Guidelines for Ramp and Interchange Spacing (2011)

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Chapter 2 Ramp and Interchange Spacing Overview This chapter provides an overview of the project development process, policy considerations, and an overview of the relationship of major published resource documents associated with ramps and interchanges. 2.1 INTRODUCTION 2.1.1 Project Development Stage There are numerous definitions of the project development stage and, regardless of the specific terms used, they generally represent an advancing sequence of activities that originate in solution concept planning and, if appropriate, culminate in a project’s implementation. Ramp and interchange spacing evaluations vary depending on the stage of the project development process. Each project stage can affect how each of the policy and technical considerations is assessed. During any project development stage, the operational, design, safety, human factors, and signing controls should be considered to make informed decisions about ramp and interchange spacing dimensions. Evaluations should also consider the design, operations, and safety tradeoffs for a particular project need. The level of analysis should be commensurate with the respective project development stage. And while early project planning activities may not typically require the same level of analysis or detailed evaluation of a later preliminary design stage, each professional should be prepared to provide a level of analysis and level of detail that is needed to support project decision making. For the sake of discussion, these Guidelines have simplified the project development process to focus on the following three stages: • Planning—This represents the earliest stages of project development when project issues are being identified and solutions concepts are being considered and evaluated. Ramp and interchange spacing values are influenced by fundamental considerations of interchange type and configuration. In retrofit situations, solutions may be greatly influenced by the constraints of the unique context of that location. At this stage, ramp and interchange spacing values are influenced by the same factors and considerations of later design stages, albeit using planning-level evaluation information. • Preliminary Design—This stage provides the balance of flexibility and design detail. Typically, a wide range of concepts and alternatives may be considered. As concepts are screened and refined, increasing For new construction, this stage may provide the most flexibility to consider and apply solutions that optimize ramp and interchange spacing in relation to design, operations, signing, and safety considerations. 7

detail is available for design, operations, safety, and signing evaluations within a project’s contextual environment. Professionals assess the solution concepts from the planning stage and quantitatively evaluate each solution using design and operations tools and techniques. Traffic operations and three-dimensional roadway design input is available to conduct analyses, and yet there is flexibility to select and refine alternative solutions in balance with the project’s contextual environment. • Final Design—There is limited flexibility in modifying solutions in a meaningful way by this project stage. A single alternative has been selected and the primary emphasis is developing design and construction documents for the project’s implementation. During this stage there may be minor design or operational adjustments; however, there is fundamentally little flexibility to positively affect ramp and interchange spacing values. 2.1.2 Common Scenarios No project is “typical,” and each has its own unique opportunities and challenges. Ramp and interchange spacing evaluations and recommendations should be predicated on that project’s contextual design environment. However, there are likely a general range of scenarios that may be common to many conditions faced by the user. Considering the variety of possible common scenarios provides insights about the flexibility or constraints of that scenario that might be applied to other projects within a similar scenario. For example, new interchange projects on new facilities may generally provide more flexibility in the types of solutions that optimize ramp and interchange spacing values. By contrast, in a complex urban environment, a retrofit project of an existing interchange may have less overall flexibility in the solutions under consideration. Evaluations in these scenarios may be focused on balancing project tradeoffs to optimize the design variables. For example, maximizing ramp spacing dimensions may be secondary to lengthening an exit ramp to minimize queue spillback on the mainline from the ramp terminal intersection. These Guidelines provide a sequential approach to considering the design aspects that influence ramp and interchange spacing and provide guidance in evaluating a range of considerations of the safety and operational performance of the possible solutions. Case Studies included in Appendix A present examples of how to consider the adequacy of ramp and interchange spacing values in different scenarios. Common ramp and interchange spacing scenarios include the following: • New interchange on a new facility; Case Studies in Appendix A illustrate spacing assessments in different environments. 8 Guidelines for Ramp and Interchange Spacing

