Accident Analysis and Prevention 42 (2010) 2099–2107 Contents lists available at ScienceDirect Accident Analysis and Prevention journal homepage: www.elsevier.com/locate/aap A comparative Full Bayesian before-and-after analysis and application to urban road safety countermeasures in New Jersey Ozlem Yanmaz-Tuzel ∗ , Kaan Ozbay 1 Rutgers Intelligent Transportation Systems (RITS) Laboratory, Department of Civil and Environmental Engineering, Rutgers, The State University of New Jersey, 623 Bowser Rd., Piscataway, NJ 08854, USA article info Article history: Received 11 December 2009 Received in revised form 28 June 2010 Accepted 30 June 2010 Keywords: Bayesian methods Safety performance functions Before-and-after analysis Hierarchical models abstract This paper develops a step-by-step methodology for the application of Full Bayes (FB) approach for before- and-after analysis of road safety countermeasures. As part of this methodology, it studies the posterior prediction capability of Bayesian approaches and their use in crash reduction factor (CRF) estimation. A collection of candidate models are developed to investigate the impacts of different countermeasures on road safety when limited data are available. The candidate models include traditional, random effects, non-hierarchical and hierarchical Poisson-Gamma and Poisson-Lognormal (P-LN) distributions. The use of random effects and hierarchical model structures allows treatment of the data in a time-series cross- section panel, and deal with the spatial and temporal effects in the data. Next, the proposed FB estimation methodology is applied to urban roads in New Jersey to investigate the impacts of different treatment measures on the safety of “urban collectors and arterial roads” with speed limits less than 45 mph. The treatment types include (1) increase in lane width, (2) installation of median barriers, (3) vertical and horizontal improvements in the road alignment; and (4) installation of guide rails. The safety perfor- mance functions developed via different model structures show that random effects hierarchical P-LN models with informative hyper-priors perform better compared with other model structures for each treatment type. The individual CRF values are also found to be consistent across the road sections, with all showing a decrease in crash rates after the specific treatment except guide rail installation treatment. The highest decrease in the crash rate is observed after the improvement in vertical and horizontal align- ment followed by increase in lane width and installation of median barriers. Overall statistical analyses of the results obtained from different candidate models show that when limited data are available, P-LN model structure combined with higher levels of hierarchy and informative priors may reduce the biases in model parameters resulting in more robust estimates. © 2010 Elsevier Ltd. All rights reserved. 1. Introduction and background Today U.S. motorists travel almost 80% more miles on urban arterials compared with rural arterials. Fatal crashes are still more likely to occur on rural roads, but most crashes, including serious ones involving occupant injuries, occur on urban streets (FHWA, 2009). According to a recent National Highway Transportation Safety Administration report (2008) urban areas account for 45% of the fatal crashes and 44% of the fatalities. Moreover, in 2006, 63% of all urban crashes occurred on roadways where the posted speed limit was 50 mph or less (NHTSA, 2008). According to 2008 highway statistics, specifically in New Jersey (NJ), 49% of all fatality crashes ∗ Corresponding author. Tel.: +1 732 445 0576; fax: +1 732 445 0577. E-mail addresses: yanmaz@rci.rutgers.edu, ozlem.yanmaz@gmail.com (O. Yanmaz-Tuzel), kaan@rci.rutgers.edu (K. Ozbay). 1 Tel.: +1 732 445 2792; fax: +1 732 445 0577. occurred on urban arterials and urban collectors, emphasizing the importance of safety analysis on urban roads with lower speed lim- its (FHWA, 2009). However, in the past, the main focus of the most of the safety research has been on intersections, major arterials and freeway sections. On the other hand, there is limited research on the safety effects of different countermeasures for urban arterials and collectors with lower speed limits (Harwood et al., 2007; Potts et al., 2007). Thus, there is an urgent need to study the effective- ness of various safety treatments on urban arterials with low speed limits. The basic approach to increase road safety consists of imple- menting a combination of different countermeasures aimed at improving vehicles, roads, and road user behavior. The choices among them are driven by analyzing their cost-effectiveness via before-and-after analysis. While conducting a before-and-after analysis expected crash rate is primarily represented by safety per- formance functions (SPFs) that relate the expected crash rate to its traffic and road characteristics. Traditionally, SPF estimation in the 0001-4575/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.aap.2010.06.023