Development of Roundabout Crash Prediction Models and Methods (2019) / Chapter Skim
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143 Research Findings This chapter presents the research findings from the project. The first section presents the final planning-level, intersection-level, and leg-level crash prediction models.
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144 Table 6-2 presents the parameter estimates and standard errors (in brackets) for the models developed for each crash type, as well as the negative binomial overdispersion parameters k.
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145 6.1.2.1 Fatal-and-Injury Crash Frequency Prediction Model, One Circulating Lane This section describes the final version of the predictive models for roundabouts with one circulating lane. These models are not applicable to roundabouts that have two circulating lanes conflicting with one or more legs.
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146 This CMF is applicable to a roundabout in an urban, suburban, or rural area. The count of access points on a leg represents the number of driveways or unsignalized access points on the leg (either side)
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147 exp 0.0415 Equation 6-18 1 1, 2 2, 3 3,S p f p f p fB sl sl sl[ ]
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148 in the equation above is obtained from Step 8. The predicted FI average crash frequency is obtained from Step 4.
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149 57% fewer crashes than a roundabout at which all legs serve two-way traffic flow. It is rationalized that the presence of an outbound-only leg reduces the number of conflict points between intersecting traffic streams, relative to a leg that serves two-way traffic flow.
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150 smaller for K, A, and B crashes at four-leg roundabouts than for three-leg roundabouts. A similar trend exists for roundabouts with two circulating lanes.
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151 This section consists of two subsections. The first subsection describes the predictive model.
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152 leg-specific CMFs, the following equation is used to compute the leg CMF. Equation 6-33, , ,CMF CMF CMF CMFj bypass j ew j cl j= × × where CMFj is the combined crash modification factor for leg j ( j = 1 to m)
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153 2.1642 Equation 6-45 1 1, 2 2, 3 3, 4 4,S exp p f p f p f p fA sl sl sl sl[ ]
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154 This equation is used once for each crash type and severity category of interest. The probabilities used in Equation 6-53 are obtained from Step 7 and Step 8.
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155 entering lane and the circulating lane(s) , which eliminates conflicts at several points through the roundabout.
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156 The proportions shown in Tables 6-6 and 6-11 can be used to examine the predicted trends associated with circulating lanes. For example, the trend associated with circulating lanes can be assessed by comparing one row in each table asso ciated with the same number of legs.
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157 SPF for four-leg roundabouts: 1.475 0.702 1000 0.168 Equation 6-55 ,4 4N exp LN EntAADT I SPF rural ( )
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158 where Np,PDO = predicted PDO average crash frequency, crashes/yr; and C = local calibration factor. Step 5: Apply Crash Type Distribution (optional)
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159 than those in urban or suburban areas with similar volume. The three-leg model was calibrated using sites with entering AADT ranging from 3,000 to 18,000 veh/d.
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160 traffic outbound from the roundabout, then the leg's AADT should not be included in the summation. Step 2: Apply CMFs.
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161 The proportion associated with a given crash type Pt is obtained from the corresponding column of the table and the row that coincides with the number of roundabout legs. Step 6: Compute Predicted Crash Frequency by Crash Type (optional)
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162 than those in urban or suburban areas with similar volume. The three-leg model was calibrated using sites with entering AADT ranging from 2,000 to 25,000 veh/d.
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163 Section 5 of the report, which describes the model development process. 6.1.3.1 Entering-Circulating Crash Models Tables 6-16 to 6-20 provide the final models developed.
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164 It should be noted that the CMF estimates often appear very small in that the value is close to 1.0. However, these values are per unit increase, meaning in practice they would likely be larger.
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165 Variable Circulating Lanes Entering Lanes Parameter Estimate Variable Min Variable Max CMFs/unit increase CMF s.e. Circulating Width 1 2 0.0319 15 42 1.0324 0.0190 2 1 -0.0141 25 45 0.9860 0.0643 2 2 -0.1375 24 45 0.8715 0.0504 ICD 1 1 -0.0068 65 236 0.9932 0.0043 1 2 -0.0082 110 314 0.9918 0.0046 2 1 -0.0148 135 426 0.9853 0.0060 Angle 1 2 -0.0234 53 182 0.9769 0.0067 2 2 -0.0134 69 182 0.9867 0.0052 Bypass Lane Present 1 1 -0.9982 0.3685 0.1856 Table 6-21.
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166 A larger ICD is associated with fewer crashes in the model for two circulating lanes and one exiting lane. An increased circulating width is associated with more crashes for the one circulating lane legs and fewer crashes for the two circulating lane legs.
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167 The model form and parameter estimates for the successfully estimated model is provided in Table 6-30. Equation 6-86 Circulating Circulating exp CircAADT expa b c CircWidth− = ( )
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168 the final total leg-level crash models for roundabouts with one circulating lane and two circulating lanes, respectively. Models were estimated separately for single and two circulating lane sites.
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169 design decisions, it is feasible to use an uncalibrated model as a basis for a relative comparison (e.g., a change in a roundabout feature results in a certain percent increase or decrease in predicted crashes)
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170 Year 1 postconstruction, an observation that does not seem generalizable. These general observations hold for all crash severity categories modeled and even for multilane and rural roundabouts for which a driver learning curve might arguably be more evident if it indeed exists.
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171 exhibit a similar trend on a per year basis (approximately 0.007 crashes/yr at single sites compared to approximately 0.025 crashes/yr at multilane sites)
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172 (Chapters 10, 11, 12, 18, and 19)
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173 effect of entry speed on crashes (and whether predicted entry speed can be used in lieu of actual measurements)
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