Clear zone or W-beam guardrail for rural highways? A Full Bayes before-after evaluation by employing the Poisson Gamma-Lindley model.

Run-off-road crashes often result in fatal and severe injuries on rural highways. Clear zones and W-beam guardrails are often installed to reduce run-off-road crashes and the extent of severe injuries. While the effectiveness of clear zone improvement and W-beams has been studied, their comparative effects in a similar empirical setting are not well known. At the same time, crashes on rural highways are often characterized by a high frequency of zero counts; however, models capable of addressing this issue-such as the Poisson-Gamma Lindley models-have not yet been tested in before-after evaluations of engineering treatments. Towards this end, this study aims to examine the safety effectiveness of clear zones and W-beam guardrails on rural highways by applying a Full Bayes approach with Poisson-Gamma Lindley model that considers heterogeneities resulting from the preponderance of zeros. A total of 82.70 km of treated roadway segments with Clear zone improvements (0-5 m to 5-10 m) implemented in 2014 and 2015 were extracted from Bruce Highway in Queensland, Australia. W-beam guardrails were installed along a total of 45.5 km of the same rural highway between 2013 and 2017. The computed Crash Modification Factors (CMFs) show that the clear zone widening significantly reduced total injury (CMF = 0.57), fatal, serious, and moderate injury (CMF = 0.58), fatal and serious injury (CMF = 0.53), and run-off-road crashes (CMF = 0.63). W-beam guardrails are also found to significantly reduce total injury (CMF = 0.75), fatal, serious, and moderate injury (CMF = 0.77), fatal and serious injury (CMF = 0.81), and run-off-road crashes (CMF = 0.56). Overall, both countermeasures significantly reduce run-off-road crashes and other injury severities, including total injury, fatal, serious, and moderate injury, and fatal and serious injury crashes. W-beam guardrails have been found to provide greater reductions in run-off-road crash occurrences, whereas clear zone improvements are more effective in reducing injury severity by offering more forgiving roadside conditions.