Development of Roundabout Crash Prediction Models and Methods (2019) / Chapter Skim
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5 Literature Review Approach and Findings This chapter presents the findings from the literature review; the findings helped inform the data collection and modeling approach. For example, findings from other research conducted in the United States and abroad were used to identify candidate crash prediction modeling techniques, geometric features for data collection, definition of a roundabout-related crash, and database formation.
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6 binomial regression model, the variance of number of crashes per year can be written as follows: VAR y E y k E y Equation 2-12[ ]
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7 overdispersion parameter that is required to apply the EB method in the HSM. Moreover, the extensive experience of negative binomial regression modeling from previous safety studies facilitated the model development in this project.
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8 Of the U.S.-based research found in the literature, Rodegerdts et al.
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9 possible, to disaggregate it from the total entering AADT at the intersection. When available, pedestrian, bicycle, and motorcycle volume information are useful data for predicting crash types unique to those modes as well as more severe crashes.
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Table 2-2. Summary of volume, geometrics, speed, and other characteristics affecting crashes [Table 2-2 is an expansion and update to Rodegerdts et al.
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Splitter island type +/- Posted speed limit * + + Free mean speed of circulating vehicles + + Entering vehicle speed I I I I Variation in vehicle speed I I + 85th percentile speed + + + + Reduction in 85th percentile speed + Sight distance + + I I + Distance to irst sight of roundabout *
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12 sideswipe crashes. Higher reductions in 85th percentile speed (i.e., the more a driver would need to reduce his/her speed to travel through the roundabout)
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13 Belgium. Findings from the study indicated that roundabouts with bicycle lanes adjacent to the circulating vehicle lane experienced a 93% increase in bicycle injury crashes.
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14 – Variation in vehicle speed (e.g., between vehicles, for one vehicle as traveling through the roundabout)
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15 Approach-Level Crash Prediction Models. The leglevel crash prediction models follow the general form below: Crashes/year AADT .
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16 to roundabouts (e.g., Persaud et al., 2001; Eisenman et al., 2004)
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17 2.2 Outreach to Public Agencies with Roundabouts As part of the literature review activities, the research team reached out to 17 public agencies with roundabouts. The purpose of these outreach activities was to identify • Existing databases and data sources that could be used in this project, • Potential focus sites for roundabouts that had been modified in the past, • Information that could be used to understand or evaluate driver learning curve, and • General information related to each agency's experiences with roundabouts that could be informative to this project.
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18 found two relevant studies. One study, Hanscom (2010)
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19 • Driver Education: Driver education across agencies included project-based informational campaigns, information in driver manuals, and an increasing number of agencies with larger general public information campaigns regarding how to drive and use a roundabout. • Coordination with Law Enforcement and Crash Reporting: Coordination activities with law enforcement agencies showed to be relatively minimal, with some focus on projectbased outreach.
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20 The base condition is no driveways, adequate sight distance, no turn lanes, and no skew. For this condition, the base model is then derived as Crashes/year 9.34 0.60 Major Road ADT 0.61 Minor Road ADT Equation 2-6 exp ln ln [ ]
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21 focus on that, with some content taken from a white paper developed by the NCHRP 17-62 project team. The probabilistic approach estimates the probability of an injury of a given severity, given that a crash has occurred, as a function of roadway and traffic characteristics and, potentially, crash and person-related variables.
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22 fitting results when the model uses either logit or probit functions (Chambers and Cox, 1967)
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23 Method Applications Strengths Weaknesses Before–after with comparison group Treatment is similar among treatment sites. Simple Dificult to account for regression-to-the-mean.
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24 changes in geometric design features, traffic operations, or other characteristics by examining the increase or reduction in crash counts between the before and after periods. Three techniques have been proposed for this kind of study: (1)
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25 The regression model was deemed inadequate on the basis of the weak effects of the speed variables, so no speed-based SPF was recommended for use. In addition to the attempt to model a speed-based SPF, the research documented in Rodegerdts et al.
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26 observed speed-based models for the advantage of expanding accessible sample size. The applicable sample size of the United States, for example, would be 33 if observed speed were applied to develop SPF.
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27 The predicted crashes correlated well to some of the existing crash prediction models. Observed data from 10 single- lane roundabouts in Indiana were then used to model conflicts in VISSIM and SSAM and a model calibrated to predict observed crashes from the estimated conflict frequencies: Crashes/year 0.796 exp 0.0486 conflicts/h Equation 2-14 ( )
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28 that describe the type of collision without reference to where it occurred on the roadway segment or within the intersection. The advantages of using these crash-type definitions in this project include requiring less time to assemble databases and develop crash prediction models; creating an easier way for practitioners to apply the crash prediction models at existing roundabouts and apply the EB method (i.e., integrate roundabout crash history into the crash prediction to improve its predictive power)
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29 2.6 Model Development Literature Review This subsection describes the findings from a review of the literature related to the topic of roundabout safety prediction. It extends the review documented in Section 2.1.3.
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30 located in several states. For total crashes, Rodegerdts et al.
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31 in the data (the findings from this comparison are provided in Section 5.2.3.3)
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32 was converted to an equivalent CMF aggregated for all crash types using Equation 2-1 and a circulating width of 25 ft. The relationship between the aggregate CMF value and circulating width is shown in Figure 2-6.
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33 Arndt, O
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34 Lyon, C., and B Persaud.
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