Guidelines for Ramp and Interchange Spacing (2011)

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62 Chapter 5 Spacing Guidance This chapter presents an overarching framework to support ramp and interchange spacing evaluations and decisions. The information supporting the framework builds upon the information of previous chapters to help guide the user though steps to evaluate ramp and interchange configurations. This chapter provides information to highlight “interchange spacing” and “ramp spacing” relationships and emphasizes that ramp spacing should be the primary consideration of ramp and interchange spacing considerations. This chapter includes qualitative, and where available from project data, quantitative input on minimum ramp and interchange spacing values. This chapter provides a simplified four step assessment method to aid the user in understanding how ramp spacing values may or may not be influenced by design, operational, safety, and signing considerations. This chapter is supported by Appendix A, which provides case studies to apply the guidelines framework and four step process outlined in this chapter and supported by the resource information in Chapters 2, 3, and 4. 5.1 GUIDELINES FRAMEWORK This section provides a simplified framework for applying the content of Chapters 2, 3, and 4 to understand a project context, consider and evaluate a range of solutions, and support efforts to select optimal ramps and interchanges for the project conditions. Exhibit 5-1 presents a framework to considering ramp and interchange configurations and illustrates how ramp and interchange spacing assessments contribute to making project decisions. The interchange spacing assessment process addresses three basic areas: Getting Started, Considering Solutions, and Selecting a Plan. The framework and subsequent tabular information integrate information from Chapters 2, 3, and 4 into a generalized sequence that can guide a user through ramp and interchange spacing assessments. The framework upon which ramp and interchange spacing decisions are made begins with establishing a broad understanding of the opportunities, constraints, and needs of a particular project context. This information helps a user understand the issues associated with the facility and helps identify the range of potential solutions. As possible solutions evolve from broad concepts to alternatives to completed designs, they become increasingly more detailed. However, as these solutions are advanced, the opportunities to affect and influence ramp and interchange spacing values diminishes. Subsequent subsections expand upon the three basic areas and tables within and decisions. Ramp and interchange spacing assessments should begin with a broad understanding of the needs of each particular project context. these subsections outline contextual, design, operational, safety, and signing considerations that can help guide ramp and interchange spacing assessments

Exhibit 5-1 Interchange Planning and Design Framework Spacing Guidance 63

5.1.1 Getting Started There are numerous factors that influence project needs and decisions. Prior to developing and considering ramp and interchange configurations the user must understand the contextual design environment for the mainline and existing interchanges or ramps, if present. The project context can influence decisions on how proposed ramp and interchange spacing configurations can help alleviate existing facility operational or safety deficiencies. A key objective is to understand how proposed ramps and interchanges and their spacing, compared to adjacent constraints, may affect or may be affected by current and forecast conditions. The project development stage, project type, area, policy, and facility characteristics can influence the approach and ability to make ramp and interchange spacing decisions. Once the context is established, gathering the available evaluation data is critical to initiating preliminary assessments and evaluations that will guide the alternative development and refinement processes. The following describes some of the project context considerations that can greatly influence freeway, interchange, and ramp design decisions. 5.1.1.1 JURISDICTION AND POLICY FHWA and state agencies provide methodologies to consider new or modified access to freeways. Federal and state policy considerations are presented in Section 2.2 and their application is illustrated in Case Study 1. At the broadest level, a primary objective of these methodologies is to understand the traffic operational effects of adding or modifying a freeway access. In addition to quantifying forecast conditions with and without the proposed ramp or interchange, the methodologies generally identify a range of considerations that users must consider before access is granted. The traffic operations and safety evaluations called for by federal and state policies can extend beyond the subject freeway segment and include analyzing the adjacent roadway network to better understand arterial network and freeway traffic operations and safety relationships. The overall intent is to consider how potential changes in ramp and interchange configurations might affect the freeway being considered. This is coupled with identifying a It can be challenging to provide ramp and interchange configurations in complex freeway networks (especially in constrained environments) that meet spacing needs and provide desired traffic and safety performance. 64 Guidelines for Ramp and Interchange Spacing

range of ramp and interchange solutions that are consistent and integrated with broader transportation system improvements. In addition to traffic and design evaluations, federal and state approval processes usually require understanding the socio-economic and environmental constraints of the improvement under consideration. Meaningful public and stakeholder outreach are valuable contributors to any successful transportation project, and two-way outreach to share information and understand concerns and input from the public and project stakeholders is a cornerstone of a successful project. 5.1.1.2 PHYSICAL ATTRIBUTES The user must understand the physical constraints of the environment and be prepared to generate potential concepts within those constraints. This requires understanding three dimensional geometric design requirements and factors that influence the ramp and interchange spacing ranges. Chapter 3 presents a number of these attributes and discusses how professionals should consider them when assessing ramp and interchange spacing. Professionals should apply tools and guidance to help them understand how sensitive system performance is to variations in design decisions associated with ramp and interchange spacing. 5.1.1.3 EXISTING AND FUTURE TRAFFIC OPERATIONS There is an iterative and dynamic relationship between geometric design choices and resulting traffic operations. Existing conditions and the contributing factors to existing traffic operations must be understood before identifying proposed solutions. Forecast traffic volumes should be used to support lane determination assessments that help establish the operational foundation of the design concepts. Alternative geometric configurations, in turn, affect predicted traffic operations as alternative designs are developed, assessed, and screened. Ramp and interchange design features will directly affect resultant traffic operations. Professionals should first establish and address appropriate basic freeway, interchange, and ramp design configurations and then evaluate how traffic operations may or may not be sensitive to ramp spacing values. Chapter 3 provides an extensive discussion on geometric design and operational relationships. Traffic operations play a key role in ramp spacing assessment in Case Studies 3, 4, and 5. 5.1.1.4 IMPACTS TO SAFETY Understanding the safety impacts associated with new or modified access alternatives to a freeway supports informed interchange and ramp spacing decisions. The general term “safety” should be considered based on crash frequency, type, and severity. General safety-related issues are presented in Section 4.4 of these Guidelines. Safety relationships of various ramp spacing The relationship between geometric design choices and resulting traffic operations is an iterative process in which forecast volumes support lane determinations and geometric configurations affect traffic operations. Spacing Guidance 65

