RES2013-28: Guidance for Site Selection, Safety Effectiveness Evaluation, and Crash Modification Factors of Median Cable Barriers in Tennessee

This project evaluated the median cable barrier safety effectiveness as experienced on Tennessee highways. Apart from descriptive statistics and questionnaire survey, before and after statistical evaluation was used to estimate the cable barriers Safety Effectiveness. The study revealed that installation of Cable Barriers in Tennessee considered the following criteria: whether the posted speed limit is equal to or greater than 45 mph, clear zone, median width, crash volume and information gathered from interviews with local officials. Questionnaire survey from other states showed that the effectiveness of the cable barriers matches those experienced in Tennessee whereby the number of fatal and severe injury crashes was highly reduced while the property damage only (PDO) crashes increased after median cable barriers installation. The study found that the statewide cable barriers Safety Effectiveness (statistical percentage change in crash frequency statewide) for fatal crashes is 94%, incapacitating injury crashes is 92% and fatal and incapacitating injury crashes combined is also 92%. The safety effectiveness for fatal and all injury crashes combined was found to be 85%. Overall, 98% of the evaluated median cable barrier segments (561 out of 571) had 100% safety effectiveness for fatal crashes while 94% of the cable barrier segments had 100% safety effectiveness based on fatal and incapacitating crashes combined. In a direct comparison through descriptive statistics, statewide fatal crashes were reduced by 82% after the cable barriers installation while the incapacitating injury crashes were reduced by 76%. In addition, head-on crashes went down by 96% and crashes involving two or more vehicles went down by 92%. Fatalities due to median crossover crashes were reduced by 83% while number of people injured went down by 71% as a result of cable barriers. The study further developed Crash Modification Factors (CMF) for median cable barriers in Tennessee. Using comparison group approach that considers the number of crashes before and after the installation of cable barriers, CMF for fatal crashes was found to be 0.04, fatal and incapacitating injury 0.07, and 0.14 for fatal and all injury crashes. The developed CMFs translate into crash reduction percentages of 96% and 86% for fatal only and fatal and all injuries combined respectively. Wider cable offsets from the travel lane and wider inside shoulders were found to help reduce the number of severe median crossover crashes while segments with high differential elevations and high speed limit had higher number of crashes compared to the opposite measures.


  • English


  • Status: Completed
  • Funding: $96713
  • Contract Numbers:


  • Sponsor Organizations:

    Tennessee Department of Transportation

    James K. Polk Building
    Fifth and Deaderick Street
    Nashville, TN  United States  37243-0349
  • Managing Organizations:

    Tennessee Department of Transportation

    James K. Polk Building
    Fifth and Deaderick Street
    Nashville, TN  United States  37243-0349
  • Performing Organizations:

    Tennessee State University

    Nashville, TN  United States  37209
  • Principal Investigators:

    Chimba, Deo

  • Start Date: 20131215
  • Expected Completion Date: 20170831
  • Actual Completion Date: 0
  • USDOT Program: Transportation, Planning, Research, and Development

Subject/Index Terms

Filing Info

  • Accession Number: 01762844
  • Record Type: Research project
  • Source Agency: Tennessee Department of Transportation
  • Contract Numbers: RES2013-28
  • Files: RIP, STATEDOT
  • Created Date: Jan 27 2021 3:36PM