Roundabouts in the United States (2007) / Chapter Skim
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Pages 34-64
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From page 34... ...
34 This chapter presents a variety of modeling and evaluation tasks related to the operational performance of roundabouts in the United States. First, the results of the investigation of the ability of existing capacity and delay models to represent U.S.
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For each site, only those data that are considered "plausible" for both entry lanes together, the left lane individually, and the right lane individually are included. (Data are "plausible" if the lane has a known queue occurring during the entire minute, either through visual verification of the video record or through examination of the critical headway and follow-up headway.)
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the MD04-E (Baltimore County, Maryland) and MD05-NW (Towson, Maryland)
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HCM Delay Analysis The HCM 2000 control delay equation was tested against delay measurements obtained from the field. To maximize the use of available field data, the following assumptions were made: • During queued minutes, the determination of the arrival flow at the back of queue was not always possible because of the limited field of view of the data collection equipment.
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From page 38... ...
Approximate origin-destination matrices were developed for each roundabout during periods of continuous queuing, which have been used to estimate the turning movements and conflicting flow at each entry. In addition, flow rates were varied using a range of flow multipliers (10% to 500%)
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From page 39... ...
less than the critical headway and accepts any headway greater than the critical headway.Hence,a driver's largest rejected headway will typically be less than the critical headway, and the accepted headway will be typically greater than the critical headway. Such theory assumes that the driver's behavior remains consistent.
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40 MD06-N (Lothian, MD) MD07-E (Taneytown, MD)
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41 MD06-N (Lothian, MD) MD07-E (Taneytown, MD)
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ways. The difference between the average estimates is approximately 0.1 s.
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conflicting lanes. An alternative technique estimates the critical headway for the entire approach, combining the entering lanes and conflicting lanes into single entering and conflicting streams, respectively.
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The opposing vehicle time is calculated based on the accepted lag or gap: • Using the accepted lag, the opposing vehicle time is the sum of the entry arrival time plus the accepted lag. • Using the accepted gap, the opposing vehicle time is the sum of the entry arrival time plus the total rejected gaps and lag.
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From page 45... ...
Despite efforts to increase the dataset, a number of the sites have fewer than 100 follow-up headway observations. While the average follow-up headway of the entry lane may change with a larger sample size, the result is indicative of the average behavior of the entry queued minutes.
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From page 46... ...
critical headway. The Australian predictions of the critical headway and follow-up headway vary based on the conflicting flow, number of lanes, diameter, and entry width.At multilane sites, the follow-up headway is also a function of the dominant and subdominant arrival flows.
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l´ Effective flare length (m) r Entry radius (m)
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• Entry angle: 2.5 to 53.0 degrees at single-lane sites; 12.0 to 34.0 degrees at multilane sites. To assess predictive quality, two attributes of each parameter have been determined: correlation and contribution.
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the significance. For example, the inscribed diameter, D, contributes to the entry capacity with a confidence level, P, of 0.999 (1 0.001)
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The critical headway and the percentage of heavy vehicles in the entering flow have a moderate negative correlation (0.52)
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The critical headway and follow-up headway as functions of the entry-lane width, entry angle, and radius are illustrated in Figures 25, 26, and 27, respectively. There is a slight and unintuitive increase in the critical headway as the entry-lane width increases.
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From page 52... ...
It has also been suggested that as drivers become more familiar with roundabouts, their critical headways and follow-up headways will decrease. Similarly, as drivers are faced with more congested situations, their behavior will become more urgent and the critical headways and follow-up headways will also decrease.
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at single-lane roundabouts has been explored independently by Mereszczak et al.
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impact all entering vehicles, and the exact extent of the influence of exiting vehicles has not been determined. To estimate the capacity, the recalculated critical headway and follow-up headways should be used together with an estimate of the conflicting plus exiting flow.
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include the influence of heavy vehicles and, in examination of these against the percentage of heavy vehicles, did not show an intuitive trend. A more detailed examination of truck factors should be performed outside the model calibration.
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lower entry capacity. As illustrated by the reduction in the RMSE, the impact of this adjustment, in addition to the revised geometry, is significant.
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It should be noted that the HCM 2000 model is not intended to predict capacity of a multilane entry. There is a moderate to large improvement in the RMSE when the field-measured critical headway and follow-up headway are used.
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In summary, the inclusion of exiting vehicles in the analysis methodology did not improve the estimate of the capacity. However, because such behavior was observed in the field, refinements to the assumptions may suggest otherwise.
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The predicted exponential regression intercept and slope of 1129 and 0.0010 compares favorably with the HCM intercept of (3600/tf 3600/3.2) 1125 and slope of [(tctf /2)
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reflected by the variation in the class mean. The coefficients of determination of the linear and exponential relationships for the left lane are 0.50 and 0.53, respectively.
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of the most heavily utilized lane, or critical lane, thus removing the number of entering lanes from the capacity model. Effectively, a capacity condition is reached when the critical lane has a constant queue.
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The following influences in the multilane data should be noted: • The critical-lane data mostly comprise right-entry-lane observations. The critical right- and left-lane observations are illustrated in Figure 35.
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63 0 250 500 750 1000 1250 1500 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Conflicting Flow (pcu/hr)
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From page 64... ...
64 Level of Service In the HCM 2000 (2) , the criteria for levels of service for stop-controlled intersections and signalized intersections differ because the intersection types create different user perceptions.
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