Cable barrier

Last updated
A cable barrier separating lanes on a 2+1 road in Sweden. E20 2plus1 west of Skara.jpg
A cable barrier separating lanes on a 2+1 road in Sweden.

A cable barrier, sometimes referred to as guard cable or wire rope safety barrier (WRSB), is a type of roadside or median safety traffic barrier/guard rail. 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. [1] Also similar to most roadside barriers, cable barriers function by capturing and/or redirecting the errant vehicle.

Contents

Because these barriers are relatively inexpensive, as opposed to concrete step barriers to install and maintain, and are 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.

A cable barrier in the Czech Republic Plana nad Luznici, zatacka.jpg
A cable barrier in the Czech Republic

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. [1]

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

In many countries of the European Union these cable barriers are not allowed to be used along highways as they are perceived to be especially hazardous for motorcyclists. However, a study of motorcyclist injury rates for several types of highway barrier did not find an appreciable difference in fatal and severe injuries between cable and W-beam barriers. Both were significantly more hazardous than concrete barriers but less hazardous than none. [2]

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 at both ends of the cable run are compressed (according to temperature) [3] 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 (3.7 metres) 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 (600 metres) 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 (600 metres) 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
SystemManufacturer
ArmorWire Armorflex International
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.

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 9,000lb (9,000 to 40,000 Newtons). 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 (2.4 metres) 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

Safety testing

A roadside safety hardware feature must undergo rigorous safety testing before it can be used on the National Highway System (NHS) in the United States. 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 (NCHRP 350). NCHRP 350 evaluates safety hardware according to three general factors: [4]

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 there 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 and pickups at 60 miles per hour (97 km/h). This category of traffic accounts for the majority of all vehicle traffic in the United States.

At TL-3, an 1,800 pounds (820 kg) car is crashed at 60 miles per hour (97 km/h) on an impact angle of 20°. Also at this level, a 4,400 pounds (2,000 kg) pickup truck impacts at 60 miles per hour (97 km/h) and 25°. TL-4 includes both these tests but adds a 17,600 pounds (8,000 kg) single-unit truck impacting at 50 miles per hour (80 km/h) 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

Slopes

Cable barrier, 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 TL-4 systems available that function as TL-3 on slopes as steep as 1V:4H. [5] [6]

Clearance

Rigid barriers such as concrete and semi-rigid barriers such as steel guardrail, exhibit impact deflections of 0 to 4 ft (1.2 metres), respectively. [1] Flexible systems such as cable barriers deflect between 8 and 12 ft (2.4 and 3.7 metres) upon impact. Given these relatively large deflections, cable barrier systems are not usually considered appropriate to shield fixed objects closer than 8 ft (2.4 metres) offset of the travelled way. Even when the available clearance exceeds 8 ft (2.4 metres), 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. [7] In places, such as Arizona, there is indication that the state government agency in charge of highway regulation failed to follow proper installation procedures. [8] Apparently there are internal government documents which show that the Arizona Department of Transportation was aware of cable barrier problems, and they may have also rushed installation of these barriers on state highways.[ citation needed ]

A major problem alleged, that reduces the effectiveness of cable barriers, is the installation below grade, especially around slopes or dips. [9] 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.[ citation needed ] Litigation has arisen in Arizona regarding the improper installation of cable barriers. [9] One wrongful death suit resulted in a one million dollar settlement with the state. [10] In Washington state, numerous letters were submitted to the state Department of Transportation complaining of cable barrier installation. [11]

Common specifications

Tension (Low/High)Containment/DeflectionNo. of ropes (1/2/3/4)Standards - TL3/TL4/otherSymmetrical/Non-Symmetrical
example 1HighDeflection4otherSymmetrical
example 2HighContainment4TL4Symmetrical

Common characteristics

Tension – High tensioned WRSB are generally tensioned to app. 2.5t during installation (subject to weather conditions, type of WRSB, and other factors). Low tensioned WRSB are not as common as they used to be, the tension is generally very low and close to 0.

Containment or Deflection – based WRSB. Deflection aimed WRSB could be tensioned to slightly higher tension and will most probably use 4 wires (ropes). The overall length of the barrier tends to be shorter. Containment based WRSB will have wire ropes spread further apart from each other (approximately 150mm - 60mm), to increase the catchment area.

See also

Related Research Articles

<span class="mw-page-title-main">Crash test</span> Form of destructive testing

A crash test is a form of destructive testing usually performed in order to ensure safe design standards in crashworthiness and crash compatibility for various modes of transportation or related systems and components.

<span class="mw-page-title-main">Road traffic safety</span> Methods and measures for reducing the risk of death and injury on roads

Road traffic safety refers to the methods and measures used to prevent road users from being killed or seriously injured. Typical road users include pedestrians, cyclists, motorists, vehicle passengers, horse riders, and passengers of on-road public transport.

<span class="mw-page-title-main">Speed bump</span> Traffic calming device

Speed bumps are a class of traffic calming devices that use vertical deflection to slow motor-vehicle traffic in order to improve safety conditions. Variations include the speed hump, speed cushion, and speed table.

<span class="mw-page-title-main">Jersey barrier</span> Modular concrete or plastic barrier for separating vehicle traffic

A Jersey barrier, Jersey wall, or Jersey bump is a modular concrete or plastic barrier employed to separate lanes of traffic. It is designed to minimize vehicle damage in cases of incidental contact while still preventing vehicle crossovers resulting in a likely head-on collision. Jersey barriers are also used to reroute traffic and protect pedestrians and workers during highway construction. They are named after the U.S. state of New Jersey which first started using the barriers as separators between lanes of a highway in the 1950s.

