A fully Bayesian multivariate approach to before–after safety evaluation

Full Bayesian evaluation of the safety effects of reducing the posted speed limit in urban residential area

“…The notable difference between the EB and the univariate FB approaches was that the estimates of the effects had much lower standard deviations for the FB approach, indicating that the precision of the estimates was much higher for the FB approach. This finding aligns with previous research findings by Lan et al (2009) andPersaud et al (2010), but contradicts the findings of Park et al (2010). Moreover, while only severe crash reduction in the EB method was statistically significant at a 95% confidence level, all the reductions in the univariate FB method were statistically significant.…”

“…In terms of the deviance information criterion (DIC), authors found that the multivariate Poisson-lognormal (MVPLN) model provided a superior fit over the univariate PLN model, which is in line with earlier research on multivariate analysis (Aguero-Valverde and Jovanis, 2009;El-Basyouny and Sayed, 2009a;Park and Lord, 2007). It is worth noting that the model form used by Park et al (2010) is different from the one used by Lan et al (2009) andPersaud et al (2010). Lan and Persaud (2012) used the univariate and multivariate FB methods to evaluate a hypothetical case.…”

“…A number of studies have used the FB method in observational before-after safety evaluations (El-Basyouny and Sayed, 2010, 2012a,b, 2013Lan et al, 2009;Lan and Persaud, 2012;Lan and Srinivasan, 2013;Li et al, 2008;Park et al, 2010;Pawlovich et al, 2006;Persaud et al, 2010). Within the FB evaluation framework, crash data of different types or severities were modelled either independently, known as the univariate approach Sayed, 2010, 2012a,b;Lan et al, 2009;Lan and Persaud, 2012;Lan and Srinivasan, 2013;Li et al, 2008;Park et al, 2010;Pawlovich et al, 2006;Persaud et al, 2010), or jointly, known as the multivariate approach (Chib and Winkelmann, 2001;Sayed, 2011, 2013;Lan and Persaud, 2012;Lan and Srinivasan, 2013;Park et al, 2010).…”

“…Within the FB evaluation framework, crash data of different types or severities were modelled either independently, known as the univariate approach Sayed, 2010, 2012a,b;Lan et al, 2009;Lan and Persaud, 2012;Lan and Srinivasan, 2013;Li et al, 2008;Park et al, 2010;Pawlovich et al, 2006;Persaud et al, 2010), or jointly, known as the multivariate approach (Chib and Winkelmann, 2001;Sayed, 2011, 2013;Lan and Persaud, 2012;Lan and Srinivasan, 2013;Park et al, 2010). The multivariate approach takes into account that crash data of different severities or types are correlated, while the univariate approach fails to do so.…”

“…Meanwhile, Park et al (2010) applied the EB and the univariate and multivariate FB methods to evaluate the posted speed limit (PSL) reduction on various Korean expressways. They found that for the low sample mean, safety effects estimated by the two methods were quite different.…”