Professionals should consider signing at the early stages to assess its role in ramp spacing decisions. Signs needs can influence potential solution concepts. parameters have been developed for these Guidelines and provide a useful aid to consider possible safety tradeoffs from various ramp and interchange configurations. Safety considerations in evaluating possible ramp and interchange evaluations are included in Case Studies 1, 2, 3, and 4. 5.1.1.5 SIGNING Geometric design and traffic operations needs often require ramps and interchanges to be far enough apart that the MUTCD’s signing requirements can be satisfied. However, this is not always the case and signing needs should be considered early in the alternatives development stages. Complex or unusual interchange forms can require complex signing with a large number of message units or sign panels. The MUTCD notes no more than three should be placed at the same location; therefore, in such cases, ramp spacing may need to be increased from minimum geometric and operational dimensions to accommodate signing that is adequate and not overwhelming to drivers. Signing considerations are presented in Section 3.9 of these Guidelines and signing evaluations are included in Case Studies 1, 2, 3, and 5. Case Study 5 illustrates a situation in which a proposed alternative is determined to be infeasible because it cannot be adequately signed consistent with MUTCD criteria 5.1.1.6 PROJECT CONTEXT SUMMARY Table 5-1 summarizes just some of the elements associated with establishing a project’s context. The table is not exhaustive and is intended to represent some common considerations. The Case Studies in these Guidelines were developed to provide an array of project conditions and contexts. Table 5-1 Understand Project Context Project Development Stage Project Type Area Planning Preliminary Design Final Design Implementation New interchange and new facility New interchange on an existing facility Extensive modifications to an existing interchange on an existing facility Type Urban Rural Suburban Issues Environmental Physical Social Reason for Modification Facility Type Policy Considerations New development Capacity Access Safety Interstate State facility County City Justification reports Interchange handbooks Design standards and guidelines Resource documents Table 5-2 provides a summary of facility geometric characteristics that can influence concept solutions and ramp and interchange spacing decisions. 66 Guidelines for Ramp and Interchange Spacing

Whether it is the type of interchange form being evaluated or a desire to ensure lane continuity or the principle of lane balance, facility characteristics and considerations will form the basis for project solutions. These and other design considerations are presented in Chapter 3. Case Study 5 includes an example of how one alternative concept would have led to unacceptable mainline lane conditions. Table 5-2 Facility Geometric Characteristics Interchange type and form Ramp Combination Lanes Mainline Network “T”, “Y”, and “X” System All directional Directional with loops Service Diamond Cloverleaf Partial Cloverleaf EN-EX EN-EN EX-EX EX-EN Number Basic Auxiliary Balance Continuity Freeway Highway Adjacent network Connections to public roads Isolated interchange Consistent series of interchanges (e.g., 1-mile spacing) Inconsistent interchange spacing In addition to understanding elements about the project’s context and facility type, there are a variety of data needs that could influence ramp and interchange configurations and, therefore, spacing. Table 5-3 includes a partial list of the types of operational data and characteristics that might influence project decisions for ramp and interchange forms. Table 5-3 Evaluation Considerations Traffic Speed Crash History Design Vehicle Volumes Existing Design Forecast Composition Trucks Passenger cars Recreation vehicles Purpose Commuter Recreation Freight Upstream Design Posted Operating Downstream Design Posted Operating Type Frequency Severity Location Isolated System Type Percentage Spacing Guidance 67

5.1.2 Considering Solutions Concept solutions should be developed based upon the overarching context considerations that might drive project documentation and decision making. During the initial sketch-planning stages of alternative development and evaluation, professionals should consider a wide range of possible solutions to compare and screen less promising alternatives. Ramp and interchange spacing limitations may directly influence screening decisions. Developing, comparing, and selecting a preferred alternative is an iterative process that integrates geometric design, operational testing, signing evaluations, and safety analyses. As more detail is available throughout the process, evaluation tools and techniques can be applied at increasing detail to aid in project decision making. For example, Section 5.3.3 includes tools to support safety assessments at two levels: A planning level assessment that considers trade offs based on ramp spacing only and a planning/preliminary design level assessment that considers key variables that should be available as the concept design evolves in increasing design detail. Table 5-4 provides an overview of the types of information that might be considered as potential solutions evolve from conceptual development to the alternative selection (sketch planning to 30% design). As solutions evolve from sketch-planning concepts to 30-percent plans, the same spacing assessments should occur, but at increasing levels of detail that are consistent with the information available. 68 Guidelines for Ramp and Interchange Spacing

Table 5-4 Alternatives Development and Refinement Alternative Development/ Evaluation Comparing Alternatives Selected Alternative Other Possible Solutions? Network Capacity Improvement Programmed Improvement Plan Select Appropriate Configurations Ramp and Interchange Form Prepare Interchange and Ramp Layout Design Alternatives Assess Conceptual Signing Needs Ramp terminal layout Parallel Taper Profile considerations Schematic sign layout Message Sequence placement Preliminary Design Refine design concepts based on governing agency Ramp and interchange spacing tradeoffs Final signing Operational Testing Per lane capacity values Ramp/Freeway capacity charts and tables Highway Capacity Software Ramp (Capacity, merge/diverge) Mainline (Segment, Weaving) Analysis iterations Microsimulation, if needed Safety Analysis Ramp combinations Short spacing highly sensitive to crash frequency Preliminary safety assessment tool Safety performance functions Crash modification factors In some cases, ramp and interchange needs and constraints simply do not allow for conventional ramp or interchange designs that meet operational and safety objectives. Professionals may be faced with challenging decisions about accepting sub-optimal ramp spacing versus alternative solutions that may be more costly and have a greater impact. Collector-distributor (C-D) roadways are effective in protecting a freeway mainline by locating merge, diverge, and weaving movements onto a secondary roadway. However, C-D roadways clearly have facility cross-section impacts and may require a more extensive longitudinal evaluation of a mainline segment. Ramp braids (grade separated ramps) are alternatives to closely spaced ramps. These configurations may also be physically impacting, costly, and aesthetically unappealing. While much is written about interchange and ramp planning and design considerations, relatively little guidance is provided to support planning and design recommendations for C-D roadways or ramp braids as options to increase ramp spacing values. Information on C-D roads and grade separated ramps is presented in Case Study 5. These design features are sometimes needed to meet project needs. Spacing Guidance 69

