Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results (2021) / Chapter Skim
Currently Skimming:
Pages 1-8
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 1... ...
1 This Guide presents the results of NCHRP Project 17-78, "Understanding and Communicating Reliability of Crash Prediction Models." The project was conducted by the Highway Safety Research Center at the University of North Carolina; Kittelson and Associates, Inc.; Persaud and Lyon, Inc.; and NAVIGATS Inc. Objectives The objectives of NCHRP Project 17-78 were to assist practitioners working in transportation and road safety analysis in the following areas: • How to quantify the impact of selecting or neglecting certain data parameters in the safety estimate predictions • How to estimate, interpret, and improve the reliability of predictions and use of crash modification factors (CMFs)
|
From page 2... ...
2 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results In this Guide, each scenario contains an overview of the method and the procedure used to assess the reliability of specific conditions, followed by an example illustrating the procedure and an explanation of how to communicate the results to managers, other practitioners, and the general public. The following scenarios are presented: • Quantifying the Reliability of CPM Estimates for Mismatches Between Crash Modification Factors and SPF Base Conditions (Chapter 2)
|
From page 3... ...
Introduction and Background 3 • Coefficient of variation (CV) for the increased root mean square error • Percent bias • Root mean square difference (RMSD)
|
From page 4... ...
4 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results where σe,I = increased root mean square error e = error in predicted crash frequency σ2abs = absolute difference of the change in variance of the predicted value kreported = reported overdispersion parameter for CPM kp,true = predicted true overdispersion parameter Np = predicted crash frequency from CPM, crashes/year Np,true = predicted true crash frequency, crashes/year Coefficient of Variation for the Increased Root Mean Square Error The increased root mean square error can be normalized by dividing it by the predicted true crash frequency. This division produces a coefficient of variation (CV)
|
From page 5... ...
Introduction and Background 5 Root Mean Square Difference The root mean square difference (RMSD) is a measure of the variability of the difference between the CPM prediction with error in input values and the CPM prediction with the estimated values bias.
|
From page 6... ...
6 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results distribution parameters, the practitioner estimates the value of absolute deviation that 85% of sites would be expected to be less than or equal to, or conversely, the value that 15% of sites may exceed. Spearman's Correlation Coefficient The Spearman's correlation coefficient (Rho)
|
From page 7... ...
Introduction and Background 7 ∑ = − MAD y y n i ii where yi = predicted crash values from the SPF yi = observed crash counts n = validation data sample size Coefficient of Variation of the Calibration Factor The coefficient of variation of the calibration factor [CV(C)
|
From page 8... ...
8 Reliability of Crash Prediction Models: A Guide for Quantifying and Improving the Reliability of Model Results • Maximum value exceeding 95% confidence limits (Max_DCure) : This measures the distance between the cumulative residuals and the 95% confidence limits if cumulative residuals are outside the confidence limits.
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.