Investigating the influence of passing relief lane sections on safety and traffic performance

As it is well known, one of the keys to improve traffic performance of two lane rural roads with high traffic volumes is to provide passing sections. However, providing a long segment in which the passing maneuver is allowed, is not always feasible, especially when suburban areas are considered. The retrofitting of some road sections by adding a passing relief lane can improve traffic performance by reducing platoons, driver delays and increasing speed. Nevertheless, the effects of this measure on safety may be controversial. With higher traffic speed, the diverging and merging conflicts may escalate and deteriorate the safety conditions of the treated sites. The present research seeks to address this dilemma by presenting an operation and safety study based on experimental data from two lane rural roads. Serious crashes with fatalities and injured people were taken into account in the estimation of the Crash Modification Factors. The Empirical Bayes before-after study was performed for a period from 2005 to 2013, with the exclusion of 2009 when road segments were retrofitted by adding a passing relief lane. Certain improvements in safety were observed for both total and target crashes. The research also encompassed traffic performance by way of investigating the changes in speed and platoon size at the beginning and at the end of the treated sections. The results have shown that the platoon reduction value depends on the length of the passing section and the share of heavy vehicles. The obtained operational results proved less beneficial than expected.