A preliminary assessment of the capacity, geometrics, safety, and signing of each alternative should be conducted to understand the existing conditions and future facility needs. The desired outcomes are preliminary ramp and interchange configurations, an understanding of their spacing characteristics, and their likely resultant performance. Table 5-5 provides an overview of the fundamental considerations in assessing ramp and interchange configurations. Their considerations are presented in Section 5.3 as sequential steps to support ramp and interchange spacing decisions. Each of the Case Studies guides the user through applications of the spacing assessment steps. Table 5-5 Fundamental Spacing Assessment Considerations Geometric Traffic Operations Safety Signing Mainline Proper Basic Lanes Lane Balance Lane Continuity Route Continuity Interchange form Single-Exit Design Exits in Advance of the Cross Street No Left Exits Ramps Combinations (EN-EX, etc.) Type (loops, diagonal, etc.) Freeway Number of lanes Weaving Merge/diverge capacity Closely spaced ramps Ramps Terminal Intersection Ramp-Freeway Junction Network Cross Streets Parallel Roads Adjacent Intersections Isolated Ramp Terminal System/Network Upstream or downstream effects Logical destination Sequencing Spacing Message units As concepts are screened and options advanced to schematic-level design, more detail and information is known about specific cross-sectional elements, the horizontal and vertical geometry and configuration of the ramps, and the number and arrangement of lanes on the mainline facility. The approximate ramp-freeway junction concepts may be more refined to reflect ramp terminal design features (taper versus parallel design and gore geometry, converge and diverge angles, gore dimensions). At this stage, the specific ramp spacing measurements can be more carefully considered, and a more thorough evaluation of the projected operational and safety performance of the ramp and mainline can be conducted. With more detailed operations analysis results, users can assess tradeoffs for various ramp and interchange spacing alternatives. A schematic sign layout Geometric and signing information is presented in Chapter 3. Operations and safety information is presented in Chapter 4. Information on C-D roads and grade separated ramps is presented in Case Study 5. These design features are sometimes needed to meet project needs. These fundamental considerations form the basis of four sequential steps for assessing ramp and interchange spacing alternatives. 70 Guidelines for Ramp and Interchange Spacing

for messages, sequencing and placement could be established to aid in evaluating and refining the design alternative. Specifically, users should consider how ramp configurations and ramp and interchange spacing dimensions affect expected operations and safety performance. During these evaluations and comparisons, inferior alternatives may be screened to advance only the most promising solutions. After thoroughly comparing the potential alternatives, the selected alternative is typically moved into the preliminary (30-percent) design phase. The design and operational concepts are refined based on governing agency guidelines and standards. The various ramp and interchange spacing tradeoffs are fully identified and reviewed. Operational analysis is likely to become more refined with additional highway capacity analyses and, in some cases, by applying microsimulation models. Ramp and interchange spacing values can be optimized and refined based on traffic operations analysis results. With detailed preliminary design and operational analysis results, signing requirements can be refined, and collectively, all of the information should be used to select the most promising alternative(s). This stage generally includes preparing and completing documentation needed for a particular governing agency, such as FHWA or state or local governments, to make project approval decisions. Additional documentation for environmental review evaluations, design deviations and possible design exceptions can be completed at this time. 5.1.3 Selecting a Plan Upon completing project documentation, project solutions are refined and advanced to the final design stages. At this stage, there is very little flexibility to influence ramp and interchange spacing decisions. As the designs are refined and advanced, minor revisions for right-of-way, utilities, or other project constraints may be necessary, but major revisions are less practical. 5.2 “INTERCHANGE” VERSUS “RAMP” SPACING This section provides information to highlight “interchange spacing” and “ramp spacing” relationships and emphasizes that ramp spacing should be the primary consideration of Spacing Guidance 71

ramp and interchange spacing decisions. This section provides information on how design, operations, safety, and signing may influence ramp and interchange spacing assessments Ramp spacing values should be the primary consideration in making interchange and ramp spacing technical decisions. “Interchange” spacing addresses the dimensions between freeway cross street centerlines. Ramp spacing addresses the dimensions of sequential ramps that are to be configured to meet geometric design, operational, safety, and signing needs. Interchange spacing dimensions generally provide limited value in most interchange and ramp spacing evaluations and should not, on their own, guide project decisions. 5.2.1 Geometric Design: Interchange Form Considerations Considering ramp element dimension ranges from Chapter 3 and how those ramp elements are applied to various service interchange forms, it is possible to conceptually assess how interchange form may influence or be influenced by cross street spacing. Interchange forms influence the length of ramp components and the type of interchange may influence spacing assessments. For example, a single exit design for a partial cloverleaf form may result in longer ramps than a diamond interchange. The interchange form should be selected before performing a spacing assessment. In turn, a spacing assessment may help determine that some interchange forms are better suited for an available interchange spacing dimension. Exhibit 5-2 presents the cross street spacing feasibility of three pairs of different forms of service interchanges based upon the interchange spacing. Exhibit 5-2 is based on geometrics only and does not account for traffic operations, safety, signing, or other factors that play a role in interchange spacing decisions. Ramps, more so than interchanges, should be the focus of spacing evaluations. Interchange spacing dimensions generally provide limited value in most interchange and ramp spacing evaluations. Interchange forms may influence interchange spacing assessment results. 72 Guidelines for Ramp and Interchange Spacing

