While cable median barriers have drastically reduced fatal and other serious crashes, the barrier’s distance from the road may impact its safety effectiveness. MnDOT has a new method to estimate the change in the frequency of crashes based on this distance, facilitating informed decisions about installing or moving cable median barriers.
Cross-median vehicular crashes can have serious or fatal consequences. MnDOT uses high-tension cable median barriers to mitigate the impacts from vehicles driving off the road and into the median toward oncoming traffic. The cable in this crash countermeasure absorbs a vehicle’s impact, reducing the vehicle’s speed and redirecting the vehicle so it remains traveling in its original direction.
Minnesota has 774 miles of high-tension cable median barriers on divided multilane highways, which have reduced fatal and life-changing crashes and supported MnDOT’s Toward Zero Deaths goals. The most effective distance away from the road to install the barriers for the best outcomes, however, was still unknown.
“Since material costs are increasing, we needed to know if installing or moving cable barriers delivered the desired safety performance. This project shows the benefits justify the costs in many locations because crashes, including severe and fatal crashes, decrease when cable barriers are present and located the appropriate distance from the road,” said Maxwell Moreland, traffic safety crash data engineer, MnDOT Office of Traffic Engineering.
Also, while cable barriers can be repaired quickly and cost less than permanent concrete barriers, material costs have risen substantially. Changing a median design or regrading slopes to accommodate appropriately placed cable barriers may add significant cost.
High-tension cable median barriers have been a successful safety tool that MnDOT will continue to use. The agency wanted to ensure cable barriers are being installed in the optimal locations and to verify barrier effectiveness in reducing all types of crashes.
What Was Our Goal?
The goals of this project were to assess the change in crash frequency or severity after installing high-tension cable median barriers and to examine the change in the frequency or severity of crashes depending on the distance between the cable barrier and the road.
What Did We Do?
Researchers amassed and analyzed over a decade’s worth of data to identify correlations between vehicle crashes and high-tension cable median barriers. Data was collected on existing cable barrier locations and characteristics. Some data was not used due to inconsistencies; other excluded data included locations with discrepancies in pavement markings.
Barrier offsets from travel lanes were needed but not readily available, so the offsets were derived from geographic information system (GIS) tools based on measurements of median widths and distances between yellow pavement markings on roadsides, pavement edges and barriers. The research team spot-checked locations against aerial photography and verified calculations with MnDOT traffic engineers.
Roadway attribute data included median dimensions and characteristics; lane, centerline and edge dimensions and markings; and pavement striping. Lastly, recent and historical traffic volume and crash data were gathered. Crashes were delineated as either target crashes (vehicles that ran off the road onto the median, not necessarily into a cable barrier, and possibly overturned or traveled into oncoming traffic) and cable barrier crashes (vehicles that ran off the road and hit a cable median barrier).
“MnDOT can now make informed decisions when planning cable barrier projects. Alternatives can be compared by calculating crash modification factors for different scenarios,” said Richard Storm, traffic safety and planning engineer, HDR Engineering, Inc.
A relational database with spreadsheet files for roadway attributes and crash details provided the basis for a statistical analysis of safety effectiveness. A cross-section regression analysis was used to examine effects of varying the distance between the road edge and cable barrier by facilitating the calculation of crash modification factors (CMFs)—the expected number of crashes after implementing a countermeasure. Both sides of the road were evaluated independently for target, barrier and total crashes.
A less complicated before-and-after analysis evaluated the safety effectiveness of high-tension cable median barriers in general. Crash types that were examined included those resulting in a fatality, incapacitating or suspected serious injury, suspected minor injury and no injury. Target crashes were also distinguished, and the frequencies of all crashes were tallied and crash numbers where no cable barrier existed were compared to those where cable barriers had been installed.
What Did We Learn?
As expected and consistent with previous studies, crashes with fatalities, serious or minor injuries, and possible injuries decreased after the installation of cable barriers. The overall number of crashes was found to increase when a barrier was added, but that increase was primarily noninjury crashes.
Unique findings in this study demonstrated that as cable barriers are placed farther away from the inside edge of the road, all crash types decrease. As part of the analyses, CMF equations for total, target and barrier crashes were developed that will allow MnDOT to enter a potential cable barrier offset from the road’s edge to see how crash frequency would change.
This study gives MnDOT confidence in the value of high-tension cable median barriers. Cost comparisons of calculated CMFs to placing new cable barrier, moving existing barrier and implementing more extensive projects such as regrading or widening medians are already underway to understand the relative value of the choices.