title: Comparison of empirical Bayes and full Bayes approaches for before-after road safety evaluations.
abstract: The empirical Bayes (EB) approach has now gained wide acceptance among researchers as the much preferred one for the before-after evaluation of road safety treatments. In this approach, the before period crash experience at treated sites is used in conjunction with a crash prediction model for untreated reference sites to estimate the expected number of crashes that would have occurred without treatment. This estimate is compared to the count of crashes observed after treatment to evaluate the effect of the treatment. This procedure accounts for regression-to-the-mean effects that result from the natural tendency to select for treatment those sites with high observed crash frequencies. Of late, a fully Bayesian (FB) approach has been suggested as a useful, though complex alternative to the empirical Bayes approach in that it is believed to require less data for untreated reference sites, it better accounts for uncertainty in data used, and it provides more detailed causal inferences and more flexibility in selecting crash count distributions. This paper adds to the literature on comparing the two Bayesian approaches through empirical applications. The main application is an evaluation of the conversion of road segments from a four-lane to a three-lane cross-section with two-way left-turn lanes (also known as road diets). For completeness, the paper also summarizes the results of an earlier application pertaining to the evaluation of conversion of rural intersections from unsignalized to signalized control. For both applications, the estimated safety effects from the two approaches are comparable.
authors.affiliation: Department of Civil Engineering, Ryerson University, 350 Victoria Street, Toronto, M5B 2K3, Canada. bpersaud@ryerson.ca
authors.forename: Bhagwant
authors.initials: B
authors.lastname: Persaud
authors.forename: Bo
authors.initials: B
authors.lastname: Lan
authors.forename: Craig
authors.initials: C
authors.lastname: Lyon
authors.forename: Ravi
authors.initials: R
authors.lastname: Bhim
mesh_terms: D000063:Accidents, Traffic / Q000517:prevention & control* / Q000706:statistics & numerical data*; D001334:Automobile Driving / Q000331:legislation & jurisprudence; D001499:Bayes Theorem; D004779:Environment Design; D006801:Humans; D018570:Risk Assessment / Q000379:methods; D012449:Safety; D012926:Social Control, Formal