Assumes single entrance and exit design for configurations with the loop in advance or beyond the cross street. Assumes ramp braids or C-D roadways are not used. Exhibit 5-2 Interchange Spacing Feasibility The generalized ranges of values in Exhibit 5-2 reflect conventional ramp configurations. Interchanges may be spaced more closely than the ranges indicted in this exhibit if ramp braids or C-D roadways are included. The spacing values of ramps that access the ramp braids or collector distributor roadway and other adjacent interchanges should be considered using the principles and tools included in this chapter. System interchanges are not included in Exhibit 5-2 and spacing needs should be assessed on a case-by-case basis. In general, interchange spacing values between system interchanges or between system and service interchanges will exceed (sometimes greatly) the ranges of Exhibit 5-2. In some cases these spacings may exceed two or more miles. Closer interchange spacing may be allowable if ramp braids and C-D roadways are included. In general, system interchange spacing needs should focus on ramp spacing considerations for supporting project decisions. Factors influencing ramp and interchange spacing include the orientation and “levels” of the intersection freeways and the number of levels a particular ramp must change. For example, a ramp changing only one level will have a completely different profile than a ramp changing three or four levels. System interchanges may include double or triple lane exits or branch type connections. The geometric design needs and associated resulting spacing needs for ramp terminal configurations of system interchanges will vary greatly based on system interchange form, ramp design, turning roadway design, and lane adds and drops. System interchange forms have unique characteristics that generally increase “interchange spacing” dimensions over service interchange forms. Spacing Guidance 73

In complex interchange environments, signing may become a critical consideration influencing alternative designs. Case Study 5 provides an example of complex signing needs influencing ramp spacing decisions 5.2.2 Traffic Operations Considerations Table 5-6 highlights some of the relationships between “interchange” and “ramp” spacing values. This table may help users correlate interchange spacing discussions and planning level evaluations to ramp spacing considerations and evaluations. Table 5-6 Interchange Spacing Effects Effects on ramp density Effects on Volume Effects on ramp design Increased Interchange Spacing Lower ramp density (fewer ramps per mile) More volume per ramp Increased exit ramp length to avoid queue spillback Multilane ramps Decreased Interchange Spacing Higher ramp density (more ramps per mile) Less volume per ramp Possible application of shorter ramp lengths for queue storage Interchange spacing and ramp density influences a freeway’s estimated free- flow speed (FFS). As interchanges are added within a segment (thus increasing ramp density), there is a corresponding decrease in FFS. The effects of the spacing between the added interchanges and ramps on FFS speed are less clear. 5.2.3 Safety Considerations The variability in interchange forms and the relatively subjective nature of spacing measurements between crossroad centerlines makes ramp spacing a better means of considering safety. The safety tradeoffs between various interchange spacing dimensions is best quantified by considering safety analysis tools of section 5.3.3 and assessing the predicted safety performance of the estimated ramp spacing values between the subject interchanges. 5.2.4 Signing Considerations Exit ramp placement and location is the primary factor in any signing assessment. The spacing between interchanges is of less importance with regard to signing. 74 Guidelines for Ramp and Interchange Spacing

5.3 RAMP SPACING ASSESSMENTS This section provides guidance to support ramp and interchange spacing decisions and assess their adequacy. The intent is to characterize the geometric design, traffic operations, safety, and signing considerations that influence the activities depicted in the Guidance Framework of Exhibit 5-1. The steps outlined in these Guidelines represent an approach to integrate design, operations, safety, and signing considerations in ramp and interchange spacing decisions. Ramp and interchange spacing design requires an iterative approach to blending geometric design, traffic operations, safety, and signing considerations within a project’s contextual design environment. Each of these four fundamental elements is interrelated and optimizing solutions comes as a byproduct of balancing and integrating the elements. “Ideal” projects result in a balance of the four elements, however, in reality, sometimes projects focus on optimizing the relationship between two or more elements. For example, one project’s conditions may require an extensive effort to balance geometric design and traffic operations considerations; another project may require emphasizing geometric design and signing considerations. Exhibit 5-3 provides graphical representation of the interdependence of the four fundamental elements and highlights examples that focus on the selected specific considerations (geometric design/traffic operations and geometric design/signing). Exhibit 5-3 Ramp and Interchange Considerations Relationships Interchange and ramp designs are complex, and no sequential process is sufficient to address a particular contextual design environment. The user is responsible for adapting and adjusting the approach provided in these Guidelines to meet project-specific needs. The Case Studies consider all of these relationships Spacing Guidance 75

There are many ways professionals can conduct ramp and interchange spacing assessments and numerous ways to develop ramp and interchange concepts. These Guidelines suggest four fundamental, sequential steps and at the same time, note the importance of flexibility in the approach for any particular project need. In addition, users should be aware of the iterative nature of the assessment process as they accomplish the activities outlined in the Interchange Planning and Design Framework of Exhibit 5-1. The four sequential steps address the following: • Geometric Design, • Traffic Operations, • Safety, and • Signing. The fundamentals that should be included in these steps are provided in Chapters 3 and 4. Appendix A contains five project case studies depicting a range of hypothetical projects that apply the concepts presented throughout this Guidelines document. AASHTO guidance has not specifically correlated the relationship between ramp spacing, interchange spacing, and interchange form. Some ramp combinations are likely to occur within the same interchange and this will dictate their spacing. Table 5-7 provides insights into the influencing factors that affect ramp spacing values. Users of these Guidelines should consider these key influencing factors as they assess ramp and interchange geometric design options. Table 5-7 Factors Influencing Minimum Ramp Spacing EN-EX EN-EN EX-EX EX-EN Primary Considerations in Spacing Evaluations Geometry Traffic Operations Safety Traffic Operations Safety Signing Geometry Relationship to Interchange Spacing Greatly influences crossroad (interchange) spacing Single entrance designs may increase spacing to adjacent interchanges Single exit designs may increase spacing to adjacent interchanges Typically none Unless stated otherwise, these Guidelines assume that ramp spacing values are between interchanges rather than within a single interchange. Appendix A includes five case studies that apply the sequential ramp assessment steps to a range of hypothetical project types. 76 Guidelines for Ramp and Interchange Spacing