• New interchange on an existing facility (illustrated in Case Studies 2, 3, and 5); • Extensive modifications to an existing interchange or interchange system on an existing corridor; • Retrofits to a partial-access controlled facility to convert an at-grade intersection to an interchange (illustrated in Case Study 1); and, • Retrofits to an existing corridor that may include partially or completely removing an existing interchange (illustrated in Case Study 4). 2.2 POLICY CONSIDERATIONS Designing, operating, and managing a roadway (including interchanges and ramps) should align with the appropriate jurisdictional policies associated with that facility. The facility location and type (Interstate, freeway, highway, and other managed access facilities) can often dictate the appropriate spacing guidelines, design parameters, and technical considerations that should be applied. 2.2.1 Interstate Freeways Interstate freeways are intended to provide the highest service levels in terms of mobility and safety. Interstate freeway access control is essential to preserving the integrity of the overall system. Therefore, the FHWA must grant approval for any new or revised access point on an Interstate freeway. The FHWA policy criteria for new or revised access is described in the August 27, 2009 edition of the Federal Register (Volume 74, No. 165) (9). This FHWA policy document describes the following eight points FHWA considers in granting an Interstate access: 1. The existing system is incapable of accommodating desired access or traffic demands; 2. All reasonable alternatives to a new interchange have been considered, including transportation system management; 3. The proposal does not have an adverse safety or operational impact on the freeway; 4. A full interchange at a public road is provided; 5. The proposal is consistent with transportation and land use plans; 6. A comprehensive interstate network study is prepared; 7. There is coordination with transportation system improvements; and, FHWA must approve new interchanges or changes of access from existing interchanges on the Interstate Highway System. Ramp and Interchange Spacing Overview 9

8. The request is considered as an alternative in environmental evaluations. Some of these points consider evaluating non-interchange alternatives to achieve transportation objectives. Others necessitate concept development at a level sufficient to appropriately analyze design tradeoffs of potential new or modified interstate access. These Guidelines support efforts to consider ramp and interchange spacing when evaluating new or revised interstate accesses. 2.2.2 Non-Interstate Freeways Non-interstate freeways are managed primarily by state and local highway agencies and toll authorities. Many state departments of transportation require some form of “interchange justification report” (IJR) or “interchange modification report” (IMR) for requests made for new or modified interchanges on their network. Highway agencies sometimes adopt the FHWA policy criteria or incorporate elements of the criteria in their own policy or procedural documents to adapt the guidelines for specific state regulations. The primary interest of transportation agencies remains consistent with the fundamental considerations of the FHWA access point policy: to objectively evaluate sound technical information about the design, operational, and safety tradeoffs of proposed changes to the controlled access highway. Policies from two transportation agencies are presented in the following sections. The considerations by these agencies may provide helpful insights to other users who are conducting ramp and interchange evaluations. The outcomes of freeway and interchange planning considerations will influence or be influenced by ramp spacing values. Florida The Florida Department of Transportation (FDOT) requires that IJRs or IMRs be prepared for most new or modified interchanges on existing limited access, non-interstate facilities. To assist with this process, FDOT has published the Interchange Handbook (10). The need for an IJR or IMR is made by considering the following criteria: • The need was previously defined by the (Florida Intrastate Highway System) planning process, master plan, and/or traffic or safety report; • The FHWA interchange modification criteria (contained in the FDOT Interchange Handbook); • The complexity of the proposal and potential impact on adjacent interchanges (spacing, operational overlap, change in traffic patterns); Florida is an example of one state that has a formal approval process for new interchanges. 10 Guidelines for Ramp and Interchange Spacing

• The potential impact on the operation and safety of the mainline (change in level of service, merge, diverge, weaving impacts, need for auxiliary lanes); • The facility jurisdiction (turnpike, FDOT, local expressway authority); • Consistency with local government transportation and land-use plans; and, • Known policy, public, or environmental issues that could affect approval of the Interchange Proposal. Once an IJR or IMR is submitted, the following criteria are considered by FDOT: • Is the analysis and documentation complete, accurate, sufficient, and consistent with the interchange process? If not, does FDOT concur with any deviations? • Is the need for the interchange fully justified and in the best interest of the public? • Does the proposal meet the eight FHWA policy criteria? • Does the proposal impact the operation and safety of the mainline, adjacent interchanges or the surrounding street network and, if so, are the impacts properly mitigated? • Has an Arterial Access Management Plan been developed and agreed to (where required)? • Are the final funding commitments consistent with the proposed opening and interim and design years, and are they in place? • Are all exceptions to policies and standards approved? • Is the proposal consistent with local government and MPO land-use and transportation plans? • Is the proposal consistent with the [Florida Intrastate Highway System] Plan? FDOT provides guidance to maximize safety and reduce conflicts at entrances and exits by doing the following: • Spreading and clarifying decision points; • Creating uniformity in design and operations; and, • Creating clear and simplified signing. The handbook notes that new interchanges should be considered only after improvements to adjacent interchanges and the arterial system have been Ramp and Interchange Spacing Overview 11

