Guidelines for the Development and Application of Crash Modification Factors (2022) / Chapter Skim
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Pages 189-224
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From page 189... ...
B-1 Contents B-2 Chapter 1 Introduction B-8 Chapter 2 Procedure B-8 Step 1 Determine Potential Overlap of Individual Treatment Effects B-11 Step 2 Determine Magnitude of Individual Treatment Effects B-11 Step 3 Define the Applicability of Individual CMFs B-11 Step 4 Select and Apply an Appropriate Method to Estimate the Combined Effect B-12 Estimating the Standard Error of the Combined Safety Effect of Multiple Treatments B-12 Extension of Method to Estimate the Combined Safety Effect of Three or More Treatments B-13 Chapter 3 Example Applications of Procedure B-20 Chapter 4 Supporting Research B-20 Combination of Centerline and Shoulder Rumble Strip Installation B-28 Combination of Lane and Shoulder Widening B-32 Combination of Intersection Skew Angle and Sight Distance Improvements B-35 References A P P E N D I X B Procedure for Estimating the Combined Safety Effect of Two Treatments
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B-2 Crash modification factors and functions (CMFs) provide users with an opportunity to quantify the safety performance of their decisions.
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-3 Method Summary of Method Dominant Effect Additive Effects Multiplicative Dominant Common Residuals Dominant Effect for Overlapping Crash Types The dominant effect method applies the CMF for only the most effective treatment (i.e., lowest CMF value)
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From page 192... ...
B-4 Guidelines for the Development and Application of Crash Modification Factors Background on the Application of Multiple CMFs The key factors to consider when applying multiple CMFs include • Selection of appropriate CMFs • Application of CMFs by type and severity • Bounds of combined treatment effect • Overlapping effects among individual treatments • Magnitude of individual treatment effects Selecting an Appropriate CMF The CMF selection process involves several considerations including availability of CMFs, applicability of available CMFs, and quality of applicable CMFs. The key to selecting an appropriate CMF is to identify the CMF that best matches the scenario at hand.
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From page 193... ...
Procedure for Estimating the Combined Safety Effect of Two Treatments B-5 bound is more difficult to establish. In theory, there is no limit on the minimum reduction in crashes, and this reduction can even be negative (i.e., an increase in crashes)
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B-6 Guidelines for the Development and Application of Crash Modification Factors 40% and 20% of total crashes, respectively. If the first treatment reduces motorcycle crashes by 50% and the second treatment reduces pedestrian crashes 50%, then the combined effect is a 30% reduction in total crashes.
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-7 of the combined effect from the four methods. Specifically, the results range from 0.40 to 0.70 (a difference of 0.30)
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B-8 This procedure is intended to guide the decision process for selecting the most appropriate method to estimate the combined effect of two treatments. Figure B1 is a flowchart that illustrates the steps of the procedure.
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-9 Step 1: Determine Potential Overlap of Individual Treatment Effects Case A: Zero overlap Case B: Some overlap Case C: Complete overlap Case D: Enhancing effects Case E: Counteracting effects Step 2: Determine Magnitude of Individual Treatment Effects Small (< 10% change) Medium (10 - 25% change)
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B-10 Guidelines for the Development and Application of Crash Modification Factors Overlap Method Case A Case D Additive Effects with Maximum Reduction of 100% (i.e., CMF = 0) Assuming no overlap among treatment effects, one would expect the full benefit of each treatment.
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From page 199... ...
Procedure for Estimating the Combined Safety Effect of Two Treatments B-11 Step 2 Determine Magnitude of Individual Treatment Effects The second step is to determine the magnitude of the individual treatment effects based on the levels defined in Table B7. If both individual treatment effects are small (i.e., less than 10% change)
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B-12 Guidelines for the Development and Application of Crash Modification Factors Estimating the Standard Error of the Combined Safety Effect of Multiple Treatments The standard error of the CMF is needed for several activities, such as assessing the quality of a CMF and combining multiple CMF estimates for the same treatment. Given that the use of individual CMFs to estimate the combined effect of multiple treatments occurs after the quality rating process and after the combination of individual CMFs for the same treatment, there is limited value or need for the standard error of the combined effect of multiple treatments.
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B-13 This section provides additional examples to illustrate how to navigate the decision process and then how to apply an appropriate method to estimate the combined effect of two treatments. Example 1 Complete Overlap among Treatment Effects and Same Applicability An analyst is considering two treatments, lane widening and shoulder widening, to address safety concerns related to run-off-road crashes on curves along a rural, two-lane, undivided road.
