title: Comparison of safety effect estimates obtained from empirical Bayes before-after study, propensity scores-potential outcomes framework, and regression model with cross-sectional data.
abstract: A variety of different study designs and analysis methods have been used to evaluate the performance of traffic safety countermeasures. The most common study designs and methods include observational before-after studies using the empirical Bayes method and cross-sectional studies using regression models. The propensity scores-potential outcomes framework has recently been proposed as an alternative traffic safety countermeasure evaluation method to address the challenges associated with selection biases that can be part of cross-sectional studies. Crash modification factors derived from the application of all three methods have not yet been compared. This paper compares the results of retrospective, observational evaluations of a traffic safety countermeasure using both before-after and cross-sectional study designs. The paper describes the strengths and limitations of each method, focusing primarily on how each addresses site selection bias, which is a common issue in observational safety studies. The Safety Edge paving technique, which seeks to mitigate crashes related to roadway departure events, is the countermeasure used in the present study to compare the alternative evaluation methods. The results indicated that all three methods yielded results that were consistent with each other and with previous research. The empirical Bayes results had the smallest standard errors. It is concluded that the propensity scores with potential outcomes framework is a viable alternative analysis method to the empirical Bayes before-after study. It should be considered whenever a before-after study is not possible or practical.
authors.affiliation: Department of Civil and Environmental Engineering, The Pennsylvania State University, 231 Sackett Building, University Park, PA 16802, USA. Electronic address: jsw277@psu.edu.
authors.forename: Jonathan S
authors.initials: JS
authors.lastname: Wood
authors.affiliation: Department of Civil and Environmental Engineering, The Pennsylvania State University, 231 Sackett Building, University Park, PA 16802, USA. Electronic address: edonnell@engr.psu.edu.
authors.forename: Eric T
authors.initials: ET
authors.lastname: Donnell
authors.affiliation: Department of Civil and Environmental Engineering, University of Utah, 110 Central Campus Drive, Suite 2000, Salt Lake City, UT 84112, USA. Electronic address: richard.jon.porter@utah.edu.
authors.forename: Richard J
authors.initials: RJ
authors.lastname: Porter
mesh_terms: D000063:Accidents, Traffic / Q000706:statistics & numerical data*; D001499:Bayes Theorem; D065187:Controlled Before-After Studies; D003430:Cross-Sectional Studies; D004779:Environment Design; D006801:Humans; D015233:Models, Statistical; D064887:Observational Studies as Topic; D057216:Propensity Score; D012044:Regression Analysis; D012107:Research Design; D012189:Retrospective Studies; D012449:Safety
keywords: Crash modification factor; Cross-sectional study; Empirical Bayes; Mixed-effects negative binomial; Potential outcomes; Propensity scores; Safety Edge treatment; Safety performance function; Study design