Evaluating Roundabout Capacity, Level of Service and Performance

Results of research on roundabouts in the United States are presented in the NCHRP Report 572. The findings and recommendations of this report are important as they will be the basis of the 2010 Highway Capacity Manual methodology for operational analysis of roundabouts. This paper discusses various findings and recommendations of the NCHRP Report 572 on roundabout capacity and level of service. Discussions include issues related to the finding of lower capacity of roundabouts in the USA compared with Australian and UK roundabouts, and the choice of Level of Service (LOS) thresholds for roundabouts compared with those used for signalized and sign-controlled intersections. Capacity and the resulting degree of saturation (v/c ratio) estimates obtained using the NCHRP Report 572 and standard SIDRA INTERSECTION methods are compared by presenting single-lane and multi-lane roundabout examples. Importance of the basic findings of research on US roundabouts as presented in the NCHRP Report 572 is discussed. Various model extensions related to the use of the NCHRP 572 model in SIDRA INTERSECTION are discussed. An example is also given for modeling Roundabout Metering signals. Finally, the paper presents a discussion of the issue of possible increases in roundabout capacities in the USA over time due to changes in driver behavior. INTRODUCTION Based on a comprehensive evaluation of roundabouts in the United States, the NCHRP Report 572 (1) presented methods of estimating the operational and safety impacts of roundabouts. This paper focuses on the operational performance aspects of the report. The NCHRP Report 572 stated "Perceived differences in driver behavior raise questions about how appropriate some international research and practices are for the United States. ... Under NCHRP Project 3-65, (the researchers) reviewed existing safety and operational models. After compiling a comprehensive inventory of roundabouts in the United States, they traveled to several representative ones to gather geometric, operational and safety data. ... They then evaluated the different analytical models to determine how well they replicate U.S. experience." On operational performance, the report concluded that "Currently, drivers in the United States appear to use roundabouts less efficiently than models suggest is the case in other countries around the world. In addition, geometry in the aggregate sense (number of lanes) has a clear effect on the capacity of a roundabout entry; however, the fine details of geometric design (lane width, for example) appear to be secondary and less significant than variations in driver behavior at a given site and between sites.". Akçelik Evaluating Roundabout Capacity, Level of Service and Performance 2 The report proposed exponential models of capacity for single-lane and two-lane roundabouts, and recommended that level of service (LOS) criteria are the same as those currently used for unsignalized intersections. The report also recommended that "Because driver behavior appears to be the largest variable affecting roundabout performance, calibration of the models to account for local driver behavior and changes in driver experience over time is highly recommended to produce accurate capacity estimates." and stated that "These models have been incorporated into an initial draft procedure for the Highway Capacity Manual (2010),which the TRB Committee on Highway Capacity and Quality of Service will continue to revise until its eventual adoption.". This paper discusses findings and recommendations of the NCHRP Report 572 on roundabout capacity and level of service. Discussions include issues related to the finding of lower capacity of roundabouts in the USA compared with Australian and UK roundabouts, and the choice of Level of Service (LOS) thresholds for roundabouts compared with those used for signalized and sign-controlled intersections. The NCHRP Report 572 recognized the SIDRA INTERSECTION software (previously named aaSIDRA) developed in Australia by the author of this paper as one of the "two major software implementations in use in the United States" and included evaluation of the capacity model used in the software (2). The original roundabout capacity model in SIDRA INTERSECTION is based on research on Australian roundabouts (3-16) thus reflecting Australian driver characteristics. When the early results of the NCHRP 3-65 research were published (17-19), indicating that capacities of roundabouts in the USA were significantly lower compared with the Australian and UK roundabouts, the SIDRA INTERSECTION capacity model was