The following subsections build upon the design and signing information in Chapter 3 and the operations and safety information in Chapter 4. Where possible, quantitative thresholds can aid professionals in integrating geometric design, traffic operations, safety, and signing considerations into ramp and interchange spacing evaluations and decision making. 5.3.1 Geometric Design As presented in Chapters 1, 2, and 3, the ramp and interchange geometric design is based upon factors such as the following: • Traffic volumes, • Interchange Form, • Terrain, and • Agency standards and preferences. These factors greatly influence interchange design from one location to another. Users must consider three-dimensional roadway design relationships to develop appropriate ramp and interchange configurations. These configurations should consider and reflect desired traffic operations. Upon understanding system and service interchange forms and basic ramp design elements (ramp-freeway junctions, ramp proper, ramp terminal intersections), a user should be familiar with the possible applications of other potential interchange elements such as turning roadways, C-D roads, and braided ramps. Understanding and applying three dimensional roadway geometric design principles, approximate dimensions (as presented in Chapter 3) can be used as a starting point in laying out interchanges and the associated ramp components. These dimensions address three dimensional geometric design principles for relatively simplified site constraints and do not consider traffic operations. These dimensions apply to single lane ramps and users can 5.3.1.1 ENTRANCE RAMP FOLLOWED BY EXIT RAMP (EN-EX) Tables 5-8 and 5-9 indicate the potential feasibility of an entry-exit ramp combination based upon the spacing between single lane diamond interchange ramps and partial cloverleaf ramps, respectively. As discussed in Chapter 1, ramp spacing is measured between painted gore stripe tips. Dimensions presented in the “potentially geometrically feasible” range generally correlate to the ability to apply these ramp configurations within one-mile cross street spacing. Approximate ramp design dimensions presented in Chapter 3 reflect simplified site constraints. Users must apply the principles of the Chapter 3 design information to their specific project context. apply these principles to investigate the influence of two lane entrance ramps. Spacing Guidance 77

Table 5-8 Diamond Interchange Ramp Entrance-Exit Ramp Combination Ramp Spacing Dimension Feasibility Less than 1,600 ft Likely Not Geometrically Feasible 1,600 ft to 2,600 ft Potentially Geometrically Feasible Greater than 2,600 ft Likely Geometrically Feasible Table 5-9 Partial Cloverleaf Interchange Entrance-Exit Ramp Combination Ramp Spacing Dimension Feasibility Less than 1,600 ft Likely Not Geometrically Feasible 1,600 ft to 1,800 ft Potentially Geometrically Feasible Greater than 1,800 ft Likely Geometrically Feasible Assumes single entrance and exit design for configurations with the loop in advance or beyond the cross street. 5.3.1.2 ENTRANCE RAMP FOLLOWED BY ENTRANCE RAMP (EN-EN) Table 5-10 indicates the potential feasibility of an entrance-entrance ramp combination. The values are primarily influenced geometrically by the freeway entrance ramp terminal design with the smallest values attributed to sharper convergence angles compared to flatter entrance designs. The smallest theoretical dimension would be when the completed entrance taper coincides with physical gore of the second entrance ramp. Table 5-10 Entrance-Entrance Ramp Combination Ramp Spacing Dimension Feasibility Less than 1,400’ Likely Not Geometrically Feasible 1,400’ to 1,800’ Potentially Geometrically Feasible Greater than 1,800’ Likely Geometrically Feasible 5.3.1.3 EXIT RAMP FOLLOWED BY EXIT RAMP (EX-EX) Table 5-11 indicates the potential feasibility of an exit-exit combination. The values are primarily influenced geometrically by the freeway exit ramp terminal design with the smallest values attributed to sharper divergence angles compared to flatter exit designs. The smallest theoretical dimension would occur when the diverge of the second exit coincides with the physical gore of the upstream exit. Note that minimum sign spacing values of 800 ft between exit-exit combinations would become a critical control if exit-exit spacing values of less than 900 ft are being considered. 78 Guidelines for Ramp and Interchange Spacing

Table 5-11 Exit-Exit Ramp Combination Ramp Spacing Dimension Feasibility Less than 900’ Likely Not Geometrically Feasible 900’ to 1100’ Potentially Geometrically Feasible Greater than 1100’ Likely Geometrically Feasible 5.3.1.4 EXIT RAMP FOLLOWED BY ENTRANCE RAMP (EX-EN) There are two primary scenarios of an exit-entrance combination. The shortest dimension would be that of an exit followed by the entrance for a “button hook” 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. This interchange form is not desirable and this combination is an unlikely configuration. Should this configuration be considered, other operations and interchange and ramp configuration policy or criteria will likely need to be considered. The second scenario would be when an exit ramp and subsequent entrance ramp are servicing grade separated ramps (ramp braids). Based on the concept depicted in Exhibit 3-6 for ramp braid vertical and horizontal relationships, the spacing values are presented in Table 5-12. The minimum values reflect a condition where both ramp profiles are changing. Table 5-12 Exit-Entrance Ramp Combination (Braided Ramps) Ramp Spacing Dimension Feasibility Less than 1700’ Likely Not Geometrically Feasible 1700’ to 2300’ Potentially Geometrically Feasible Greater than 2300’ Likely Geometrically Feasible 5.3.2 Traffic Operations The spacing dimensions presented previously are based on the lengths of the various components of ramps and interchanges. They do not account for traffic volumes and may result in geometrics that do not adequately serve forecast volumes. Fundamental traffic operations and capacity considerations should be considered in the earliest stages of developing interchange and ramp configurations. The following elements should be evaluated and the findings incorporated into geometric layouts. As alternatives are refined, evaluated, and screened, ramp and interchange configurations should be refined, and these elements should be re-evaluated using tools and techniques at increasing levels of detail. Chapter 3 provides an overview of the many ways traffic operations can be evaluated using a variety of techniques at the earliest planning stages through more detailed geometric design evaluations. Spacing Guidance 79

