Determining Guidelines for Ramp and Interchange Spacing (2011)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Chapter 1 Introduction

NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing Chapter 1: Introduction 1-1 Chapter 1 INTRODUCTION The purpose of NCHRP Project 3-88 was to investigate the impact of ramp and interchange spacing on traffic operations and safety and to develop a set of guidelines to help users consider ramp and interchange spacing needs when planning and assessing new or reconstructed freeway facilities. The project team used simulation models, calibrated with field data, to assess the impact of ramp spacing on freeway speed. The project team studied two combinations of ramps: an entrance ramp followed by an exit ramp (with and without an auxiliary lane) and an entrance ramp followed by an entrance ramp. The project team constructed a crash database to assess the impact of ramp spacing on crash frequency. The project safety models also addressed crash type and crash severity. Research activates were primarily focused on ramps, not interchanges, because the wide variety of interchange forms and ramp designs in existence can greatly impact ramp spacing dimensions at any given interchange spacing dimension. Research activities and the resulting guidelines were focused on ramp- freeway junctions and the effects of the spacing between ramps on the performance of the freeway. Ramp design and performance of ramp-terminal intersections were outside the scope of this project and was not directly addressed in research conducted as part of this project or within the Guidelines. 1.1 PURPOSE OF THIS FINAL REPORT This Final Report documents the research effort including the literature review, research methodology, research findings, Guidelines development, recommended changes to the major resource documents, and recommendations for future research. The Guidelines themselves are being published separately from this Final Report. A key finding of the literature review that influenced subsequent project activities is that the origins of existing ramp and interchange spacing guidance, some of which date back to the early days of freeway building in America, are somewhat uncertain yet have been incorporated into numerous policy documents and have become standards in some cases. The project Guidelines avoid a one-size-fits-all approach and allow ramp and interchange spacing dimensions to be customized for a particular project condition. 1.2 SUMMARY OF PHASE I RESEARCH ACTIVITIES The project team conducted extensive information gathering at the onset of the project. In addition to a literature search, the team conducted a focus group meeting with practitioners from across the country and the wealth of

Final Report NCHRP 3-88 Chapter 1: Introduction Guidelines for Ramp and Interchange Spacing 1-2 experience from the project’s panel was accessed. Information gathering activities are fully discussed in Chapter 2 of this report. The literature search was conducted by searching online databases such as the Transportation Research Information Service (TRIS), and the Institute of Transportation Engineers (ITE) Journal archive. Additionally, some team members had extensive libraries of relevant literature. The literature search included reviewing the current editions of the AASHTO Policy on Geometric Design of Highways and Streets (Green Book), (HCM), and Manual of Uniform Traffic Control Devices (MUTCD) as well as all past editions of the HCM and AASHTO Policy (Green Book, Blue Book, and Red Book). The project team identified and reviewed state-level documents and policies by searching state DOT websites. Many of the documents reviewed were several decades old and dated to a time when construction of new freeways in America was more common than it is today. A focus group meeting, held with designers, planners, and operators of freeways, helped to identify concerns or needs in the current practice of ramp and interchange spacing. The issues raised by the participants ultimately helped the project team craft the Guidelines to be more useful to practitioners. The project team sought similar feedback from the Panel, who provided meaningful contributions to improve the early draft Guidelines. The project team also developed the Phase II work plan, which consisted of an operations and a safety component, during Phase I. The project team presented the work plan to the Panel as part of Interim Report #1 and at the panel meeting held at TRB headquarters at the conclusion of the Phase I. The Panel reviewed and approved the work plan. The project team also drafted an annotated outline of the Guidelines during Phase I and presented it to the Panel as part of Interim Report #1. This allowed the project team to receive valuable feedback on the Guidelines content before drafting a complete version of the document during Phase II 1.3 SUMMARY OF PHASE I I RESEARCH ACTIVITIES Phase II activities consisted of execution of the work plan, producing of the Guidelines, and the drafting of recommended changes to major resource documents. The work plan results, a draft of the Guidelines, draft recommendations of resource document changes were presented to the Panel as part of Interim Report #2. After receiving comments from the Panel, the project team produced the two final deliverables of NCHRP Project 3-88: the Guidelines document and this Final Research Report. 1.3.1 Traffic Operations Work Plan The traffic operations work plan consisted of several elements: • Analysing datasets from previous NCHRP projects to supplement the limited field data collected for this project

NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing Chapter 1: Introduction 1-3 o Project 3-37: Capacity of Ramp-Freeway Junctions o Project 3-75: Analysis of Freeway Weaving Sections o Project 3-92: Production of the 2010 Highway Capacity Manual • Reviewing the HCM to identify any minimum ramp or interchange spacing values or other thresholds dictated by traffic operations considerations. • Collecting field data and conducting simulation modelling The project team’s review of data from previous NCHRP projects revealed little about the impact of ramp spacing or interchange spacing on mainline freeway speed. It did, however, help to reaffirm the project team’s assertion that ramp spacing has a greater effect on traffic operation and interchange spacing. Reviewing the HCM produced planning-level guidance, although only for a fairly specific situation: identifying minimum spacing values needed to achieve a desired level of service between and entry ramp and an exit ramp on six-lane freeways. Collecting field data and conducting simulation modeling formed the largest component of the traffic operations work plan. Collected data included lane- by-lane speed and volume data at an entry-exit (without auxiliary lane) ramp combination and an entry-entry ramp combination in Phoenix, Arizona. The data was collected using side-mounted digital wave radar and video cameras. Data collection site features included modern design elements, variations in traffic volume, vantage points for cameras, and the availability of supplemental data from Arizona Department of Transportation sensors. Data at each site was collected from 2 p.m. to 6 p.m. to capture off-peak and peak conditions. The project team constructed a calibrated VISSIM mode of each data collection site. The team then varied traffic volume and the distance between the ramps to assess the impact of ramp spacing on mainline freeway operating speed. The project team modeled combinations of the following scenarios: • Mainline freeway volumes of 1250, 1500, and 1750 vehicles per hour per lane (vphpl) • Ramp volumes of 750, 1250, and 1750 vehicles per hour • Short (700’ for entry-entry and 1000’ for entry-exit) and long (2500’ for both ramp combinations) ramp spacing dimensions The models provided speed estimates at five different locations: at the painted merging and diverging tips and at three locations in between. Model results of the entry-entry ramp combination indicated that ramp spacing has an impact on mainline freeway speeds except at high (1750 vphpl) mainline volume. Model results of the entry-exit ramp combination

Final Report NCHRP 3-88 Chapter 1: Introduction Guidelines for Ramp and Interchange Spacing 1-4 indicated that ramp spacing has an impact on mainline freeway speeds except at moderate to high (1500+ vphpl) mainline volume. For the entry-exit ramp combination, the project team conducted a second set of model runs with an auxiliary lane between the ramps. Model results indicated that the benefits (in terms of mainline freeway speed) of an auxiliary lane were greatest with shorter ramp spacing and higher mainline and exiting volume. 1.3.2 Safety Work Plan The safety work plan focused on examining the impact of ramp spacing on crash frequency, type, and severity. This analysis was performed by examining 650 directional miles of freeway in Washington State using crash data from the Highway Safety Information Service (HSIS), interchange diagrams, and aerial photography. The project team concluded that that many previous studies of ramp spacing were limited by the accuracy and quality of the datasets. For this project, the project team placed an emphasis on including detailed geometric and volume data for all interchanges. The project team initially sought to develop safety performance functions and accident modification factors for all ramp combinations, but ultimately developed them for only entry-exit ramps and entry-entry ramps. The sample size of exit-exit ramps was not sufficient for performing analysis. Exit- entrance ramps were generally part of the same interchange, and the project team concluded there was limited value in analyzing such ramps. Analysis results for entry-exit ramp combinations found that ramp spacing has a negligible impact on crash frequency at spacing values greater than 2600 feet. Below this dimension, crash frequency increases with increasing sensitivity as ramp spacing decreases. Crash severity was also found to increase as ramp spacing decreased. Introducing an auxiliary lane between the ramps was found to reduce crashes by approximately 20 percent. Analysis results for entry-entry ramps produced similar findings. Ramp spacing was found to have a negligible impact on crash frequency at spacing values greater than 2200 feet. As ramp spacing decreased from this value, increases in total and severe crashes occurred. For entry-entry analysis, the team supplemented Washington State data with California data to increase the sample size. 1.3.3 Guidelines The Guidelines, published separately from this document, incorporate the findings of the information gathering activities in Phase I as well as work plan activities in Phase II. The Guidelines are intended to assist users as they consider new or modified ramps and interchanges. The following summarizes the general content of each Guidelines chapter: • Chapter 1 defines ramp and interchange spacing

NCHRP 3-88 Final Report Guidelines for Ramp and Interchange Spacing Chapter 1: Introduction 1-5 • Chapter 2 presents relevant federal, state, and local policies. • Chapter 3 discusses elements of geometric design and freeway signing relevant to ramp and interchange spacing • Chapter 4 discusses traffic operations and safety considerations for ramp and interchange spacing • Chapter 5 presents ranges of recommended ramp and interchange spacing dimensions • Chapter 6 presents five scenario-based case studies that apply the principles presented in the preceding chapters • The Appendix presents tools and findings developed as part of the operations work plan 1.3.4 Resource Document Revisions Based upon the findings of this project, the project team revisited relevant sections of the AASHTO Green Book, HCM, MUTCD, and HSM to determine if changes should be made. The project team concludes that no changes should be made the HCM or the MUTCD. The HSM does not contain quantitative information related to ramp and interchange spacing. However, the models and findings of NCHRP 3-88 will be directly relevant to future updates of the HSM. The safety performance functions and crash modification factors developed here could be combined with additional research to form the basis of a greatly expanded HSM chapter on the subject. The project team recommends changes to the interchange spacing and ramp spacing guidance in Chapter 10 of the 2004 AASHTO Green Book. For interchange spacing, the team recommends including a table developed as part of NCHRP 3-88. The table provides ranges of interchange spacing dimensions that are “likely not geometrically feasible”, “potentially geometrically feasible”, “likely geometrically feasible” for different interchange forms. For ramp spacing, the team recommends replacing Exhibit 10-68 with a ranges of ramp spacing dimensions that are “likely not geometrically feasible”, “potentially geometrically feasible”, “likely geometrically feasible” for each ramp combination.