Evaluating the Performance of Corridors with Roundabouts (2014)

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Evaluating the Performance of Corridors with Roundabouts Chapter 5–Conclusions and Suggested Research Page 5-1 CHAPTER 5. CONCLUSIONS AND SUGGESTED RESEARCH Corridors of roundabouts have developed as roundabouts have become more common and more distributed across the United States; 58 corridors were identifed at the start of this project. Based upon an in depth examination of nine of those corridors, the use of roundabouts in series appears to have been successful in a wide variety of contexts throughout the United States. The following sections summarize the major conclusions from this study, and further research is recommended in a number of areas. 5.1. CONCLUSIONS It is clear that roundabouts in series can be successful in a variety of contexts, although success can be measured in a variety of ways, including improvement in safety, improvement in operations, improvement in pedestrian and bicycle access, and community acceptance. The authors believe a case by case evaluation is the preferred approach for evaluating the performance of corridors, and the Corridor Comparison Document (CCD) developed in this project is intended to facilitate this process. The CCD (Appendix A) provides a framework for comparing alternative corridor configurations and should objectively inform project decisions based on the unique context of each corridor. The following elements are included: Information on different users of arterials, including passenger cars, buses, pedestrians, bicycles, trucks, and emergency vehicles. An overview of the project planning process wrien from the perspective of a practitioner evaluating alternatives for reconstructing an existing corridor or constructing a new roadway where the alignment has already been determined. Typical performance measures, assessment techniques for performance measures, and methods for selecting and prioritizing performance measures. The performance measures presented in the CCD are grouped in the broad categories of quality of service, safety, environmental, costs, community values, and others. Four fictional example applications illustrating use of the CCD on the following corridors: o A new suburban arterial being buit in a greenfield to create access to undeveloped land and to provide increased connectivity; o A community enhancement project on an existing urban arterial; o An existing two lane highway in a rural, context sensitive environment that is beginning to experience suburban style development as it transforms into a vacation and second home community; and

Evaluating the Performance of Corridors with Roundabouts Page 5-2 o An existing suburban corridor being evaluated for safety and operational improvements due to changing context and a need for pavement rehabilitation. The use of the CCD for these four examples results in roundabout control being selected for three of the corridors, and signal control being selected for the fourth. Field data collection for this project was focused on travel time, in part because this project’s literature review identified a lack of methods of corridor level operational evaluations of roundabouts. Vehicles equipped with GPS probes drove through and left turn routes at different times of the day on nine roundabout corridors. Data from seven of the nine corridors were used to develop the following models: Free flow speed models for roadway sub segments upstream and downstream of roundabouts. Roundabout influence area (RIA) models for roadway sub segments upstream and downstream of roundabouts. The RIA is the distance upstream and downstream of a roundabout in which unimpeded speeds are below free flow speed due to acceleration into and deceleration out of the roundabout. Field data indicate that upstream RIA lengths generally vary more from site to site than downstream RIA lengths. Geometric delay (upstream and downstream of the roundabout). Field data indicate upstream geometric delay generally varies more than downstream geometric delay from site to site. Impeded delay (upstream and downstream of the roundabout). These models were incorporated into the framework of the Urban Streets procedure in Chapter 17 of the HCM 2010 and can be used to estimate travel time and facility level of service on a roundabout corridor. Field travel times were generally recorded during the a.m. peak, off peak, and p.m. peak periods. Aggregating data within these times periods together for each corridor indicates the following: Study corridors operated at LOS A through C based on travel speed as a percent of free flow speed (the HCM 2010 performance measure for Urban Streets). Most routes had the same LOS for the three time periods. Some changed by one le“er grade. Travel speed and LOS for through routes and left turn routes were generally similar, and there was no pa“ern of one performing be“er than the other. Data from the two Carmel, Indiana, corridors were reserved for validation and not used in the development of the models. The validation exercise showed the developed corridor methodology correctly predicted the LOS for all four analysis routes for the Spring Mill corridor, and the LOS was within one le“er grade for the Old Meridian corridor. The resulting percent free flow speed estimates (the metric used to determine LOS) matched the field observed data within 10 Chapter 5–Conclusions and Suggested Research

