Modeling Roundabouts : Lessons Learned in Idaho

Modern roundabouts are a relatively new intersection treatment in the United States and particularly in Idaho. Due to a lack of data and the complex geometry of modern roundabouts, calibrating and validating models to evaluate capacity at roundabouts has proven difficult. Several choices exist to perform such analysis ranging from empirical models like RODEL and the Highway Capacity Manual, to analytical models such as aaSIDRA, to micro-simulation models including VISSIM and Paramics. Each method and software has its advantages and disadvantages, as well as its proponents and critics. This paper presents a summary of lessons learned over the course of the Ada County Roundabout Study, which consisted of the development of the Ada County Highway District Roundabout Application Guidelines, and the Amity Road Corridor Study. These projects involved the analysis of roundabouts using a gap acceptance-based micro-simulation model (VISSIM) and an empirical-based model (RODEL), as well as comparing roundabouts to signalized intersections. These lessons are fairly generic and can be applied to modeling roundabouts as a whole. They include calibration: the importance of knowing how empirical models are calibrated and issues with calibrating and validating gap-acceptance based models; and roundabouts vs. signalized intersection comparisons: why and why not to make these comparisons and how it can be done. INTRODUCTION In recent years, the authors have been involved with several roundabout design, planning, and design policy development projects. Inevitably the question arises concerning the capacity of roundabouts and how that capacity compares to other intersection types. This has required the use of a computer model to estimate the capacity of this intersection type and has led to an evolutionary learning process paralleling the national debate regarding the subject of modeling roundabouts. This paper is not intended to serve as an exhaustive analysis of roundabout analysis software; neither is it a recommendation to use one modeling software package over the other. Rather, the purpose of this paper is to present some observations made by the authors that may be useful to other practitioners when modeling roundabouts. In the interest of providing some background information, an overview of each project follows with a description of the objectives of the project and the tools used to meet those objectives. The next section will summarize the lessons learned from these projects and will examine specific aspects of each in more depth. ACHD Roundabout Application Guidelines This project, the first phase of the Ada County Roundabout Study, involved the development of roundabout siting and design guidelines for Ada County, Idaho. These guidelines are meant to assist the Ada County Highway District (ACHD) as well as other agencies, developers, and consultants in implementing roundabouts at intersections across the county. Highlights of the study include research of existing best practices nationwide, application and siting guidelines, design parameter guidelines, sample roundabout configurations, guidelines for designing roundabouts for expandability from single-lane to dual-lane operation, VISSIM and RODEL modeling to test the design parameters, a cost comparison between roundabouts and signals, research into future Americans with Disabilities Act (ADA) issues affecting roundabouts, and a National Roundabout Conference 2 Transportation Research Board Ryan W. Peterson, PE, PTOE, PEC 2 Stephen J. Lewis, PE, PTOE, PEC Kent J. Fugal, PE, PTOE, PEC peer review by a national expert. Amity Road Corridor Study This was the second phase of the Ada County Roundabout Study and consisted of a concept study of six arterial intersections on Amity Road (McDermott Rd., Black Cat Rd., Ten Mile Rd., Linder Rd., Locust Grove Rd., and Eagle Rd.) in Ada County, Idaho. The intent of the project was to use the newly developed siting and design guidelines to evaluate the appropriateness of constructing roundabouts at the study intersections. The analysis included a comparison of roundabout alternatives and signal alternatives at study intersections and required a capacity analysis. VISSIM was used to perform the roundabout analysis and Synchro was used to perform the signal analysis.