed is the speed at which drivers are observed operating their vehicles during free-flow conditions. The 85th percentile of the distribution of observed speeds is the most frequently used measure of the operating speed associated with a particular location or geometric feature (AASHTO, 2018). Posted speed Posted speed is the highest speed that can legally be traveled on a section of roadway. Posted speeds are usually set to approximate the 85th percentile speed of traffic as determined by measuring the speeds of a sizable sample of vehicles. Running speed Running speed is the speed at which an individual vehicle travels over a section of highway. The running speed is the length of the highway section divided by the time for a typical vehicle to travel through the section. Peak and off-peak running speeds are used in design and operation, while average running speeds for an entire day are used in economic analyses (AASHTO, 2018).

15 Ramps Configurations: Diagonal ramp (Figure 6) A diagonal ramp is a turning roadway that connects two intersecting roadways at an interchange. Diagonal ramps 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. Figure 6. Diagonal Ramp (AASHTO, 2018) Loop ramp (Figure 7) A loop ramp is a turning roadway that connects two intersecting roadways at an interchange. Its shape is circular in nature. A loop ramp may have single turning movements (left or right) or double turning movements (left and right) at either or both ends. Loop ramps usually involve more indirect travel distance than other types of ramps. Figure 7. Loop and Semidirect Connection Ramps (AASHTO, 2018) Semidirect connection (Figure 7) A semidirect connection is a ramp where the driver exits to the right first, heading away from the intended direction, gradually reversing, and passing around other interchange ramps before entering the intersecting road with a free-flow movement. A semidirect connection may also be used for right turns, but there is little reason for its use if a conventional diagonal ramp 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 typically less than for a comparable loop and more than for a direct connection.

16 Direct connection (Figure 8) A direct connection is defined as a ramp that does not deviate greatly from the intended direction of travel. Direct connections are designed to encourage high-speed merging with freeway traffic and are generally designed with higher design speeds than semidirect connections. Direct connections typically are designed with parallel-type terminals at both ends. Figure 8. Direct Connection Ramp (AASHTO, 2018) Outer connection (Figure 9) An outer connection is a turning roadway where the driver exits in the intended direction and merges with the other leg of the interchange and another ramp (normally a loop ramp) is located between the outer connection and center of the interchange. Outer connections are designed to encourage high-speed merging with traffic. Figure 9. Outer Connection Ramp (AASHTO, 2018) Characteristics of Ramps: Auxiliary lane An auxiliary lane is defined as the portion of the roadway adjoining the through lanes for speed change, turning, storage for turning, weaving, truck climbing, and other purposes that supplement through-traffic movement (AASHTO, 2018). Speed- change lane (Figure 5) A speed-change lane is an auxiliary lane, including tapered areas, primarily for acceleration or deceleration of vehicles entering or leaving through traffic. The term speed-change lane applies broadly to the added lane that joins the traveled way of the highway to the turning roadway and does not necessarily imply a definite lane of uniform width. For an entrance ramp the speed-change lane length is measured from the gore point to the end of the taper, and for an exit ramp the speed-change length is measured from the beginning of the taper to the gore point. A speed-change lane should have sufficient length to enable a driver to make the appropriate change in speed between the intersecting highway and the turning roadway.

17 Acceleration length (LAcc Length) (Figure 5) The acceleration length is associated with the freeway mainline ramp terminal of an entrance ramp and merging maneuvers of a system interchange and is the distance needed for acceleration upon exiting the controlling feature of the ramp proper or final section of the ramp proper upstream of the gore point to the point of convergence with the primary highway or freeway. This distance for acceleration is governed by the speed differential between the operating speed on the entrance curve or other controlling feature of the ramp and the operating speed of the primary highway. 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. Point A may be located upstream of the gore point, but it cannot be located downstream of the gore point. The acceleration length should not start on a curve of the ramp unless the radius equals 1,000 ft or more and a driver on the ramp has a clear view of freeway right-lane traffic. Gap acceptance length (LGap Acpt) (Figure 5) The gap acceptance length is associated with the freeway mainline ramp terminal of an entrance ramp and merging maneuvers of a system interchange and is the distance necessary to allow motorists to evaluate gaps in the freeway traffic and position their vehicles to use the gap. The gap acceptance length begins at the gore point and ends at the end of the acceleration length where the width of the auxiliary lane reduces to less than 12 ft. The gap acceptance length is always less than or equal to the acceleration length. Deceleration length (LDec Length) (Figure 5) The deceleration length is associated with the freeway mainline ramp terminal of an exit ramp and diverging maneuvers of a system interchange and 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 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. Divergence zone length (LDiv Zone) (Figure 5) 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 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.