• Mainline Freeway—The overarching design and operational relationships of Section 3.1 (lanes and uniformity) should first be assessed and fundamentals incorporated into the earliest geometric configurations. Interchange and ramp configurations for proposed improvements should be prepared within the objectives of the preserving basic lanes, lane continuity, lane balance, and other lane elements noted in section 3.1.1. Traffic operations for the mainline must address basic segment capacity needs. As noted in Table 4-1, the capacity of a single freeway lane ranges from 2,250 to 2,400 passenger cars per hour. • Ramp Terminal Intersections—Ramp terminal intersection treatments affect capacity and queuing, and ramp terminal intersection evaluations should be conducted early in the evaluation process. Ramp terminal intersection queue lengths will vary based on the lane numbers and arrangements that are provided at that location. Stopping sight distance and deceleration lengths to the back of those queues directly affect ramp horizontal alignments and, ultimately, ramp-freeway junction locations. • Isolated Merge or Diverge—Ramp-freeway junctions should be investigated in isolation to check basic capacity needs and as part of a system to check the impact of close spacing. As noted in Table 4-1, the capacity of a single merging or diverging influence area (ramp plus right two lanes of freeway) ranges from 4,400 to 4,600 passenger cars per hour. • Closely Spaced Merges and Diverges—Research conducted when developing these Guidelines examined the impact of ramp spacing on mainline freeway speed. In general, ramp spacing has the greatest impact when traffic volumes (of the freeway, the ramps, or both) are near but not at capacity. Under low to moderate volume, changes in ramp spacing generally have little effect on freeway operations. At capacity, a freeway will operate poorly regardless of ramp spacing. This general relationship is illustrated in Exhibit 5-4. Specific findings are presented in the following sections and in Appendix B. Appendix A provides a variety of planning level operational analysis tools to assess possible tradeoffs of ramp spacing values 80 Guidelines for Ramp and Interchange Spacing

Exhibit 5-4 Conceptual Effect of Changes in Ramp Spacing on Freeway Speed For entrance-exit ramp combinations without an auxiliary lane, research conducted in developing these Guidelines found that: • Ramp spacing significantly affects mainline segment speed for mainline segments with low entering volumes and high exit-ramp volumes. • Ramp spacing significantly affects mainline segment speed for mainline segments that have moderate and high mainline entering ramp volumes and moderate and high exit-ramp volumes. Closely spaced entry-exit ramps may be designed with or without an auxiliary lane. If an auxiliary lane is present, an HCM weaving analysis is needed to evaluate operations. Any application of auxiliary lanes should include a review of lane balance provisions. Auxiliary lanes provide operational and safety benefits, and the benefit can potentially be maximized by providing lane balance at the down stream exit ramp. Adding an auxiliary lane creates minor speed increases at low mainline and exit volumes. However, the increase becomes significant as traffic volumes increase. Adding an auxiliary lane to a longer ramp spacing generally has less benefit than adding an auxiliary lane to shorter ramp spacing. For entrance-entrance ramp combinations, research conducted in developing these Guidelines found that: • With low to moderate mainline volumes upstream of the first ramp, ramp spacing generally has little effect on mainline speed regardless of ramp volume levels. The operational effects of auxiliary lanes are quantified in Section 4.3.1 and Appendix B. At low to moderate volumes, ramp spacing generally has little effect on freeway operation. Spacing Guidance 81

• Ramp spacing has a significant impact on mainline segment speeds with high mainline volume upstream of the first ramp and moderate to high ramp volumes. 5.3.3 Safety A comprehensive ramp spacing safety assessment should consider: • Safety impacts on the freeway mainline (addressed in this section); • Safety associated with speed-change lane presence and design (can be addressed with HSM or ISAT—see the following discussion); • Safety along the ramp proper (can be addressed with ISAT); • Safety at ramp terminal intersections (can be addressed with ISAT); and • Safety on surrounding highways and streets (capabilities that intertwine travel demand modeling and safety are somewhat limited). Users should access published safety documents and tools to make comprehensive evaluations of freeway and interchange design alternatives to augment the tools developed for these Guidelines. This includes: • Applying the HSM crash modification factors for designing interchanges with crossroads above (versus below) the freeway, speed change lane lengths, and modifying lane arrangements at merge and diverge areas (7). Professionals should also access the qualitative safety discussions to guide decisions regarding ramp type/configuration, right-side versus left-side entrances and exits, interchange spacing, weaving area length, ramp proper alignment and width, and the provision of bicycle and pedestrian facilities at ramp terminals. • Using FHWA’s Interchange Safety and Analysis Tool (ISAT) safety performance functions for predicting crash frequencies along the freeway mainline, at freeway-ramp terminals, at ramp-cross street terminals, and along the ramp proper (32). Like the HSM, ISAT will continue to evolve and expand as supplemental research findings become available over time. Research conducted in developing these Guidelines identified a consistent trend: reductions in ramp spacing are generally associated with an increase in crashes along the freeway mainline (all else, including measures of exposure, being equal). Similar to the previous planning-level information for considering geometric design requirements, users may estimate ramp spacing safety impacts for specific ramp combinations. This information may be used to provide A complete safety assessment of proposed ramps and interchanges should include more than spacing impacts, as discussed in Section 4.5 Reductions in ramp spacing are generally associated with an increase in crashes along the freeway mainline (all else, including measures of exposure, being equal). 82 Guidelines for Ramp and Interchange Spacing

insights into the safety considerations and potential trade offs of alternative ramp and interchange spacing values. The tools do not address rear-end crashes that may occur far upstream of the entrance gore as a result of queue formation during congested conditions. 5.3.3.1 ENTRANCE RAMP FOLLOWED BY EXIT RAMP (EN-EX) The information provided in these Guidelines can be used to conduct a ramp spacing safety assessment at two levels: An early planning level assessment and a planning/preliminary design level assessment. The planning level tool depicting the relationship between EN-EX ramp spacing and relative crash risk is shown in Exhibit 5-5. The relative safety impacts of ramp spacing alternatives can be assessed without gathering data on freeway volumes, ramp volumes, and detailed geometrics. Relative crash risk is measured by the percent difference in crashes, of all types and severities, at some ramp spacing value compared to a ramp spacing of 1,600 ft. The shaded regions of Exhibit 5-5 graphically summarize the following: • Up to 10% more crashes are expected for ramp spacing values between 1,200 and 1,600 ft when compared to the baseline of 1,600 ft; • Between 10-25% more crashes are expected for ramp spacing values between 900 and 1,200 ft when compared to the baseline of 1,600 ft; • More than 25% more crashes are expected when ramp spacing is less than 900 ft when compared to the 1,600 ft baseline; • The incremental safety benefits of ramp spacing values greater than 1,600 ft are relatively minor • Up to 10% less crashes are expected for ramp spacing values between 1,600 and 2,600 ft when compared to the 1,600 ft baseline; • The incremental safety benefit of providing ramp spacing values longer than 2,600 ft are relatively negligible. Spacing Guidance 83