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B-14 Guidelines for the Development and Application of Crash Modification Factors Step 4 Select and Apply an Appropriate Method to Estimate the Combined Effect The fourth step is to select and apply an appropriate method to estimate the combined effect of the treatments. The following is a summary of the considerations from steps 1–3 for this example.
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-15 Step 1 Determine Potential Overlap of Individual Treatment Effects The first step is to determine the potential overlap of the individual treatment effects and select the category from Table B6 that best matches the scenario at hand. Edgeline pavement markings target run-off-road right crashes, but may affect other crash types such as head-on, sideswipe opposite direction, and run-off-road left.
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B-16 Guidelines for the Development and Application of Crash Modification Factors undivided roads range from 0.53 to 0.86. This suggests that there is an enhancing effect of the combined treatment.
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-17 Step 4 Select and Apply an Appropriate Method to Estimate the Combined Effect The fourth step is to select and apply an appropriate method to estimate the combined effect of the treatments. The following is a summary of the considerations from steps 1–3 for this example.
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B-18 Guidelines for the Development and Application of Crash Modification Factors median barrier targets cross-median crashes, including head-on and opposite direction sideswipe. Shoulder rumble strips target run-off-road crashes as well as the head-on and opposite direction sideswipe crashes related to vehicles crossing the median.
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-19 assume the estimated crashes without treatment is 8.9 crashes per year, which includes 2.3 crossmedian head-on crashes, 1.3 cross-median sideswipe opposite direction crashes, and 5.3 run-offroad crashes.
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B-20 This section on supporting research provides details related to the CMF development effort to develop the procedure for estimating the combined safety effect of two treatments at the same location. The team for Project 17-63 developed CMFs for the following three combination treatments, including CMFs for both the individual and combined treatment effects.
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From page 209... ...
Procedure for Estimating the Combined Safety Effect of Two Treatments B-21 where, m = expected number of crashes before the treatment w = weight to estimate EB expected crashes P = sum of annual SPF estimates "before" treatment at the treatment sites α = inverse of the dispersion parameter from the SPF. Note the value of α is estimated from the SPF calibration process with the use of a maximum likelihood procedure, and a larger value of α indicates less dispersion.
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B-22 Guidelines for the Development and Application of Crash Modification Factors where, length = segment length (miles) Constant = constant estimated during modeling process Log(AADT)
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-23 Variable Coefficient Standard Error Zscore Pvalue Lower 95% Conf.
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B-24 Guidelines for the Development and Application of Crash Modification Factors Variable Coefficient Standard Error Zscore Pvalue Lower 95% Conf.
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-25 Variable Coefficient Standard Error Zscore Pvalue Lower 95% Conf.
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B-26 Guidelines for the Development and Application of Crash Modification Factors Variable Coefficient Standard Error Zscore Pvalue Lower 95% Conf.
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-27 Variable Coefficient Standard Error Zscore Pvalue Lower 95% Conf.
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B-28 Guidelines for the Development and Application of Crash Modification Factors Combination of Lane and Shoulder Widening For this analysis, the team employed a rigorous cross-sectional method to estimate CMFs for the individual and combined treatment effects. The two combined treatments of interest for tangent sections are (1)
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-29 The multivariate models were developed by identifying base models with traffic volume only, exploring the effects of adding other predictor variables to the models, and then selecting the final model. Having developed the base models for each crash type (traffic volume only)
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B-30 Guidelines for the Development and Application of Crash Modification Factors Variable Coefficient Standard Error Zscore P-value Lower 95% Conf.
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-31 Variable Coefficient Standard Error Zscore P-value Lower 95% Conf.
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B-32 Guidelines for the Development and Application of Crash Modification Factors Combination of Intersection Skew Angle and Sight Distance Improvements For this analysis, the team employed a cross-sectional modeling approach to estimate CMFs for the individual treatment effects as well as the combined treatment effect. The two individual treatment effects were defined as: • Individual intersection sight distance (ISD)
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Procedure for Estimating the Combined Safety Effect of Two Treatments B-33 additional variables were considered. Once a variable was included in the model, the estimated parameters and associated standard errors were examined to determine: • Is the direction of effect (i.e., expected decrease or increase in crashes)
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B-34 Guidelines for the Development and Application of Crash Modification Factors Variable Coefficient Standard Error Zscore P-value Lower 95% Conf.
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B-35 American Association of State Highway and Transportation Officials (AASHTO)
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