18 Taper length (LTaper) (Figure 5) The taper length is a transition length of roadway where the lane width gradually increases to add a lane or gradually narrows to remove a lane. The taper length typically begins or ends where the auxiliary lane transitions to a width less than 12 ft. Taper-type entrance and exit (Figure 10) A taper-type entrance and exit is a general form of a speed-change lane that provides direct entry or exit at a flat angle. For exit ramps, the divergence angle is usually between 2 and 5 degrees. Figure 10. Taper-Type Entrance and Exit Ramps Parallel- type entrance and exit (Figure 11) A parallel-type entrance and exit is a general form of a speed-change lane that has an added lane for changing speed adjacent to the freeway mainline. Figure 11. Parallel-Type Entrance and Exit Ramps Acceleration lane An acceleration lane is a speed-change lane of sufficient length to enable a driver to increase speed from the speed of the turning roadway to the higher speed of operation of the highway or street prior to merging. Moreover, 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 maneuver into the stream before the acceleration lane ends. The terms “speed-change lane” and “acceleration lane” can be used synonymously for an entrance ramp. Deceleration lane A deceleration lane is a speed-change lane of sufficient length to enable a driver to reduce speed from the speed of operation on the highway or freeway to the lower speed on the turning roadway. The terms “speed-change lane” and “deceleration lane” can be used synonymously for an exit ramp.

19 Painted nose (Figure 12) The painted nose is a point, having no dimensional width, where the pair of solid white pavement edge markings from the ramp and intersecting roadway meet. Figure 12. Typical Exit Gore Area Characteristics (adapted from AASHTO, 2018) Gore point (Figure 12) 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 gore point is often used to define the beginning or ending milepost of the ramp. The gore point is also used to define the beginning of the gap acceptance length and speed-change lane length for an entrance ramp and the end of the divergence zone length and speed-change lane length for an exit ramp. Physical nose (Figure 12) The physical nose is a point upstream from the gore, having some dimensional width (typically 4 to 8 ft) that separates the ramp from the intersecting roadway. Neutral area (Figure 12) The neutral area refers to the triangular area between the painted nose and the gore nose and incorporates the gore point and physical nose. Gore nose (Figure 12) The gore nose is a point delineating the downstream end of the neutral area at the shoulder intersection points. The gore nose generally is the location where the neutral area pavement ends. Gore (Figure 12) The term “gore” indicates an area downstream from the shoulder intersection points. Painted Nose Physical Nose Gore Nose Shoulder GoreNeutral Area Neutral Area 4-8 ft Gore Point (2 ft)

20 Crossover crown line At the boundary between the right lane of the freeway and the adjacent auxiliary lane of a freeway mainline ramp terminal, the cross slope rates of the freeway lane and auxiliary lane may differ. This boundary location is referred to as the crossover crown line. The crossover crown line (not to be confused with the crown line normally provided at the centerline of a roadway) is measured as the algebraic difference in cross slope rates of the two adjacent lanes. Traveled way The portion of the roadway for the movement of vehicles, exclusive of shoulders and bicycle lanes (AASHTO, 2018). Ramp meter A traffic signal installed on an entrance ramp to control the number of vehicles entering the freeway. Diverge A movement in which a single stream of traffic separates into two streams without the aid of traffic control devices (TRB, 2010). Merge A movement in which two separate lanes of traffic combine to form a single lane without the aid of traffic signals or other right-of-way controls (TRB, 2010). Horizontal Alignment: Simple curve A circular arc with a single, constant radius. Compound curve A compound curve is where two or more circular arcs of different radii are joined in succession, turning in the same direction, without any tangent between adjoining arcs. Spiral curve The Euler spiral, which is also known as the clothoid, is used in the design of spiral transition curves. The radius varies from infinity at the tangent end of the spiral to the radius of the circular arc at the end that adjoins that circular arc. By definition, the radius of curvature at any point on an Euler spiral varies inversely with the distance measured along the spiral. In the case of a spiral transition that connects two circular curves having different radii, there is an initial radius rather than an infinite value (AASHTO, 2018). Controlling curve 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 that significantly affects vehicle speed is considered the controlling curve.

