A cable barrier is also called a guard or wire rope safety barrier (WRSB). They are becoming more prevalent on divided highways, especially in my state of Georgia.
They are roadside or median safety traffic barriers/guard rails of steel wire ropes mounted on weak posts. As with any roadside barrier, its primary purpose is to prevent a vehicle from leaving the highway and striking a fixed object, a terrain feature, or an oncoming vehicle. Also similar to most roadside barriers, cable barriers function by capturing and redirecting the errant vehicle.
Compared to concrete step barriers these barriers are inexpensive. Cross-median crashes are particularly severe given the opposing directions of traffic on divided highways making them increasingly prevalent worldwide.
While median width plays a large role in the occurrence of these crashes, increased width alone does not eliminate them and quite often, the median must be shielded with a barrier. Cable barriers provide a cost-effective solution to the shielding issue.
The system is more forgiving than traditional concrete (Jersey) barriers or steel barriers used today and remains effective when installed on sloping terrain. The flexibility of the system absorbs impact energy and dissipates it laterally, which reduces the forces transmitted to the vehicle occupants.
There are two types of cable barrier systems in use today, low-tension and high-tension. Low tension simply means the cables themselves are tensed only enough to eliminate sag between posts. Large springs at both ends of the cable run are compressed (according to temperature) to maintain the tension in the system.
When a vehicle impacts the low-tension system under normal conditions, the cable moves as much as 12 ft from its original location. This movement is known as dynamic deflection. Given the lack of tension in the system, individual installations, or “runs”, of cable are limited to 2,000 ft with an anchor assembly at each end.
Due to the low tension of the system, the cables tend to lie on the ground in the event that an impact damages multiple posts. As such, there is no residual safety value within the undamaged remainder of the 2,000 ft installation and that entire section of barrier will remain nonfunctional until repaired. The high tension barriers, in appearance, look the same as the low.
High-tension cable consists of three or four pre-stretched cables supported by weak posts. During installation, the cables are placed on the posts and then tightened to a specific tension according to temperature. The tension values range between approximately 2,000 and 9,000lb.
Due to this tightening, the cable installations can be of indefinite length. In fact, the lengths of the runs are usually only limited by the presence of obstacles such as median openings or bridge columns.
When a vehicle impacts the high-tension system under normal conditions, the cable deflects as little as 8 ft from its original location. The inherent tension within the system also allows the cables to remain strung, even after an impact that removes several posts, thus allowing the remainder of the run to function normally. The cable barrier is intended for use on slopes with a 1:6 vertical to horizontal ratio.