Cable barrier - AbsoluteAstronomy.com

Cable barrier, sometimes referred to as guard cable, is a type of roadside

Roadside

Many things may be found along the roadside. These include:* Roadside attractions* Roadside memorials* Japanese Roadside Stations- Music :* Roadside Monument, a defunct US-based rock band* "Roadside", a song by Rise Against from The Sufferer & the Witness...

or median barrier. It consists of steel

Steel

Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...

wire rope

Wire rope

thumb|Steel wire rope Wire rope is a type of rope which consists of several strands of metal wire laid into a helix. Initially wrought iron wires were used, but today steel is the main material used for wire ropes....

s mounted on weak posts. As is the case with any roadside barrier, its primary purpose is to prevent a vehicle

Vehicle

A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

from leaving the traveled way and striking a fixed object or terrain

Terrain

Terrain, or land relief, is the vertical and horizontal dimension of land surface. When relief is described underwater, the term bathymetry is used...

feature that is less forgiving than itself. 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, their use is becoming increasingly prevalent worldwide. 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)

Jersey barrier

A Jersey barrier or Jersey wall is a modular concrete barrier employed to separate lanes of traffic. It is designed to both minimize vehicle damage in cases of incidental contact while still preventing crossover in the case of head-on accidents....

barriers or steel barriers used today and remains effective when installed on sloping terrain. The flexibility of the system absorbs impact energy

Energy

In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...

and dissipates it laterally, which reduces the forces transmitted to the vehicle occupants.

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 called the “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

Spring (device)

A spring is an elastic object used to store mechanical energy. Springs are usually made out of spring steel. Small springs can be wound from pre-hardened stock, while larger ones are made from annealed steel and hardened after fabrication...

at both ends of the cable run are compressed (according to temperature) 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. Thousands of miles of the generic system remain in use today in countries worldwide.

High-tension


Available High-Tension Cable Barriers
System	Manufacturer
Brifen	Brifen USA
CASS	Trinity Industries, Inc.
Gibraltar	Gibraltar
Safence	Blue Systems, AB
NU-CABLE	Nucor Steel Marion, Inc.

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

Manufacturing

Manufacturing is the use of machines, tools and labor to produce goods for use or sale. The term may refer to a range of human activity, from handicraft to high tech, but is most commonly applied to industrial production, in which raw materials are transformed into finished goods on a large scale...

.

During installation, the cables are placed on the posts, and then tightened to a specific tension according to temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

. 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.

Safety testing

A roadside safety hardware feature must undergo rigorous safety testing before it can be used on the National Highway System (NHS)

National Highway System (United States)

The National Highway System is a network of strategic highways within the United States, including the Interstate Highway System and other roads serving major airports, ports, rail or truck terminals, railway stations, pipeline terminals and other strategic transport facilities.Individual states...

in the United States. Most states have adopted the same testing criteria for highway

Highway

A highway is any public road. In American English, the term is common and almost always designates major roads. In British English, the term designates any road open to the public. Any interconnected set of highways can be variously referred to as a "highway system", a "highway network", or a...

s 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 (NCHRP 350).
NCHRP 350 evaluates safety hardware according to three general factors:

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
Passenger
A passenger is a term broadly used to describe any person who travels in a vehicle, but bears little or no responsibility for the tasks required for that vehicle to arrive at its destination....

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 common
as it establishes safety criteria for both small cars

Automobile

An automobile, autocar, motor car or car is a wheeled motor vehicle used for transporting passengers, which also carries its own engine or motor...

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

Pickup truck

A pickup truck is a light motor vehicle with an open-top rear cargo area .-Definition:...

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

Semi-trailer truck

A semi-trailer truck, also known as a semi, tractor-trailer, or articulated truck or articulated lorry, is an articulated vehicle consisting of a towing engine , and a semi-trailer A semi-trailer truck, also known as a semi, tractor-trailer, or (in the United Kingdom and Ireland) articulated truck...

Slopes

Cable barrier, like most roadside safety hardware

Hardware

Hardware is a general term for equipment such as keys, locks, hinges, latches, handles, wire, chains, plumbing supplies, tools, utensils, cutlery and machine parts. Household hardware is typically sold in hardware stores....

, is intended for use on slope

Slope

In mathematics, the slope or gradient of a line describes its steepness, incline, or grade. A higher slope value indicates a steeper incline....

s with a 1:6 vertical to horizontal ratio. The 1V:6H requirement is based in both computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

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 TL-4 systems available that function as TL-3 on slopes as steep as 1V:4H.

Clearance

Rigid barriers such as concrete and semi-rigid barriers such as steel guardrail, exhibit impact deflections

Deflection (engineering)

In engineering, deflection is the degree to which a structural element is displaced under a load. It may refer to an angle or a distance.The deflection distance of a member under a load is directly related to the slope of the deflected shape of the member under that load and can be calculated by...

of 0 to 4 ft, respectively. 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.

Defective Installation and Accidents

Median Cable Barriers have been studied for safety, and they are arguably effective deterrents to serious highway accidents. However, a lack of proper installation and testing has led to severe collisions and even death. In places, such as Arizona

Arizona

Arizona ; is a state located in the southwestern region of the United States. It is also part of the western United States and the mountain west. The capital and largest city is Phoenix...

, there is indication that the state government agency in charge of highway regulation failed to follow proper installation procedures. Apparently there are internal government documents which show that the Arizona Department of Transportation

Arizona Department of Transportation

The Arizona Department of Transportation is an Arizona state government agency charged with facilitating mobility within the state. In addition to managing the state's highway system, the agency is also involved with public transportation and municipal airports...

was aware of cable barrier problems, and they may have also rushed installation of these barriers on state highways.

A major problem alleged, that reduces the effectiveness of cable barriers, is the installation below grade, especially around slopes or dips. Without any compensation for a slope in the median, a car can actually jump the top of a barrier, and therefore be exposed to a potential cross-over collision. Litigation has arisen in Arizona regarding the improper installation of cable barriers. One wrongful death suit resulted in a one million dollar settlement with the state. In Washington state, numerous letters were submitted to the state Department of Transportation

Department of Transportation

The Department of Transportation is the most common name for a government agency in North America devoted to transportation. The largest is the United States Department of Transportation, which oversees interstate travel. All U.S. states, Canadian provinces, and many local agencies also have...

complaining of cable barrier installation.