Cable Median Barrier with Inside Shoulder Rumble Strips on Divided Roads Raghavan Srinivasan, Bo Lan, & Daniel Carter, UNC Highway Safety Research Center

Background Cable median barriers introduced in the United States in the 60’s Initially low tension; later higher tension became more common Can take multiple hits before requiring repair Many states have started using cable barrier instead of concrete barrier Very few studies have examined the combination of cable barrier with inside shoulder rumble strips Study funded by FHWA as part of the DCMF contract

Objectives Develop crash modification factors (CMFs) for combination of cable median barrier and inside shoulder rumble strips Specific combination may vary depending on the state Disaggregate analysis to determine if CMFs are associated with: AADT Median width Expected crashes in the before period

Method Empirical Bayes before-after evaluation Identify a reference group of similar sites (without a treatment) and develop safety performance functions (SPFs) Estimate the expected number of crashes in the after period had the treatment not been implemented Estimate CMFs along with the standard error of CMFs

Data Expressway, freeway, or interstate 4 states Kentucky: 323 miles (R), 66 miles (T) Illinois: 401 miles (R), 100 miles (T) Wisconsin: 745 miles (R), 185 miles (T) Missouri: 288 miles (T) (system-wide implementation) Not exactly the same “treatment” in all states Kentucky, Illinois, and Wisconsin Before period: rumble strip without barrier After period: rumble strip with barrier Missouri Before period: no rumble strip or barrier After period: rumble strip with barrier

Total Injury and fatal (KABC) Injury and fatal (KAB) Cross median Injury and fatal cross median (KABC) Median-related Area type (urban vs rural) and crashes during winter (Dec, Jan, Feb) All types mentioned above Definition of cross- median crashes Some previous studies did a manual review of crash reports We relied on coded crash types o Cross median indicator (Missouri) o Head on o Opposite direction sideswipe Multiple definitions possible Definition of median related Crash Types

Data Issues in Kentucky Initial results in Kentucky showed large increases (more than 20%) in injury and fatal Crashes Conversation with Kentucky DOT and Kentucky Transportation Center (University of Kentucky) o Detailed discussion over many weeks o We found that ramp crashes in the more recent years need to be excluded based on the last three digits of the route code o Problems with the older data in Jefferson County (one of the largest counties in the state) – Had to remove 58 sites (about 50 miles) from the analysis (out of a total of 116 miles)

Kentucky Crash TypeEB Expected AfterObserved AfterCMFS.E. of CMF Total KABC KAB Cross median Cross median (KABC) Note: Cross median based on head-on and opposite direction sideswipe

Illinois Crash TypeEB Expected After Observed After CMFS.E. of CMF Total KABC KAB Cross median Cross median (KABC) Note: Cross median based on head-on and opposite direction sideswipe

Wisconsin Crash TypeEB Expected After Observed After CMFS.E. of CMF Total KABC Cross median Cross median (KABC) Note: Cross median based on head-on and opposite direction sideswipe

Missouri Crash TypeEB Expected After Observed After CMFS.E. of CMF Total KABC KAB Cross median Cross median + head- on Cross median + head- on (KABC) Note: Cross median is a crash type in Missouri

Summary of Findings Cross median crashes decreased significantly in all states except Wisconsin Largest reduction in cross median crashes were in Missouri Combined effect of rumble strips and barrier? Different definition for cross median? Total crashes increased significantly in all states (due to collisions with barriers) except Wisconsin No significant change in injury crashes in Kentucky Currently working on disaggregate analysis