Determining Guidelines for Ramp and Interchange Spacing (2011)

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Chapter 5 Resource Document Revisions

NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing Chapter 5: Resource Document Revisions 5-1 Chapter 5 RESOURCE DOCUMENT REVISIONS For many years, transportation professionals have heavily relied upon three primary resource documents when planning and designing highway facilities: The AASHTO Green Book, the HCM, and the MUTCD. In 2010, a fourth primary resource document joined this list – the Highway Safety Manual (HSM). These documents incorporate the results of hundreds of research projects that have contributed to the knowledge base of the transportation profession. While this research project was being conducted, an effort was underway to update (or publish for the first time) each one of these documents. By the time this project was completely, new editions of the HCM and the MUTCD had been published, the initial edition of the HSM had been published, and content for the next edition of the Green Book was finalized. As a result, it may be many years before any content from this project is incorporated into these major resource documents. 5.1 AASHTO POLICY ON GEOMETRIC DESIGN OF HIGHWAYS AND STREETS (GREEN BOOK) At the time of NCHRP Project 3-88 research, the 2004 Green Book was being revised. It is expected to be finalized and published in 2010. A member of the NCHRP 3-88 Panel was involved with the update to Chapter 10: Grade Separations and Interchanges and coordinated with the project team. No significant changes to this chapter are anticipated in the 2010 edition, although one change will be made to the 2004 version of Exhibit 10-68 (reproduced in this report as Exhibit 2-9). Currently, Exhibit 10-68 notes that the recommended minimum ramp terminal spacing values are measured “between like points, not necessarily ‘physical’ gores.” In the 2010 edition, this note will be changed to indicate that spacing values should be measured between painted tips. This change will make the spacing measurement definition in Exhibit 10-68 consistent with these Guidelines and the 2010 HCM. Based upon the findings of NCHRP Project 3-88, more significant changes to the Green Book are proposed by the NCHRP Project 3-88 team. Changes to interchange spacing and ramp spacing guidance are outlined below in relation to the 2004 edition of the Green Book. The project team recommends that these changes be made to the next edition of the Green Book after 2010, recognizing that significant changes to the 2010 edition are not possible at this point.

Final Report NCHRP 3-88 Chapter 5: Draft Resource Document Revisions Guidelines for Ramp and Interchange Spacing 5-2 5.1.1 Interchange Spacing 5.1.1.1 EXISTING The 2004 Green Book offers the following guidance on interchange spacing: Interchange spacing has a pronounced effect on freeway operations. In areas of concentrated urban development, proper spacing usually is difficult to attain because of traffic demand for frequent access. Minimum spacing of arterial interchanges (distance between intersecting streets with ramps) is determined by weaving volumes, ability to sign, signal progression, and lengths of speed-change lanes. A general rule of thumb for minimum interchange spacing is 1.5 km [1 mi] in urban areas and 3.0 km [2 mi] in rural areas. In urban areas, spacing of less than 1.5 km [1 mi] may be developed by grade-separated ramps or by adding collector-distributor roads. 5.1.1.2 PROPOSED The project team recommends the following changes to the text (changes shown in red italics) and the inclusion of two supporting exhibits: Interchange spacing has a pronounced effect on freeway operations. less of an impact on freeway operations than ramp spacing. Interchange spacing is measured between crossroads as shown in Exhibit 5-1, Exhibit 5-1 Definition of Interchange Spacing In areas of concentrated urban development, proper spacing usually is difficult to attain because of traffic demand for frequent access. Minimum spacing of arterial interchanges (distance between intersecting streets with ramps) is determined by interchange form, lane configuration, weaving volumes, ability to sign, signal progression, and lengths of speed-change lanes. A general rule of thumb for minimum interchange spacing is 1.5 km [1 mi] in urban areas and 3.0 km [2 mi] in rural areas. The feasibility of interchange spacing values near 1.5 km is provided in Exhibit 5-2 for various interchange forms. Feasibility is based upon the resulting ramp spacing.

NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing Chapter 5: Resource Document Revisions 5-3 Assumes single entrance and exit design for configurations with the loop in advance or beyond the cross street. Exhibit 5-2 Interchange Spacing Feasibility In urban areas, sSpacing of less than 1.5 km [1 mi] the values in Exhibit 5-2 may be developed by grade-separatinged 5.1.2 Ramp Spacing ramps or by adding collector- distributor roads. 5.1.2.1 EXISTING The 2004 Green Book offers guidance on ramp spacing: On urban freeways, two or more ramp terminals are often located in close succession. To provide sufficient weaving length and adequate space for signing, a reasonable distance should be provided between successive ramp terminals. Spacing between successive outer ramp terminals is dependent on the classification of the interchanges involved, the function of the ramp pairs (entrance or exit), and weaving potential. The five possible ramp-pair combinations are: (1) an entrance followed by an entrance (EN-EN), (2) an exit followed by an exit (EX-EX), (3) an exit followed by an entrance (EX-EN), (4) an entrance followed by an exit (EN- EX) (weaving), and (5) turning roadways. Exhibit 10-68 presents recommended minimum ramp terminal spacing for the various ramp-pair combinations as they are applicable to interchange classifications. Where an entrance ramp is followed by an exit ramp, the absolute minimum distance between the successive noses is governed by weaving considerations. The spacing policy for EN-EX ramp combinations is not

Final Report NCHRP 3-88 Chapter 5: Draft Resource Document Revisions Guidelines for Ramp and Interchange Spacing 5-4 applicable to cloverleaf loop ramps. For these interchanges, the distance between EN-EX ramp noses is primarily dependent on loop ramp radii and roadway and median widths. A recovery lane beyond the nose of the loop ramp exit is desirable. When the distance between the successive noses is less than 450 m [1,500 ft], the speed-change lanes should be connected to provide an auxiliary lane. This auxiliary lane improves traffic operation over relatively short sections of the freeway route and is not considered an addition to the basic number of lanes. See the section “Auxiliary Lanes” in this chapter for alternate methods of dropping these lanes. 5.1.2.2 PROPOSED The project team recommends replacing the current Green Book text and Exhibit 10-68 with the following (changes shown in red italics): On urban freeways, two or more ramp terminals are often located in close succession. To provide sufficient weaving length and adequate space for signing, a reasonable distance should be provided between successive ramp terminals. Spacing between successive outer ramp terminals is dependent on the classification of the interchanges involved, the function of the ramp pairs (entrance or exit), and weaving potential. Ramp spacing is measured between painted tips, as shown in Exhibit 5-3.

NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing Chapter 5: Resource Document Revisions 5-5 Exhibit 5-3 Definition of Ramp Spacing The five possible ramp-pair combinations are: (1) an entrance followed by an entrance (EN-EN), (2) an exit followed by an exit (EX-EX), (3) an exit followed by an entrance (EX-EN), (4) an entrance followed by an exit (EN- EX) (weaving), and (5) turning roadways. Minimum recommended spacing values for each of these five combinations are provided in the following sections. A range, rather than a single number, is provided in order to account for differences in convergence and divergence angles, taper and parallel designs, and other gore-area elements. Traffic operations needs must also be considered in addition to the geometric minimums presented here. Entrance Ramp Followed by Exit Ramp (EN-EX) Where an entrance ramp is followed by an exit ramp, the absolute minimum distance between the successive noses is governed by weaving considerations. The spacing policy for EN-EX ramp combinations is not applicable to cloverleaf loop ramps. For these interchanges, the distance between EN-EX ramp noses is primarily dependent on loop ramp radii and roadway and median widths. A recovery lane beyond the nose of the loop ramp exit is desirable. When the distance between the successive noses is less than 450 m [1,500 ft], the speed-change lanes should be connected to provide an auxiliary lane. This auxiliary lane improves traffic operation over relatively short sections of the freeway route and is not considered an addition to the basic number of lanes. See the section “Auxiliary Lanes” in this chapter for alternate methods of dropping these lanes. Table 5-1 Diamond Interchange Entrance-Ex it Ramp Combination Ramp Spacing Dimension Feasibility Less than 1600’ Likely Not Geometrically Feasible 1600’ to 2600’ Potentially Geometrically Feasible Greater than 2600’ Likely Geometrically Feasible Table 5-2 Partial Cloverleaf Interchange Entrance-Ex it Ramp Combination Ramp Spacing Dimension Feasibility Less than 1600’ Likely Not Geometrically Feasible 1600’ to 1800’ Potentially Geometrically Feasible Greater than 1800’ Likely Geometrically Feasible Assumes single entrance and exit design for configurations with the loop in advance or beyond the cross street.

Final Report NCHRP 3-88 Chapter 5: Draft Resource Document Revisions Guidelines for Ramp and Interchange Spacing 5-6 Entrance Ramp Followed by Entrance Ramp (EN-EN) Table 5-3 Entrance-Entrance Ramp Combination Ramp Spacing Dimension Feasibility Less than 1400’ Likely Not Geometrically Feasible 1400’ to 1800’ Potentially Geometrically Feasible Greater than 1800’ Likely Geometrically Feasible Exit Ramp Followed by Exit Ramp (EX-EX) Table 5-4 Ex it-Ex it 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 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. The second scenario would be when an exit ramp and subsequent entrance ramp are servicing grade separated ramps (ramp braids). Based on the vertical and horizontal relationships of this configuration, the spacing values in Table 5-5 are recommended. The minimum values reflect a condition where both ramp profiles are changing. Table 5-5 Ex it-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 Turning Roadways A turning roadway is a configuration where the entrance and exit ramps from or to multiple origins or destinations merge or diverge prior to or after exiting a mainline segment. This is a common attribute of “single exit” designs that provide a single exit or entrance that serves multiple destinations or origins. Exhibit 5-2 depicts turning roadways.

NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing Chapter 5: Resource Document Revisions 5-7 Exhibit 5-2 Turning Roadway Examples Table 5-6 Turning Roadways Ramp Spacing Dimension Feasibility Less than 600’ Likely Not Geometrically Feasible 600’ to 800’ Potentially Geometrically Feasible Greater than 800’ Likely Geometrically Feasible 5.2 HIGHWAY CAPACITY MANUAL (HCM) After several years of preparation that occurred concurrent to this project, and update to the 2000 HCM was published in 2010. The NCHRP 3-88 project team reviewed the most recent draft chapters of the 2010 HCM to which ramp and interchange spacing are potentially relevant: • Chapter 10: Freeway Facilities • Chapter 11: Basic Freeway Segments • Chapter 12: Freeway Weaving Segments • Chapter 13: Freeway Merge and Diverge Segments

Final Report NCHRP 3-88 Chapter 5: Draft Resource Document Revisions Guidelines for Ramp and Interchange Spacing 5-8 Chapter 10 provides background information on freeways and does not contain any analytical procedures. Chapter 11 contains procedures for analyzing freeway segments which are outside the immediate vicinity of ramps and interchanges. The project team does not recommend changes to these chapters. Chapter 12 provides a procedure for analyzing weaving segments. As defined in the HCM, a weaving segment must have an auxiliary lane between the entry ramp and the exit ramp that define the segment. The HCM does not provide a procedure for analyzing a similar segment without an auxiliary lane, nor does it provide any information on the applicability of the weaving (with auxiliary lane) procedures to a non-auxiliary lane segment. Simulation modeling by the project team analyzed “weaving” segments with and without an auxiliary lane to investigate the difference in freeway speed associated with each case. The NCHRP 3-88 project team recommends that research efforts in support of the next edition of the HCM (after 2010) consider a means of analyzing “weaving” sections without auxiliary lanes. This could be in the form of an entirely separate methodology or in the form of a factor that adjusts the weaving methodology for non-auxiliary lane designs. The findings of NCHRP 3-88 efforts could play a role in considering and developing such a methodology. Chapter 13 provides a procedure for analyzing the “influence area” on a two-, three-, or four-lane freeway associated with a single merging ramp or a single diverging ramp. Only on three-lane freeways does the methodology take into account the presence of adjacent ramps. However, simulation models conducted for this project have identified an impact on the operating speed of a four-lane freeway due to ramp spacing. Based upon this finding, the project team suggests this impact be more thoroughly investigated with additional field data as part of a project exclusively focused on traffic operations. The results of such a project could lead to updates to the HCM (after 2010) that could be used to quantify the impact of adjacent ramps on merges and diverges on four-lane freeways. 5.3 MANUAL OF UNIFORM TRAFFIC CONTROL DEVICES (MUTCD) In December 2009, FHWA issued a new (2009) edition of the MUTCD. Information potentially relevant to this project is primarily found in Chapter 2E: Guide Signs – Freeways and Expressways. Chapter 2E recommends limits on the number of signs that should be located at a single point, the distance between signs, and the distance in which signs should be located prior to an exit. These guidelines effectively limit the number of exits that can be placed within a short distance of each other and still be adequately

NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing Chapter 5: Resource Document Revisions 5-9 and properly signed. Human factors and other considerations that form the basis of this guidance were not explicitly investigated as part of NCHRP 3- 88. The project team does not recommend any changes to the MUTCD. 5.4 HIGHWAY SAFETY MANUAL (HSM) The first edition of the HSM was published in June 2010. The HSM contains procedures for assessing the safety performance of a number of different roadway types and components. Interchange-related information is contained within Chapter 15. That chapter presents accident modification factors (AMFs) for design elements, traffic control and operational elements, pedestrian- and bicycle-related elements, and other elements associated with interchanges. 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. Chapter 15 provides a general statement that decreases in interchange spacing appears to increase crashes, but the magnitude of the effect on crash frequency is uncertain. The AMFs and the safety performance functions developed in this project could form the basis of a future HSM procedure that quantifies the impact of interchange and ramp spacing on crash frequency.