1 Relative crash risk is measured by the percent difference in crashes, of all types and severities, at some ramp spacing value compared to a ramp spacing of 1,600 ft Exhibit 5-5 Preliminary Safety Assessment Tool for Ramp Spacing, Entrance Ramp Followed by Exit Ramp The solid line behind the shaded regions can be used to compare relative crash risk of two specific ramp spacing values. For example, a ramp spacing of 2,000 ft corresponds to a relative crash risk (value on the y-axis) of -5%. A ramp spacing of 1,600 ft corresponds to a relative crash risk of zero percent (the baseline). Therefore, one could expect 0 - (-5) = 5% more crashes for a ramp spacing of 1,600 ft than for a ramp spacing of 2,000 ft. As more detailed preliminary design information is available, users may apply a planning/preliminary design tool to conduct a more detailed safety assessment of ramp spacing than Exhibit 5-5, with explicit consideration of freeway volumes, ramp volumes, and the presence of an auxiliary lane between the entrance ramp and exit ramp. The key variables for this level of safety analysis are defined below and illustrated in Exhibit 5-6: • L = segment length (in miles) defined from the physical gore of the entrance ramp to the physical gore of the exit ramp; • S = ramp spacing (in feet) defined from the painted entrance gore to the painted exit gore; • DADT = the average daily traffic (in vehicles per day) on the freeway mainline upstream of the entrance gore in the analysis direction; The use of Exhibit 5-5 in the context of analyzing impacts of a new interchange on a freeway between two existing interchanges is illustrated in Case Studies 1, 2, 3 and 4. Case Study 3 presents a detailed illustration of the spacing and auxiliary lane interaction 84 Guidelines for Ramp and Interchange Spacing

• ADTEN = the average daily entering traffic (in vehicles per day); • ADTEX = the average daily exiting traffic (in vehicles per day); • AuxLn = a variable indicating whether there is a continuous auxiliary lane between the entrance ramp and exit ramp provided for weaving (1 = auxiliary lane present; 0 = auxiliary lane not present); and • TOTAL = number of crashes (of all types and severities) expected to occur between the physical entrance gore and physical exit gore on the freeway mainline Exhibit 5-6 Key Variables for Planning and Preliminary Design Safety Assessment Professionals may use Equation 5-1, developed for these Guidelines, to estimate the total number of crashes (of all types and severities) that might be expected to occur between the physical entrance gore and physical exit gore on the freeway mainline research with all variables defined previously. ( ) ( ) ( ) ×−×= − AuxLn S ADTADTDADTLTOTAL EXEN 23.0 450 exp107.9 02.018.012.10.16 Equation 5-1 Estimating the total number of crashes between an entrance and exit A complete safety picture requires understanding crash type and severity as discussed in Section 4.5.4. Crash type refers to the manner of vehicle collision. At the highest level, crash types are classified by the number of motor vehicles involved in the crash. Single-vehicle crashes involve only one motor vehicle. Examples include single-vehicle, overturn and single-vehicle, sideswipe, rear-end, head-on, and angle collisions. Once total expected crashes are estimated by Equation 5.1, a professional can estimate the expected percentage of predicted total crashes that will involve more than one vehicle, with the remaining percentage being single- The relationship between ramp spacing and crash type is discussed in Section 4.5.4 and fixed object collisions. Multiple-vehicle crashes involve more than one vehicle. Examples include same-direction-sideswipe, opposite-direction- Spacing Guidance 85

vehicle crashes. Exhibit 5-7 (developed for these Guidelines) graphically summarizes the following: • The expected percentage of total crashes that will involve more than one vehicle, with the remaining percentage being single-vehicle crashes. For example, approximately 66% of crashes are expected to be multiple vehicle collisions when the ramp spacing equals 2,000 ft; approximately 77% are expected to involve more than one vehicle when ramp spacing equals 1,000 ft. • The expected percentage of severe (injury or fatal) crashes. For example, approximately 30% of crashes are expected to result in at least one fatality or injury when the ramp spacing equals 2,000 ft; approximately 26% are expected to be a fatal or injury crash when ramp spacing equals 1,000 ft. Exhibit 5-7 Crash Type and Severity Distributions as a Function of Ramp Spacing Closely spaced EN-EX ramp combinations may be designed with or without an auxiliary lane, and Exhibit 5-5 includes data from both situations. However: • The presence of an auxiliary lane corresponded to approximately 20% fewer expected crashes for any given ramp spacing and projected level of traffic volumes. This overall reduction in crashes is due to reduction in multiple vehicle collisions. Case studies 3, 4, and 5 illustrate the use of Exhibit 5-7. 86 Guidelines for Ramp and Interchange Spacing

• The presence of an auxiliary lane has no effect on single vehicle collisions. The presence of an auxiliary lane was also found to have an equal reduction in injury and non-injury crashes. In the example comparing 2,000 foot and 1,600 foot spacing, the larger expected number of crashes for the 1,600 foot spacing is likely to be offset if an auxiliary lane is provided. The combined applications of equation 5.1 and Exhibit 5-5 to perform safety assessments of ramp spacing alternatives are illustrated in Case Studies 3, 4, and 5. 5.3.3.2 ENTRANCE RAMP FOLLOWED BY ENTRANCE RAMP A planning level tool depicting the relationship between EN-EN spacing and relative crash risk is shown in Exhibit 5-8. The exhibit can be used in the same way as Exhibit 5-5. The shaded regions of Exhibit 5-8 graphically summarize the following relationships between EN-EN spacing and crash frequency: • Up to 10% more crashes are expected for ramp spacing values between 1,100 and 1,400 ft when compared to the baseline of 1,400 ft; • Between 10-25% more crashes are expected for ramp spacing values between 800 and 1,100 ft when compared to the baseline of 1,400 ft; • More than 25% more crashes are expected when ramp spacing is less than 800 ft when compared to the 1,400 ft baseline; • Up to 10% fewer crashes are expected for ramp spacing values between 1,400 and 2,200 ft when compared to the 1,400 ft baseline; • The incremental safety benefits of providing ramp spacing values longer than 2,200 ft are relatively small. The information provided in this section can be used to conduct a ramp spacing assessment for two consecutive entrance ramps. Spacing Guidance 87

