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APPENDIX N RESEARCH PROBLEM STATEMENT Refinement of Roundabout Operational Models A recent NCHRP study (NCHRP 3-65) produced new capacity and safety analysis procedures for modern roundabouts. The procedures were developed based on data collected during a nationwide data collection effort. Despite that extensive effort, it was difficult to find roundabouts that were operating at capacity, with standing queues for extended periods of time. It was also challenging to find roundabouts with widely varying geometric conditions that were operating at capacity. The implication is that more data are needed at a point in time when there are a greater number of roundabouts operating at capacity. This is particularly true for multilane roundabouts, where very few were found to be operating at a capacity condition across all entry lanes of a given approach. This, coupled with the many possible variations of entry and circulating lane assignment, origin-destination patterns, and overall geometry, limited the potential scope of the model developed under the previous NCHRP effort. In addition, the previous NCHRP effort confirmed that data collection and extraction is an expensive undertaking, particularly when approaching an operational modeling effort that intends to examine both analytical and regression models. In addition, not all of the data collected under 3-65 was dedicated to the development of operational models. Some sites were selected for other purposes, such as speed measurements where free flow conditions are needed. A project dedicated to the development of operational models can optimize site selection to only those sites with queuing, as well as to time periods and seasons where queuing is maximized. In addition, it is possible that a more efficient data collection and extraction method can be used. The data collection employed in the previous NCHRP effort was primarily restricted to the summer months, which limited the ability to measure higher traffic volumes in other seasons. In addition, the weather during the summer months in much of the country (thunderstorms, windstorms) was found to be frequently incompatible with the free-standing mast used as a camera platform, thus limiting the potential collection of afternoon peak hour data. In addition, even with the elevated platform used in the previous NCHRP effort, it was difficult in some cases to visually observe the back of queue on all approaches, thus limiting the ability to accurately measure delays and queues. The previous NCHRP effort found that the act of finding appropriate sites to study is not trivial. Although a thorough survey was conducted to find the most useful roundabouts nationwide and most of these were included in the study, it is still unclear whether all of the best sites were found. Since expectations are high that a diverse set of the most ârepresentativeâ sites will be studied, a way to make sure that that has happened is needed. Therefore, the key issues are these: 1. Additional sites with queuing are needed. Since most sites in the US are still relatively new, this will likely require additional time to pass for sites to experience traffic volume increases that generate sustained queuing. 2. More multilane sites are needed to produce a more comprehensive model, and additional single-lane sites are needed to validate the current single-lane model. These sites need to have variation in lane configuration, use of striping, truck percentages, and overall NCHRP Web-Only Document 94: Appendixes to NCHRP Report 572: Roundabouts in the United States N-1
geometric arrangement, plus be operating at capacity. This will require additional time to pass, as few sites existed in 2003 that could meet these criteria (see #1). 3. Innovative data collection strategies are needed to allow the efficient collection and extraction of data during optimal seasons, days of week, and times of day over sites that are widely dispersed geographically. 4. Different techniques are needed to capture backs of queue, particularly for sites where the approach roadway curves or becomes visually obstructed. While the loss of back of queue does not necessarily affect the estimation of entry capacity, it does affect the estimation of delay and queue length. 5. An innovative method is needed to identify the best sites for use in model development. Hence, this project has three objectives. The first is to find a way to canvas the traffic engineering community to ensure that all the best sites are studied. The second is to find a better way to instrument the facilities so that better data can be collected. The third is to find an efficient procedure to do the data extraction necessary for developing operational models. The objective in this follow-on effort is to build on the work of NCHRP 3-65 and address the outstanding issues that could not be completely addressed with that effort. To achieve these objectives, the research should produce: a) A definitive way to ensure that the best sites are being studied, b) A method for instrumenting the sites that is simple, straightforward, perhaps do-able by local traffic engineers, and capable of providing the best possible data, and c) A technique for extracting the data that is accurate and cost effective. The objectives will be met through the following tasks: 1) Review of prior site identification techniques, 2) Refinement of a mechanism to find the best sites, 3) Identification of the best sites to study, 4) Investigation of better instrumentation techniques, 5) Demonstration of the data collection technique, 6) Data collection, 7) Data extraction, 8) Data summary and preliminary analysis, 9) Model refinement, 10) HCM recommendations, and 11) Final report. NCHRP Web-Only Document 94: Appendixes to NCHRP Report 572: Roundabouts in the United States N-2
