Feasibility of “Subjective” Engineering Assessments of Road Safety Improvements: Bayesian Analysis Development

Abstract

Regional safety program managers face a daunting challenge in the attempt to reduce deaths, injuries, and economic losses that result from motor vehicle crashes. This difficult mission is complicated by the combination of a large perceived need, small budget, and uncertainty about how effective each proposed countermeasure would be if implemented. A manager can turn to the research record for insight, but the measured effect of a single countermeasure often varies widely from study to study and across jurisdictions. The challenge of converting widespread and conflicting research results into a regionally meaningful conclusion can be addressed by incorporating “subjective” information into a Bayesian analysis framework. Engineering evaluations of crashes provide the subjective input on countermeasure effectiveness in the proposed Bayesian analysis framework. Empirical Bayes approaches are widely used in before-and-after studies and “hot-spot” identification; however, in these cases, the prior information was typically obtained from the data (empirically), not subjective sources. The power and advantages of Bayesian methods for assessing countermeasure effectiveness are presented. Also, an engineering evaluation approach developed at the Georgia Institute of Technology is described. Results are presented from an experiment conducted to assess the repeatability and objectivity of subjective engineering evaluations. In particular, the focus is on the importance, methodology, and feasibility of the subjective engineering evaluation for assessing countermeasures.

Evaluating traffic safety network screening: an initial framework utilizing the hierarchical Bayesian philosophy

“…For over 20 years, a technique known as empirical Bayesian (EB) has been proposed to address limitations of conventional SICL methods (Davis and Yang 2001;Hauer 1986Hauer ,1996Hauer et al 1986Hauer et al ,1988Hauer et al ,2002bHigle and Hecht 1989;Higle and Witkowski 1988;ITE 1999;Melcher et al 2001;Morris 1988;Pendleton 1988;Persaud 1988;Persaud and Hauer 1984;Saccomanno et al 2001;Schluter et al 1997). EB helps identify sites where mitigation might be most effective, increases confidence in estimates, and provides information on the relative safety of ranked sites.…”

“…findings, data) knowledge to obtain posterior (e.g., expected crashes) knowledge (Melcher et al 2001). Bayesian formalism allows adjustment of analyses based on external data, such as expert opinion.…”

“…Classical confidence intervals only produce a probability that the value lies somewhere in a range. Bayesian developments within traffic safety have focused on empirical Bayesian methods, but hierarchical Bayesian methods have also been explored recently (Davis and Yang 2001;Melcher et al 2001). Both merit further examination.…”

“…Melcher et al In 2001, Melcher, Dixon, Washington, and Hu assessed the feasibility of "subjective" engineering assessments of road safety improvements using Bayesian methods (Melcher et al 2001). They assert that regional safety program managers are challenged by the need to reduce impacts (i.e., fatalities, injuries, cost) of crashes while dealing with large perceived need, limited budgets, and uncertainty of countermeasure effectiveness.…”

“…Multiple software development efforts have been made or are in process (MRI 2002a,b,c,d,e,f,g;2003a,b,c,d,e), but use by practitioners is limited. Lately, researchers have begun to use hierarchical Bayesian methods more (Davis and Yang 2001 ;Melcher et al 2001), and the development of hierarchical Bayesian software has progressed (BUGS 2003).…”