Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results (2021) / Chapter Skim
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Pages 70-80
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From page 70... ...
70 Introduction CPMs or SPFs may include only traffic volumes as predictor variables, traffic volumes plus a limited number of geometric and traffic control variables, or traffic volumes and a large number of geometric and traffic control variables. Depending on the data that are used for the estimation, the CPMs are considered more reliable within the range of the independent variables that were available in the data used for the estimation of the CPMs.
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From page 71... ...
Reliability Associated with Predicting Outside the Range of Independent Variables 71 The procedure comprises two major steps: Step A Assemble the data needed for the procedure for the examination of different options for improving the predictions outside the range of AADT.
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From page 72... ...
72 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results This equation can also be used for CPMs for specific base conditions, where roadways meet certain conditions. When the site-specific conditions do not meet the base conditions, the HSM recommends adjusting the crash estimate from this equation by applying CMFs (also called SPF adjustment factors)
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From page 73... ...
Reliability Associated with Predicting Outside the Range of Independent Variables 73 measures the overall distance between the CURE and the 95% confidence limits for those outside the confidence limits. Similar to Max_DCure, a smaller average value exceeding 95% indicates less bias in the SPF.
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From page 74... ...
74 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results There is no current guideline on what this adjustment factor should be, and a trial and error approach is recommended by investigating multiple adjustment factors (e.g., 1.5, 1.25, 0.75, and 0.5)
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From page 75... ...
Reliability Associated with Predicting Outside the Range of Independent Variables 75 Option 5. Estimate Calibration Function or SPF with Different Parameters for AADT and the Other Factors, and Perform Calibration Option 5 is a combination of Option 3 and Option 4.
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From page 76... ...
76 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results Step B Execute One or More of the Five Options for Improving the Predictions Outside the Range of AADT, and Compare the GOF Statistics to Select the Best Option Compute the GOF measures for one or more of five options and compare the statistics to select the best option.
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From page 77... ...
Reliability Associated with Predicting Outside the Range of Independent Variables 77 Figure 7. CURE plots for MV crashes: Option 2.
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From page 78... ...
78 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results Figure 8. CURE plots for MV crashes: Option 3.
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From page 79... ...
Reliability Associated with Predicting Outside the Range of Independent Variables 79 Except for modified R2, the lower the value of each GOF measure, the better the performance for the option. The best option depends on the GOF measure that is chosen for consideration.
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From page 80... ...
80 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results As noted, except for modified R2, the lower the value of each GOF measure, the better the performance for the option. The best option depends on the GOF measure that is chosen for consideration.
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