1 Relative crash risk is measured by the percent difference in crashes, of all types and severities, at some ramp spacing value compared to a ramp spacing of 1,400 ft Exhibit 5-8 Preliminary Safety Assessment Tool for Ramp Spacing, Entrance Ramp Followed by Entrance Ramp Case Study 2 (Appendix A) applies Exhibit 5-8 in the context of analyzing impacts of a new interchange on a freeway between two existing interchanges, resulting in an EN-EN ramp sequence. A planning/preliminary design tool to conduct a more detailed safety assessment of EN-EN ramp spacing is also available as Equation 5-2. The key variables for this tool are defined below and illustrated in Exhibit 5-9: • L = segment length (in miles) defined from the physical gore of the first (upstream) entrance ramp to the end of the acceleration lane taper of the second (downstream) entrance ramp; • S = ramp spacing (in feet) defined from the painted tip of the first entrance ramp to the painted tip of the second entrance ramp; • DADT = the average daily traffic (in vehicles per day) on the freeway mainline upstream of the first entrance gore in the analysis direction; • ADTEN-1 = the average daily entering traffic (in vehicles per day) from the first entrance ramp; • ADTEN-2 = the average daily entering traffic (in vehicles per day) from the second entrance ramp; and • TOTAL = number of crashes (of all types and severities) (crashes per year) expected to occur between the physical gore of the first 88 Guidelines for Ramp and Interchange Spacing

(upstream) entrance ramp to the end of the acceleration lane taper of the second (downstream) entrance ramp. Exhibit 5-9 Key Variables for Planning and Preliminary Design Safety Assessment: Entrance Ramp followed by Entrance Ramp Equation 5-2, developed for these Guidelines, may be used to estimate the total number of crashes (of all types and severities) expected to occur between the physical gore of the first (upstream) entrance ramp to the end of the acceleration lane taper of the second (downstream) entrance ramp. ( ) ( ) ( )×= −− − S ADTADTDADTLTOTAL ENEN 420 exp100.5 09.02 34.0 1 81.00.15 Equation 5-2 Estimating the total number of crashes between an entrance and entrance The percentage of total crashes expected to be multiple vehicle on these segments changes very little as a function of ramp spacing and is generally between 70% and 80%. The expected percentage of severe (injury or fatal) that for the EN-EX ramp sequence. Therefore, the fatal plus injury curve in Exhibit 5-7 can be used to predict the percentage of the total crashes on the EN-EN segments expected to be severe. 5.3.4 Signing Upon evaluating geometric design, traffic operations, and safety evaluations, users should identify signing needs and placement options. A planning-level sign placement drawing, such as those shown in the Case Studies can help identify whether an interchange concept may require signing in excess of what a driver can process and thus potentially be an infeasible design. In some cases the analysis of signing will need to extend several interchanges upstream and downstream from the interchange in study. As illustrated in Exhibit 5-10, some designs are more likely than others to be impacted by signing considerations. crashes as a function of ramp spacing for the EN-EN sequence is similar to Spacing Guidance 89

Exhibit 5-10 Impact of Signing on Ramp Spacing Decisions Case Study 5 provides an example of how geometric design and signing considerations could ultimately influence ramp configuration project decisions. In that example, because signing needs can not be met, and there are other project alternatives, the configuration that could not be signed was recommended to be dismissed. Section 3.9 addresses signing needs for advance signing, and the number of message units can influence the effectives of communicating guidance and navigation tasks to drivers. Signing needs primarily play two roles in spacing assessments, both of which involve exit ramps only. 5.3.4.1 SPACING BETWEEN SUCCESSIVE EXIT RAMPS Exit ramps should be spaced at least 800 ft apart to satisfy the MUTCD’s recommendation. In most cases, other factors such as interchange form and ramp/gore design will place successive exit ramps more than 800 ft apart. 5.3.4.2 MAXIMUM NUMBER OF EXIT RAMPS ON A FREEWAY SEGMENT As discussed in Section 3.9, the MUTCD recommends that a certain number of advance guide signs be placed prior to an exit and that no more than three sign panels be placed side by side. This effectively creates a limit of three single-exit interchanges per mile. If one of the three interchanges were a double-exit design, it could be possible to sign both exits of the interchange using a single advance guide sign, which would raise the limit to four exit ramps per mile. In most cases, signing needs will not determine ramp spacing requirements, and factors such as geometry and traffic operations will not permit three or four exit ramps per mile. However, there are cases where signing needs are more complex than usual and are more likely to dictate ramp spacing. The thresholds of three or four exit ramps per mile assume that the exits do not require any type of special signing. System interchanges, exits signed with diagrammatic signs, and exits serving a large number of roadways or destinations are examples of situations where three ramps per mile may be Successive exit ramps should be at least 800 ft apart to meet basic signing needs. Case Study 5 illustrates a situation in which signing determines ramp location 90 Guidelines for Ramp and Interchange Spacing

infeasible and a more detailed analysis of sign and message unit requirements should be conducted. Case Study 5 illustrates such a situation. 5.4 SPACING GUIDANCE SUMMARY following: 1. Understand Project Context, 2. Document Existing and Future Conditions, 3. Develop Concept Solutions, 4. Perform Spacing Assessment, and 5. Optimize Project Considerations. Developing concept solutions and performing spacing assessments is an iterative process. In the earliest stages of planning, spacing assessments can be performed with limited data. Interchange spacing assessment tools such as those in Exhibit 5-2 may be appropriate at an initial planning stage. As concepts are refined, more in-depth analyses of spacing should be performed. Due to the wide variety of interchange forms and a multitude of project-specific ramp design features, ramp spacing assessments are more useful than interchange spacing assessments and will play a larger role in determining the adequacy of a ramp or interchange concept. Ramp spacing assessments, discussed in Section 5.3, should include analyzing geometric design, traffic operations, safety, and signing. Such analyses should be performed before the final design stage, as there is little flexibility with spacing at this point. Five scenario-based Case Studies in Appendix A illustrate how to conduct ramp spacing assessments. Evaluating a proposed ramp and interchange concept is an iterative, multiple stage process. As presented in Figure 5-1, the actions in the process are the Spacing Guidance 91