21 Superelevati on Superelevation is the rotation of the pavement on the approach to and through a horizontal curve. Superelevation is intended to assist the driver by counteracting the lateral acceleration produced by tracking the curve (TRB, 2010). Superelevati on transition section The superelevation transition section consists of the superelevation runoff and tangent runout sections. The superelevation runoff section consists of the length of roadway needed to accomplish a change in outside-lane cross slope from zero (flat) to full superelevation, or vice versa. The tangent runout section consists of the length of roadway needed to accomplish a change in outside- lane cross slope from the normal cross slope rate to zero (flat), or vice versa (AASHTO, 2018). Curve radius The radius of the circular arc used to specify the horizontal alignment of the roadway. For ramps, the minimum radii used for design should preferably be measured from the inner edge of the traveled way rather than the middle of the vehicle path or the centerline of the traveled way. Vertical Alignment: Grade and profile 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 (AASHTO, 2018). Vertical curve Vertical curves effect gradual changes between tangent grades. For simplicity, a parabolic curve with an equivalent vertical axis centered on the Vertical Point of Intersection (VPI) is usually used in roadway profile design. Sight Distance: Stopping sight distance (SSD) Stopping sight distance is the sum of two distances: (1) the distance traversed by the vehicle from the instant the driver sights an object necessitating a stop to the instant the brakes are applied, and (2) the distance needed to stop the vehicle from the instant brake application begins. These are referred to as brake reaction distance and braking distance, respectively (AASHTO, 2018). Decision sight distance (DSD) Decision sight distance is the distance needed for a driver to detect an unexpected or otherwise difficult to-perceive information source or condition in a roadway environment that may be visually cluttered, recognize the condition or its potential threat, select an appropriate speed and path, and initiate and complete complex maneuvers. Because decision sight distance offers drivers additional margin for error and affords them sufficient length to maneuver their vehicles at the same or reduced speed, rather than to just stop, its values are substantially greater than stopping sight distance (AASHTO, 2018).

22 Horizontal sight distance Sight distance is the length of the roadway ahead visible to the driver. On horizontal curves, the obstruction that limits the driver’s sight distance may be some physical feature outside of the traveled way, and on a crest vertical curve the obstruction may be the road surface at some point. Vertical sight distance Sight distance is the length of the roadway ahead visible to the driver. On a tangent roadway, the obstruction that limits the driver’s sight distance may be the road surface at some point on a crest vertical curve. 1.5 Outline of Document The remainder of this document is organized as follows: Section 2—Ramp Elements Related to Ramp Design Speed Section 3—Guide for Designing Ramps in a Consistent Manner Based on the Selected Ramp Design Speed Section 4—Design Tool to Evaluate Ramp Designs for Consistency with the Selected Ramp Design Speed Section 5—Case Studies Section 6—Summary of Design Guide and Future Research Needs Section 7—References Section 8—Abbreviations, Acronyms, Initialisms, and Symbols

Next: Section 2. Ramp Elements Related to Ramp Design Speed »
Selecting Ramp Design Speeds, Volume 1: Guide Get This Book
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Selection of a design speed should be based upon the anticipated operating speed, topography, adjacent land use, modal mix, and functional classification of the roadway.

The TRB National Cooperative Highway Research Program's NCHRP Web-Only Document 313: Selecting Ramp Design Speeds, Volume 1: Guide provides further detail for selecting an appropriate ramp design speed than presented in the 2018 Green Book, to address several overarching challenges that may lead to confusion or inconsistent interpretation of existing AASHTO guidance for selecting an appropriate ramp design speed.

Supplemental to the document are NCHRP Web-Only Document 313: Selecting Ramp Design Speeds,Volume 2: Conduct of Research Report and Ramp Speed Profile Model worksheets.

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