Other penetration causes were identified that led to penetrations on low-tension cable median barrier systems, including impacts with large vehicles (i.e., tractor trailers-or single-unit trucks), and bouncing override penetrations due to median slopes. However, as with the high- tension cable median barrier counterparts, bouncing override penetrations were impact condition dependent, and no cable barrier system manufactured had been tested and approved to redirect large vehicles (i.e. tractor-trailers).
Causes for bounce-over of low-tension systems were virtually identical to other systems. Since the arguments and outcomes were similar, the causes are not discussed here. Out of a recorded 70 cable median barrier penetrations on low-tension cable median barriers, only 2 resulted in a bounce-over. This could be the result of barrier placement, which was frequently near the center of the median or on the shoulder instead of up the median slope where vertical vehicle rebound was highest. Bounce-over penetrations may also be reduced by easy release of the top cable. Without sufficient additional crash information, the low frequency of bounce-over penetration crashes cannot be explained.
7.3.2.6 Discussion
Low-tension, 3-cable median barriers were the most susceptible barrier type to prying and diving underride penetrations on a fixed median terrain and with a fixed barrier placement, and the least susceptible to override penetration based on crash results obtained in this research effort. Strong cable-to-post attachments are essential for bottom and middle cables in low- tension cable median barrier systems. Reducing the vertical upward compliance of the lower cable could reduce the number of diving penetrations. However, care must be taken to prevent excessive stiffening of the cable against downward vertical motion on the post, or the lower cable will become a trip point for rollovers.
The advantage of the weak top cable-to-post connection is that with a low vertical release load, underriding vehicles did not experience occupant compartment crushing due to the top cable, and overrides were very infrequent when the cables quickly released from the post but remained engaged with the vehicle. This stratification of cable-to-post attachment strengths has not been optimized by any cable median barrier system in use to date and could be necessary to further improve cable median barrier design.
There is no evidence yet that many (if any) penetration crashes were caused due to cables which drooped in one location due to a previous crash. This may be due to excellent DOT response of rapid barrier repair at crash sites, which prevented these conditions from occurring. However, crashes in which two or more vehicles struck the cable median barrier only resulted in penetration if one of the vehicles was either a tractor-trailer or was towing a trailer, based on the available database. In both of these crash types, the vehicles which struck the barrier were not within the designed performance limits of the barriers. Further, crashes in which multiple vehicles struck the cable median barrier only constituted approximately 3% of all crashes in the database. If such a correlation existed and penetration propensity was higher when vehicles struck near previous crash locations, insufficient data was present in the database to indicate this increase. Nonetheless, states with low-tension cable median barrier systems often mandate barrier maintenance and repair within 48 hours of the crash notification. Even when repairs happen up to a two weeks after a crash, the repair timeframe appears to be adequate to prevent penetration events caused by previous cable barrier crashes from occurring. Shorter repair windows may be required during winter months when icy road conditions increase crash frequency.
8 CAUSES OF CABLE MEDIAN BARRIER ROLLOVERS 8.1 Overview of Crash Data
Unlike cable median barrier penetrations, rollovers caused by cable median barriers have not been well-studied. Manufacturers have noted cable barrier rollovers, but findings from these studies have not generally been made public. Frequently when rollovers were observed during full-scale tests on cable median barriers, results were largely dismissed as specific to an impact configuration or design concept failure [e.g. 41, 42]. Nonetheless, rollovers are real concerns for impacts with cable median barriers. In most states, rollover crashes were more severe but occurred less often than penetrations. Rollovers typically occurred in 3% to 8% of all cable median barrier crashes.
Rollover events were particularly cumbersome to reconstruct since rollover crashes were subject to many more factors than were penetration crashes. An accurate determination for the causes of the rollover was extremely difficult to obtain as median profile and smoothness, vehicle roof stiffness, angle of roll eccentricity, vehicle weight, and trip speed all affect the path of a rolling vehicle and the predominant locations of vehicle damage. Further, many scene diagrams are the approximate representations of a crash site and are drawn by the responding officer. As such, these diagrams were frequently inexact. One trait of all rollover crashes on cable median barriers is that, at the time of the rollover, all vehicles were non-tracking. Vehicles involved in rollovers which initially contacted the barrier with tracking impact conditions all yawed to non-tracking conditions before tripping.
Despite the difficulty in gauging trip causation, common factors were identifiable through narrative, scene diagram, photographic, and median slope evidence. Common factors associated with rollovers enable researchers and manufacturers to recommend improvements addressing
largely independent of which system was installed, although rollover frequencies varied between systems. This finding was reasonable as all systems rely on three basic components: support posts to maintain cables at desired heights; cable-to-post attachments to maintain cable heights and transmit lateral and vertical load from cables to posts; and multiple tensioned cables. Similar to the determination of the causes of penetrations, a detailed investigation and analysis was conducted to determine the causes of rollovers on cable median barriers. A summary table of rollover causes is shown in Table 25.
Table 25. Causes of Rollovers with Cable Median Barriers