considered, as well as TSM and alternative travel modes. The handbook also notes that in “rare circumstances” travel demand will not be the primary justification for an interchange. Minneapolis-St. Paul, Minnesota In the Minneapolis-St. Paul metropolitan area, new interchanges and major modifications to existing interchanges must be approved by the Metropolitan Council, which is the local metropolitan planning organization. This approval process applies to all existing freeways in the metropolitan area regardless of which agency operates them. In some cases, interchanges on facilities that are not full freeways must also be approved. The following criteria must be met for a new or modified interchange to be approved (11): 1. Additional interchange capacity should support the Metropolitan Council’s transportation plans. 2. The need for capacity or safety improvements must be documented. 3. Interchanges should only connect to metropolitan highways, minor arterials, or collectors. 4. New or expanded interchanges are not to be provided as a convenience for short trips, to compensate for the lack of an adequate minor arterial and collector system, or to compensate for deficient minor arterials and collectors. 5. The operational integrity of mainlines and weaving sections must be maintained, and 6. Interchanges should be spaced a minimum of one mile center-to- center. If a spacing less than this is determined to be appropriate, safe operation of the mainline must be preserved. The Metropolitan Council also provides design criteria for ramps and interchanges. 1. Whenever possible, standard ramp and interchange configurations should be used in design. 2. Interchange ramp configuration and design should be based on traffic forecasts. 3. Traffic backups resulting from interchange ramp designs must occur on cross streets and frontage roads rather than on the mainline. 4. Selected collector and minor arterial roadways connecting with the proposed interchange must be adequate for the anticipated volumes on the interchange. 5. Ramp configurations must be capable of being signed for safe and expeditious movement prior to construction approval. 12 Guidelines for Ramp and Interchange Spacing

6. Interchange ramp configuration and design should provide for preferential treatment of transit and rideshare vehicles. 7. Cross-street improvements, if needed, should be coordinated with interchange construction. 2.3 MAJOR PUBLISHED RESOURCE DOCUMENTATION A ramp and interchange spacing evaluation typically requires applying operational, design, signing, and safety guidance provided in the Highway Capacity Manual (HCM), A Policy on Geometric Design of Highways and Streets (Green Book), the Manual on Uniform Traffic Control Devices (MUTCD), and the Highway Safety Manual (HSM). Since 1984, the AASHTO Green Book has provided a general rule of thumb for minimum interchange spacing and values for minimum ramp terminal spacing. These Guidelines provide additional information and considerations to accompany the Green Book’s guidance, and underscore the relationship between traffic volumes and preferred minimum ramp spacing dimensions. In short, these Guidelines emphasize the importance of integrating traffic operations, design, safety, and signing when considering decisions that influence ramp and interchange spacing decisions (4). Among the vast array of traffic control guidance, the HCM provides analysis procedures for weaving sections and ramp-freeway junctions. These Guidelines include planning-level advice on minimum ramp spacing dimensions based on HCM procedures (5). The MUTCD specifies how many advance guide signs should be placed prior to an exit and how far in advance of an exit they should be placed. The MUTCD and the Texas Freeway Signing Handbook provide guidance on how many signs should be placed at the same point (6, 12). The HSM does not yet provide quantitative information for many of the elements associated with interchanges and there is no quantitative information regarding the effect of interchange spacing. Future editions of the HSM will likely provide additional information on topic areas such as freeways and interchanges. The research conducted to develop these Guidelines has resulted in safety prediction aids that can support ramp and interchange spacing evaluations consistent with the intent and principles of the HSM (7). The Green Book provides guidance on minimum interchange and ramp spacing values that is discussed in Chapter 3. The HCM provides operational analysis procedures for various interchange elements that is discussed in Chapter 4. The MUTCD provides signing recommendations that are relevant to exit ramp spacing discussed in Chapter 3. The HSM has limited interchange-related information; safety guidance in these Guidelines is discussed in Chapter 4. Ramp and Interchange Spacing Overview 13