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In this study, the effect of median guardrails in no-passing zone sections of a two-lane two-way highway was investigated. In this regard, the Firuzkuh-Qaemshahr highway in Mazandaran province of Iran which has 5.5 km median guardrails was selected. All types of crashes studied in two periods, before-and after-the implementation of median guardrails, were considered, and using crash predictive models for the two-way two-lane highway, the effect of implementation was explored. To evaluate the economic effects of this treatment, various crash cost factors were converted to monetary equivalents to compare the cost of crashes before-and after-applying the treatment. This study revealed that using median guardrails caused the monthly average of fatal and injury crashes to decrease by 84% and 29%, respectively. Moreover, the proportion of fatal and injury crashes to total crashes had a 55% drop, indicating that the severity of crashes was reduced, leading to fewer deaths or injuries. Also, this treatment had an impressive reduction in the average monthly cost of crashes, so that the largest reduction among all cost factors belonged to the cost of fatalities in traffic crashes with an 85% reduction, followed by the cost of lost time due to the traffic jams caused by fatal crashes with an 84% decrease. Furthermore, the economic analysis of the implementation of median guardrails revealed that this treatment reduced the average monthly traffic crash cost by 65%. Therefore, the median guardrails in no-passing zone sections of the two-way two-lane Firuzkuh-Qaemshahr highway had a remarkable economic benefit. [ABSTRACT FROM AUTHOR]