Guidelines for Ramp and Interchange Spacing (2011)

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1.1 RAMP AND INTERCHANGE SPACING DEFINITIONS The terms “ramp” spacing and “interchange” spacing are sometimes used interchangeably when, in fact, they differ considerably. Ramp and interchange spacing dimensions are the result of completely different measurements. When applied to ramp and interchange configuration design concepts and considering traffic operations, safety, and signing there is relatively little correlation between “ramp” and “interchange” spacing. Interchanges and their historic rule-of-thumb “one-mile” spacing in urban areas are ultimately a byproduct of the traditional spacing of urban street networks. The networks and their grid vary, but it is relatively common to have major streets and roadways set upon a one-mile grid, with minor streets placed uniformly at values of 8 to 10 streets per mile. During the early days of freeway and interchange planning, the one-mile spacing in urban areas was a result of balancing total system travel demand. In major cities, early traffic models and studies showed that interchanges placed one mile apart balanced traffic flow on the arterials. Spacing values greater than one mile resulted in overly congested conditions on those arterials that interchanged with the freeway (1, 2, 3). Ramp spacing values are primarily a byproduct of individual ramp operational and design requirements and elements. The ramp spacing dimensions between interchanges are fundamentally what remains after combining the individual ramp components from a cross street to the freeway mainline (in the case of an onramp) or the freeway mainline to the cross street (in the case of an offramp). Properly designed ramp elements account for: three-dimensional roadway design needs; appropriate horizontal alignment that facilitates appropriate speed change; appropriate vertical alignment that provides roadway profiles that facilitate grade changes and sight distance needs; and cross-section design considerations that reflect cut and fill slopes or retained earth construction costs within the ramp footprint. Operational considerations of an onramp include providing appropriate speed-change characteristics between interchanging facilities or the need to meet operational demand. For example, a cross-street double left turn feeding a diamond interchange onramp requires two receiving lanes for some distance before a lane is dropped at an appropriate rate to create a single-lane entry. On exit ramps, the ramp length should adequately serve the anticipated queue storage at the ramp terminal intersection and provide adequate sight distance and deceleration lengths to the back of that queue. The exit ramp should have sufficient tangent length or transition curve beyond the physical gore to meet deceleration requirements for the controlling curve just as the Chapter 1 Introduction 3

entrance ramp should have sufficient tangent length (or transition curve) to facilitate acceleration after the controlling entry curve. The appropriate consideration of design and operational details has a great influence on the driving characteristics and performance of the ramps. Ramp design, in turn, directly influences ramp spacing values. These spacing values influence mainline operations and safety and solidify the need for an integrated and dynamic approach for developing ramp and interchange configurations and resultant spacing values. Attaining ramp characteristics appropriate for a given project context helps establish the ramp terminal locations on the highway and, therefore, the spacing values between ramps. Considering sign placement and attaining appropriate stopping and decision sight distances can influence exit ramp terminal locations. “Interchange” spacing merely provides the general framework of the dimension between freeway cross-street centerlines. Ramp spacing is derived after the sequential ramps are appropriately configured to meet geometric design, traffic operational, signing, and safety needs. But from where are ramp spacing values measured? The American Association of State Highway and Transportation Officials (AASHTO) Policy on Geometric Design of Highways and Streets (Green Book) provides guidance on ramp spacing values (4). The Green Book dimensions had been historically noted to be from “like points.” This means from gore to gore or from painted tip to painted tip. Yet each of these ramp design event points may yield considerable differences between the associated dimensions and, therefore, predicted operations. Further, each agency was able to choose to measure their respective “like points” differently than another. Therefore, it was possible that two adjacent states may have evaluated and considered ramp spacing values differently for the same interstate freeway that passes within their respective states. These Guidelines define a new vernacular for the profession. In developing these Guidelines, the authors have reviewed and considered ramp and interchange spacing values and measurements from seminal documents such as the Green Book, the Highway Capacity Manual (HCM), and the Manual on Uniform Traffic Control Devices (MUTCD) (4, 5, 6). These Guidelines have created the following definitions for interchange and ramp spacing: Interchange Spacing: “The distance measured between the respective centerlines of freeway cross streets that include ramps to or from that freeway” (see Exhibit 1-1). Design configurations based on capacity and operational needs are the largest single determinant of ramp length. 4 Guidelines for Ramp and Interchange Spacing

Exhibit 1-1 Definition of Interchange Spacing Ramp Spacing: “The distance between the tips of the actual or theoretical convergence of the painted gore stripes (painted tips)” (Exhibit 1-2). Exhibit 1-2 Definition of Ramp Spacing 1.2 INTENDED USERS These Guidelines are intended to be a useful resource that can be applied to planning, operational, feasibility, and design studies of freeway, highway and interchange facilities. They are intended for individuals who are considering new ramps or interchanges on existing or new freeways and highways, or are considering modifications to existing ramps and interchanges. These Guidelines are meant to be used by engineers, planners, students, researchers, and policymakers. 1.3 SCOPE OF GUIDE These Guidelines focus on fundamental ramp and interchange spacing principles for general-purpose, uninterrupted-flow facilities. The Guidelines were developed primarily with freeway (fully controlled access) facilities in mind. However, the same concepts and principles presented in this document could also be applied to partially controlled access highways and expressways that have a mixture of interchanges and at-grade intersections. For simplicity, “freeway” is used in this document when referring to a facility with interchanges. These Guidelines present considerations about elements that influence ramp and interchange spacing decisions to help users understand planning, operational, signing, safety, and design tradeoffs in making informed ramp and interchange spacing decisions. These Guidelines are not rigid standards, and users are responsible for applying professional judgment in making appropriate planning and design decisions for their unique contextual environment. Introduction 5

These Guidelines do not explicitly or comprehensively address the following topics. However, many of the principles and fundamental considerations presented in these Guidelines may be applicable and useful to users who are considering the following topics: • High-Occupancy Vehicle applications, • Weigh stations, • Rest areas, • Collector-distributor roadways, • Turning roadways, • Loop ramp design, and • Spacing between ramps/interchanges and at-grade intersections on non-freeway highways. These Guidelines will provide the most value in the planning and pre-design stages of a project when the most opportunity exists for investigating ramp and interchange configuration and spacing options. The greatest flexibility to evaluate design, operational, safety, and potential tradeoffs occurs in the early stages of project development. The guidance this document provides is based on design, operational, and safety principles that should be carried through planning, design, construction, and maintenance and can be reviewed at any project stage. 1.4 RELATIONSHIP TO RECOMMENDED RESOURCE DOCUMENTS These Guidelines complement ramp and interchange spacing principles and objectives that are contained in well-known, established resource documents. The following resource documents should be used when evaluating ramp and interchange spacing in addition to and in conjunction with these Guidelines: • A Policy on Geometric Design of Highways and Streets (AASHTO Green Book) (4); • Highway Capacity Manual (HCM) (5); • Manual on Uniform Traffic Control Devices (MUTCD) (6); and • Highway Safety Manual (HSM) (7). In addition, many of the freeway and interchange planning and design principles provided in these Guidelines are documented in the ITE Freeway and Interchange Geometric Design Handbook (8). As projects move further into the preliminary and final design stages, the flexibility to make meaningful adjustments in ramp and interchange spacing values diminishes greatly. 6 Guidelines for Ramp and Interchange Spacing