Cable barrier


Cable barrier

Cable barrier, sometimes referred to as guard cable, is a type of roadside or median barrier. It consists of steel wire ropes mounted on weak posts. As is the case with any roadside barrier, its primary purpose is to prevent a vehicle from leaving the traveled way and striking a fixed object or terrain feature that is less forgiving than itself. [American Association of State Highway and Transportation Officials. Roadside Design Guide. Third Edition, 2006. Task Force on Roadside Safety, 2006.] Also similar to most roadside barriers, cable barriers function by capturing and/or redirecting the errant vehicle.

Because these barriers are relatively inexpensive to install and very effective at capturing vehicles [Missouri Department of Transportation. Study on Median Guard Cable Performance in Relation with Median Slope: Preliminary Results on I-44. Organizational Results Division, 2006.] , their use is becoming increasingly prevalent among states’ departments of transportation in the United States. By far, the most popular use of the cable barrier system occurs in the medians of divided highways. Given the opposing directions of traffic on divided highways, cross median crashes are particularly severe. 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. [American Association of State Highway and Transportation Officials. Roadside Design Guide. Third Edition, 2006. Task Force on Roadside Safety, 2006.]

Although cable barriers have been used since the 1960s it wasn’t until the mid 1990s that many departments of transportation began to deploy them with any regularity.

Types

There are two types of cable barrier systems in use today, low-tension and high-tension. Each system has its advantages and disadvantages, but in general, a high-tension system has a higher initial cost with lower long-term maintenance costs and concerns.

Low-tension

During the expansion of cable barrier use throughout the 1980s and 1990s, the low-tension system was specified almost exclusively. This system is also commonly called the “U.S. generic” system, referring to the fact that it is not exclusively manufactured by any single producer.

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) [Missouri Department of Transportation. Missouri Standard Plans for Highway Construction. Design Division, Engineering Policy Section, 2006.] to achieve the low 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 the 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.

Despite these perceived shortcomings, low-tension cable barrier, until recently, was arguably the workhorse of the industry. Many states built extensive cable barrier programs using the U.S. generic system and hundreds of miles remain in use today.

High-tension

In appearance, high-tension cable is very similar to low-tension. In most other aspects, the two systems are very different.

High-tension cable consists of three or four pre-stretched cables supported by weak posts. Currently, all high-tension systems are proprietary, that is, marketed under exclusive right of a specific manufacturer. There are five systems being marketed in the United States at present.

There is a generic high-tension system currently under development at the Midwest Roadside Safety Facility at the University of Nebraska–Lincoln.

During installation, the cables are placed on the posts, and then tightened to a specific tension according to temperature. The tensions values range between approximately 2,000 and 9,000 lb. 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.

Performance limits

afety testing

A roadside safety hardware feature must undergo rigorous safety testing before it can be used on the National Highway System (NHS). Most states have adopted the same testing criteria for highways that are not on the NHS. The standard by which all roadside safety features are measured is contained within the National Cooperative Highway Research Program Report No. 350 ( [http://safety.fhwa.dot.gov/roadway_dept/road_hardware/nchrp_350.htm NCHRP 350] ).NCHRP 350 evaluates safety hardware according to three general factors [Ross, H.E. Jr., D.L. Sicking, R.A. Zimmer, and J. Michie. Recommended Procedures for the Safety Performance Evaluation of Highway Features. National Cooperative Highway Research Program Report 350. Transportation Research Board. Washington D.C., 1993.] :
* Structural Adequacy: the system must contain and redirect the vehicle with no underriding, overriding, or penetration.
* Occupant Risk: fragments of the system cannot penetrate the passenger compartment, the vehicle must remain upright during and after the collision, and the passenger must not undergo excessive impact or deceleration.
* Vehicle Trajectory: after the impact, the vehicle should not intrude into adjacent traffic lanes nor should it exit the system at an angle greater than 60% of the entry angle.

Test levels

Within NCHRP 350 are six separate test levels (TL) representing different vehicles, impact angles and speeds.Test level three (TL-3) is probably the most commonas it establishes safety criteria for both small cars and pickups at 60 mph. This category of traffic accounts for the majority of all vehicle traffic in the United States.

At TL-3, an 1800 lb car is crashed at 60 mph on an impact angle of 20°. Also at this level, a 4400 lb pickup truck impacts at 60 mph and 25°.

TL-4 includes both these tests but adds a 17,600 lb single-unit truck impacting at 50 mph and 25°.

All cable barrier systems available today are approved at either TL-3 or TL-4. There is a great deal of anecdotal evidence, however, that many of these systems are performing at a higher level in the field capturing vehicles as large as semi truck-trailer combinations.

Environmental limits

lopes

Cable barrier, like most roadside safety hardware, is intended for use on slopes with a 1:6 vertical to horizontal ratio. The 1V:6H requirement is based in both computer modeling and full-scale crash testing and represents sound theory. In practice, however, slopes as flat as 1V:6H are often the exception.

In these cases, there are three systems available that function at TL-3 on slopes as steep as 1V:4H. [Baxter, J.R. to D.W. Muir. May 2006. Brifen WRSF on 1V:4H slope. Federal Highway Administration. Roadside Hardware: Acceptance Letters. HSA-10 / B82-B1.] [Baxter, J.R. to B. Neusch. July 2006. Gibraltar Cable Barrier on 4:1 slope @ TL-3. Federal Highway Administration. Roadside Hardware: Acceptance Letters. HSA-10 / B137C.]

Clearance

Rigid barriers such as concrete and semi-rigid barriers such as steel guardrail, exhibit impact deflections of 0 to 4 ft, respectively. [American Association of State Highway and Transportation Officials. Roadside Design Guide. Third Edition, 2006. Task Force on Roadside Safety, 2006.] Flexible systems such as cable barriers deflect between 8 and 12 ft upon impact.Given these relatively large deflections, cable barrier systems are not usually considered appropriate to shield fixed objects closer than 8 ft offset of the traveled way. Even when the available clearance exceeds 8 ft, the public seems to have a greater level of confidence in a more robust barrier.

References

ee also

* Crash barrier
* Concrete step barrier
* Jersey barrier
* Guard rail
* Impact attenuator


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