Evaluating the Performance of Corridors with Roundabouts Chapter 5–Conclusions and Suggested Research Page 5-3 percent for Spring Mill Road (all four routes) and for the northbound routes on Old Meridian Road. The Old Meridian southbound routes matched within a 20 percent difference. Comparisons of field measured vehicle travel times and simulated “equivalent” corridors with signalized or two way stop control indicated the following: Neither roundabout nor signalized/stop controlled corridor configurations consistently result in reduced travel time or intersection delays for through routes. Evidence suggests roundabout corridors have a good likelihood of improving travel time performance, but site specific operational conditions may favor signalization or stop control. This finding reinforces the need for a case by case evaluation. Corridors with irregular intersection spacing (MD 216, Spring Mill Road, Avon Road) show a higher likelihood for having beŠer travel times under a roundabout configuration rather than a signalized configuration. Corridors that can use two way stop controlled intersections in the place of roundabouts or signals generally produce beŠer end to end travel times, even if intersection delays are lower under a roundabout configuration. The corridor having the lowest travel times under an equivalent non roundabout configuration (La Jolla Boulevard) was designed with mixed signalized and unsignalized control. As a result, end to end travel times are more favorable under an equivalent non roundabout configuration, even though the roundabout configuration resulted in lower intersection delays. The corridor analyzed with large intersection spacing and higher speeds (SR 539) showed virtually no difference in travel time between the two alternatives, despite the observation that the intersection delay increases in a roundabout configuration. For corridors where turning movements entering or departing the corridor are of similar or greater importance than end to end travel times, roundabout corridors appear more likely to improve those travel times. This may be due to the fact that cycle lengths and offsets for signalized corridors are typically prioritized for movement of through traffic over left turns and side street movements. This is, of course, a variable that should be evaluated when determining the most appropriate operational strategy for a signalized corridor, further emphasizing the need for a case by case evaluation. Of the 20 through route combinations analyzed, approximately half resulted in lower travel time under a roundabout configuration and approximately half resulted in lower travel time under a non roundabout configuration. Of the 60 route combinations analyzed, travel time differences ranged from 54 percent (roundabout corridor travel time lower) to +80 percent (roundabout corridor travel time greater). The top 16 routes in which roundabouts had the greatest improvement in travel time were all left turn routes. Forty four routes showed a decrease in travel time with roundabouts, and 16 routes showed an increase in travel time with roundabouts.

Evaluating the Performance of Corridors with Roundabouts Page 5-4 Some findings for specific corridors: o Approach delay was lower with roundabouts for all intersections in both major street directions except for SR 539. o Through route travel time (average of both directions) increased with roundabouts on La Jolla Boulevard, Old Meridian Street, and Golden Road; decreased with roundabouts on MD 216, Spring Mill Road, Avon Road, and SR 67; and remained virtually unchanged on SR 539. o Travel time for routes with a left turn off the major street (average of both directions) increased with roundabouts on La Jolla Boulevard; decreased on MD 216, Old Meridian Street, Spring Mill Road, SR 539, Avon Road, and SR 67; and remained virtually unchanged on Golden Road. o Travel time for routes with a left turn onto the major street (average of both directions) increased with roundabouts on La Jolla Boulevard and decreased on the other corridors. o The La Jolla Boulevard corridor performs quite differently from the other corridors studied in this project. It is the most urban of the corridors studied, with considerable pedestrian, bicycle, and on street parking activity. As a result, through vehicular traffic experiences more friction than was observed for other corridors. As confirmed in the corridor interviews, this outcome is consistent with the multimodal focus desired for this particular corridor. The comparisons were entirely operations focused due to limitations of this project. However, safety comparisons would provide additional insights into which form of intersection control is preferred on each corridor. Section 3.5 of this report provides information on key comparative safety studies of roundabouts and signalized/stop controlled intersections to date, and guidance on how to compare the expected number of crashes at a signalized or stop controlled intersection and a roundabout. In general, the findings of this project indicate a need for corridor specific evaluations to determine which form of intersection control is operationally preferred on a given corridor. Furthermore, there are many performance measures other than traffic operations that are used to choose intersection control on a corridor. 5.2. SUGGESTED RESEARCH While the overall corridor comparison framework developed for this project is comprehensive, the detailed data collection and analysis conducted within this project was focused on automobile traffic operations. Future studies of roundabout corridors should focus on other elements, including the following: Pedestrian and bicycle quality of service procedures for urban streets that include roundabouts: Like the auto mode methodology, the pedestrian and bicycle mode methodologies of the HCM 2010 Urban Streets Chapter 5–Conclusions and Suggested Research

Evaluating the Performance of Corridors with Roundabouts Chapter 5–Conclusions and Suggested Research Page 5-5 procedure were developed using data collected on signalized arterials. The applicability of these methodologies to roundabout corridors should be assessed, and models and/or user preference surveys should be conducted for any roundabout specific aspects of corridors Before/after safety analysis: Several studies have established crash modification factors (CMFs) for the conversion of signalized or stop controlled intersections to roundabouts. However, corridor level changes in safety performance have not been quantified, including the confounding effect of associated changes in access management. Predictive safety analysis: An upcoming NCHRP project (17 70) is planned to develop safety performance functions (SPFs) for roundabouts and enable greater confidence in safety comparisons of roundabouts to other intersection forms. This project should include data from roundabouts on corridors. Automobile traveler perception scores: In addition to level of service, the HCM 2010 Urban Streets procedure provides a traveler perception score for the automobile mode. Public support: There is anecdotal evidence that some roundabout corridors are favored by the public over other alternatives, but no known scientific surveys have been conducted. Wayfinding within the corridor: There is anecdotal evidence that travelers get lost within a roundabout corridor. Guidance on providing a system of signs within a corridor would improve the overall traveler experience.