Roundabouts in the United States (2007) / Chapter Skim
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Pages 65-101
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From page 65... ...
65 This chapter presents the design findings for this project. The following sections discuss an analysis of predicted versus observed speeds, pedestrian behavior, and bicyclist behavior.
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where V predicted speed (mph) R radius of vehicle path (ft)
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67 Mean Deviation Root Mean Square Error Average (Mean) Speed 2.1 mph (3.4 km/h)
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enced by hesitation on entry, which, over time, could be reasonably expected to reduce as drivers become more comfortable. Therefore, the current method for estimating V2 is generally conservative at the present time but reasonable, and no changes are proposed.
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at sites with 15 or more observations. Only 85th-percentile speeds from sites with 15 or more observations were used in the analysis.
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70 Characteristic Through-Movement Exit Speed, V 3 Left-Turn- Movement Exit Speed, V 6 Total Range of measured speeds 8 – 37 mph (13 – 60 km/h)
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assigned a predicted exit speed of 45 mph. However, the cluster of data points with predicted speeds in the 30- to 40-mph range (for which such arbitrary assignments were not made)
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Using this new formulation, 85th-percentile speeds were plotted against adjusted predicted speeds for all sites, the subset of single-lane sites, and the subset of multilane sites, as shown in Figures 49, 50, and 51, respectively. Estimates for mean deviation and RMSE for both the unadjusted and adjusted predictions are shown in Table 55.
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From page 73... ...
Entry Speed Table 57 summarizes the characteristics of the entry speed (V1) data.
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74 Site Movement Type Number of Observations 85th-Percentile Field Speed (mph) Raw Predicted Speed (mph)
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75 Characteristic ThroughMovement Entry Speed, V1 Left-Turn-Movement Entry Speed, V1L Total Range of measured speeds 8 – 35 mph (13 – 56 km/h)
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where V1 V1 speed (km/h) V1pbase V1 speed predicted based on path radius (km/h)
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From page 77... ...
77 y = 0.6237x + 6.0316 R2 = 0.3491 0 5 10 15 20 25 30 35 40 45 50 0 5 10 15 20 25 30 35 40 45 50 Adjusted Predicted Speed, V1p1 (mph) , All Sites A ct u a l S pe ed , V 1a (m ph )
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From page 78... ...
• Current speed prediction methods significantly overestimate entry and exit speeds, particularly for entry paths and exit paths that are tangential or nearly tangential. These prediction methods are significantly improved by incorporating acceleration and deceleration effects as they relate to predicted circulating speeds.
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79 Site Movement Type Number of Observations 85th-Percentile Field Speed (mph) Raw Predicted Speed (mph)
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From page 80... ...
roundabout did not match the number of lanes at the location of the crosswalk. For example, one leg had two lanes at the yield line for motor vehicles but only a single lane at the point where the crosswalk was located.
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• Retreats: Pedestrian began crossing and then retreated to the curb or splitter island because of an approaching vehicle. • Runs: Pedestrian ran across the leg because of an oncoming vehicle.
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For crossings that were initiated on the exit side (see Figure 60) , the overall percentage of crossings that were coded as normal is lower than what was observed for entry side starts.
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The motorist yielding behavior results for pedestrian crossings that started on the exit side are shown in Figure 63. For one-lane sites, 29% of the motorists did not yield to the pedestrian on the exit side, and 10% did not yield on the entry side.
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pedestrians conflicting vehicle bus Figure 66. Pedestrian Conflict 3 at MD05SW-S (Towson, MD)
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a conflict, both parties had to respond correctly. The pedestrian had to reject gaps when the motorist did not yield, and the motorist had to yield when the pedestrian was crossing.
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motorist yielding behaviors, and pedestrian waiting and crossing times. In an effort to provide more insight into these results, the roundabout findings were compared to results from intersections with other types of traffic control that are more commonly found in the United States, such as signalization and stop signs.
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intersections. It did occur for 4% of the crossings at signalized intersections and for 27% of the crossings at uncontrolled intersections.
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yield on the exit side compared to 33% not yielding on the entry side. For one-lane sites, the exit and entry non-yield percentages were 22% and 13%, respectively.
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to the previous questions, the two design elements that correlate to differences in behaviors are the number of lanes and the directional side of the site (entry lanes versus exit lanes)
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The analysis of bicyclist events covers the following topics: • Bicyclist position • Bicyclist behaviors • Motorist behaviors • Bicycle-motor vehicle conflicts • Other bicyclist behaviors Refer to Appendix L for images to help describe some of the behaviors observed and described in the subsequent sections. Bicyclist Position Bicyclist position refers to the location of the bicyclist's path as the bicyclist enters, exits, or traverses the roundabout.
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From page 91... ...
circulating lane of the roundabout and crossing at the crosswalk. Behaviors were categorized as one of the following: • Normal (passes through without stopping because there is no vehicle in the vicinity or vehicle yields)
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there are not significant safety problems with bicyclists entering roundabouts. Bicyclist Behaviors at Crosswalks There were 81 events where a bicyclist crossed at the crosswalk.
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Unlike the pedestrian study, behaviors of the motorist were only recorded for interactions when there was an accepted gap by the bicyclist. For this reason, Table 68 does not provide information on the percentage of motorists not yielding to bicyclists.
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where a bicyclist interacted with a pedestrian. Each interaction occurred when the bicyclist was on the roadway and yielded to a pedestrian on the crosswalk.
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The safety and geometric data identify several trends related to the early roundabout experience in the United States. Overall, the crash experience has been positive (showed an overall reduction in crash frequency)
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roundabouts, and the research team believes that this is the primary cause for the high crash frequency. Multilane Roundabout Evaluation Approximately one-third of the sites in the safety database are multilane roundabouts.
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accommodate the natural vehicle path yet still exhibited a higher than anticipated crash frequency. This site exhibits narrower lane widths than other sites reviewed and than recommended by the FHWA Roundabout Guide.
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have as strong a relationship in the United States. As demonstrated in the operational analysis work, while there appears to be a relationship between the additional width added as part of entry flare (see Chapter 4)
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of exiting vehicles. As a result, this factor has not been included in the recommended capacity models.
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greater than the 6.2 s value measured by Kyte et al.
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FHWA Roundabout Guide in 2000 (1) and continued in a number of state guides (e.g., Kansas [37]
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