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Pages 57-69

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From page 57...
... 57   Introduction In HSM Part C, a simple two-stage approach is used when estimating average predicted crash frequencies by type and severity using an SPF: 1. The total crash frequency is estimated for a specific facility type.
From page 58...
... 58 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results Scenario 4, Case B There are a few SPFs for specific/given rare crash types and severities with Low confidence because Ivan et al. (under NCHRP Project 17-62)
From page 59...
... Reliability Associated with Using a Crash Prediction Model to Estimate Frequency of Rare Crash Types and Severities 59   Step 6. Compute the CV of the estimated calibration factor for each parent SPF and the CURE plots.
From page 60...
... 60 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results factor for each SPF or SPF and CMF combination. The Calibrator can be downloaded from the link: (https://safety.fhwa.dot.gov/rsdp/toolbox-content.aspx?
From page 61...
... Reliability Associated with Using a Crash Prediction Model to Estimate Frequency of Rare Crash Types and Severities 61   Outline of Solution Step 1. Select the Parent SPF(s)
From page 62...
... 62 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results Step 5. Calibrate the Parent SPF(s)
From page 63...
... Reliability Associated with Using a Crash Prediction Model to Estimate Frequency of Rare Crash Types and Severities 63   outside the 95% confidence limits indicate a poor fit over that range in the variable of interest. If these residuals are frequently outside the confidence limits, possible bias in the SPF is indicated.
From page 64...
... 64 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results Step 3. Assemble data needed to develop the crash type and severity proportions.
From page 65...
... Reliability Associated with Using a Crash Prediction Model to Estimate Frequency of Rare Crash Types and Severities 65   Step 6. Calibrate the Parent SPF(s)
From page 66...
... 66 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results Outline of Solution Step 1. Select the Uncalibrated and Unreliable SPF Several base-condition SPFs developed under NCHRP Project 17-62 did not validate well or had poor GOF statistics.
From page 67...
... Reliability Associated with Using a Crash Prediction Model to Estimate Frequency of Rare Crash Types and Severities 67   Step 5. Apply the Crash Type and Severity Proportions to the Respective Parent SPF(s)
From page 68...
... 68 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results Figure 3. The Calibrator CURE plot of residuals based on NCHRP Project 17-62 estimated base-condition SPF (Step 1)
From page 69...
... Reliability Associated with Using a Crash Prediction Model to Estimate Frequency of Rare Crash Types and Severities 69   In this case, the CURE plots suggest that Option 1 or Option 2 would be somewhat superior to the original model in that the cumulative residuals for these options oscillate closer to the x-axis and stay within the 2σ limits. The Calibrator output indicated that the percentage of points in the CURE plot exceeding the 2σ limits is 29% for the original model, 19% for Option 1, and 13% for Option 2.

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