calibrated for US applications to provide capacity estimates closer to those observed in the USA. For this purpose, the Environment Factor parameter of the model was set to 1.2 as the default for US versions of SIDRA INTERSECTION while the value of this parameter for the original capacity model is 1.0. For the purpose of this paper, the US version of the SIDRA INTERSECTION roundabout capacity model with the Environment Factor value of 1.2 will be referred to as the "SIDRA Standard" model. In SIDRA INTERSECTION Version 4 released recently, the roundabout capacity models proposed in the NCHRP Report 572 report have been implemented directly as an alternative to using the SIDRA Standard model. This model will be referred to as the "NCHRP 572" model. This paper presents comparison of capacity and the resulting degree of saturation (v/c ratio) estimates obtained using the NCHRP 572 and SIDRA Standard models by presenting single-lane and multi-lane roundabout examples. Various model extensions related to the use of the NCHRP 572 model in SIDRA INTERSECTION are discussed. An example is also given for modeling Roundabout Metering signals (20-22). Importance of the basic findings of research on US roundabouts as presented in the NCHRP Report 572 is discussed. Finally, the paper presents a discussion of the issue of possible increases in roundabout capacities in the USA over time due to changes in driver behavior. Akçelik Evaluating Roundabout Capacity, Level of Service and Performance 3 EXAMPLES Two examples shown in Figures 1 and 2 are used to compare estimates of capacity and degree of saturation (v/c ratio) from the SIDRA Standard and the NCHRP 572 capacity models. For the SIDRA Standard capacity model, Environment Factor of 1.2 is used. For the NCHRP 572 capacity model, Origin-Destination factors or adjustment factors for Entry /Circulating Flow Ratio are not used, and the Capacity Constraint method applies in one case (see the discussion in the following sections). For the NCHRP 572 model, the capacity equation for "single-lane roundabouts" is applied to all single-lane circulating road cases, including multilane approaches. For multi-lane approach lanes with multi-lane circulating roads, different equations apply. For both capacity models, lane flows are determined according to the SIDRA INTERSECTION principle of equal degrees of saturation which assigns lower flow rates to lanes with lower capacity. Analyses are carried out for 15-min peak period. The hourly flow rates calculated from 15-min peak volumes are shown in Figures 1 and 2. Peak Flow Factors are 1.0 due to the use of known peak flow rates. Geometric parameters other than number of lanes and lane disciplines are not used in the NCHRP 572 model. Geometric parameters are applicable to the SIDRA Standard model only. Although geometric parameters have been shown in both metric and US customary units, the latter system is used in the analysis reported in this paper. The parameter values in metric and US customary units are not necessarily precise converted values. Example 1: Single-Lane Four-Way Roundabout This one-lane roundabout case (Figure 1) is based on the example described in Highway Capacity Manual 2000, Chapter 17, Part C (23). The entry flows represent a fairly balanced origin-destination flow pattern. The results from the SIDRA Standard and NCHRP 572 capacity models for this example are shown in Table 1. The capacities and degrees of saturation estimated by the two models are seen to be very close. This indicates that the use of the Environment Factor value of 1.2 in the SIDRA Standard model approximates the NCHRP 572 model closely, with slightly (about 3-4 %) lower capacities estimated by the SIDRA Standard model. When the Environment Factor value of 1.0 is used, capacities are about 30 per cent higher for this example, corresponding to the Australian driving characteristics as represented by the original SIDRA model. Akçelik Evaluating Roundabout Capacity, Level of Service and Performance 4 Figure 1 Example 1: single-lane four-way roundabout given in HCM 2000, Chapter 17 (23) Table 1 Capacity results for Example 1 (single-lane four-way roundabout) App. ID Approach Name Approach Flow (veh/h) Circulating Flow (pcu/h) Capacity (veh/h) Degree of saturation (v/c ratio) SIDRA Standard Capacity Model (Environment Factor = 1.2) S El Moro NB 430 695 543 0.79 E Buena Vista WB 510 495 671 0.76 N El Moro SB 500 535 643 0.78 W Buena Vista EB 540 450 703 0.77 NCHRP 572 Capacity Model S El Moro NB 430 695 564 0.76 E Buena Vista WB 510 495 689 0.74 N El Moro SB 500 535 662 0.76 W Buena Vista EB 540 450 721 0.75 8 m / 26 ft 20 m / 66 ft 95 255 150 500 120