<span class="mw-page-title-main">Rumble strip</span> Road safety feature

Rumble strips are a road safety feature to alert inattentive drivers of potential danger, by causing a tactile vibration and audible rumbling transmitted through the wheels into the vehicle interior. A rumble strip is applied along the direction of travel following an edgeline or centerline, to alert drivers when they drift from their lane. Rumble strips may also be installed in a series across the direction of travel, to warn drivers of a stop or slowdown ahead, or of an approaching danger spot.

The National Highway Traffic Safety Administration is an agency of the U.S. federal government, part of the Department of Transportation, focused on transportation safety in the United States.

Standards for Interstate Highways in the United States are defined by the American Association of State Highway and Transportation Officials (AASHTO) in the publication A Policy on Design Standards: Interstate System. For a certain highway to be considered an Interstate Highway, it must meet these construction requirements or obtain a waiver from the Federal Highway Administration.

<span class="mw-page-title-main">Guard rail</span> Freestanding fixture meant to aid in pedestrian and vehicle safety

Guard rail, guardrails, or protective guarding, in general, are a boundary feature and may be a means to prevent or deter access to dangerous or off-limits areas while allowing light and visibility in a greater way than a fence. Common shapes are flat, rounded edge, and tubular in horizontal railings, whereas tetraform spear-headed or ball-finialled are most common in vertical railings around homes. Park and garden railings commonly in metalworking feature swirls, leaves, plate metal areas and/or motifs particularly on and beside gates.

The Midwest Roadside Safety Facility is a research organization in Lincoln, Nebraska that conducts automotive crash testing. MwRSF is operated by the University of Nebraska–Lincoln (NU) and researches highway design and safety with a particular emphasis on safety performance evaluations of roadside appurtenances. Since its establishment in 1974, the facility has evaluated existing barriers and frequently developed new design concepts and technologies for use on public roadways.

<span class="mw-page-title-main">Curb</span> Edge where a sidewalk meets a road

A curb, or kerb, is the edge where a raised sidewalk or road median/central reservation meets a street or other roadway.

<span class="mw-page-title-main">2+1 road</span> Design of road

2+1 road is a specific category of three-lane road, consisting of two lanes in one direction and one lane in the other, alternating every few kilometres, and usually separated with a steel cable barrier. The second lane allows faster-moving traffic to overtake slower vehicles at regular intervals. Traditional roads of at least 10 metres (33 ft) width can be converted to 2+1 roads and reach near-motorway safety levels at a much lower cost than an actual conversion to motorway or dual carriageway.

<span class="mw-page-title-main">Impact attenuator</span> Safety device in traffic management used to soften the blows of car crashes

An impact attenuator, also known as a crash cushion, crash attenuator, or cowboy cushion, is a device intended to reduce the damage to structures, vehicles, and motorists resulting from a motor vehicle collision. Impact attenuators are designed to absorb the colliding vehicle's kinetic energy. They may also be designed to redirect the vehicle away from the hazard or away from roadway machinery and workers. Impact attenuators are usually placed in front of fixed structures near highways, such as gore points, crash barrier introductions, or overpass supports. Temporary versions may be used for road construction projects.

<span class="mw-page-title-main">Barrier transfer machine</span> Heavy vehicle used to transfer concrete lane dividers during rush hours

A barrier transfer machine, also known as zipper machine or road zipper, is a type of heavy vehicle that is used to transfer concrete lane dividers, such as Jersey barriers, in order to relieve traffic congestion during rush hours. Many other cities use them temporarily during construction work. The lanes created by the machine are sometimes referred to as "zipper lanes".

<span class="mw-page-title-main">Cycle track</span> Bikeway between a road and sidewalk, protected by barriers

A cycle track, separated bike lane or protected bike lane is an exclusive bikeway that has elements of a separated path and on-road bike lane. A cycle track is located within or next to the roadway, but is made distinct from both the sidewalk and general purpose roadway by vertical barriers or elevation differences.

<span class="mw-page-title-main">Traffic barrier</span> Barrier installed within medians of and next to roads to prevent vehicle collisions

Traffic barriers keep vehicles within their roadway and prevent them from colliding with dangerous obstacles such as boulders, sign supports, trees, bridge abutments, buildings, walls, and large storm drains, or from traversing steep (non-recoverable) slopes or entering deep water. They are also installed within medians of divided highways to prevent errant vehicles from entering the opposing carriageway of traffic and help to reduce head-on collisions. Some of these barriers, designed to be struck from either side, are called median barriers. Traffic barriers can also be used to protect vulnerable areas like school yards, pedestrian zones, and fuel tanks from errant vehicles.

<span class="mw-page-title-main">Road collision types</span> Overview of the various types of road traffic collision

Road traffic collisions generally fall into one of five common types:

<span class="mw-page-title-main">Roadway departure</span> Type of single-vehicle collision

A roadway departure is a type of incident that occurs when a vehicle leaves the roadway. Such incidents can lead to a single-vehicle collision.

<span class="mw-page-title-main">Geometric design of roads</span> Geometry of road design

The geometric design of roads is the branch of highway engineering concerned with the positioning of the physical elements of the roadway according to standards and constraints. The basic objectives in geometric design are to optimize efficiency and safety while minimizing cost and environmental damage. Geometric design also affects an emerging fifth objective called "livability," which is defined as designing roads to foster broader community goals, including providing access to employment, schools, businesses and residences, accommodate a range of travel modes such as walking, bicycling, transit, and automobiles, and minimizing fuel use, emissions and environmental damage.

The flail space model (FSM) is a model of how a car passenger moves in a vehicle that collides with a roadside feature such as a guardrail or a crash cushion. Its principal purpose is to assess the potential risk of harm to the hypothetical occupant as he or she impacts the interior of the passenger compartment and, ultimately, the efficacy of an experimental roadside feature undergoing full-scale vehicle crash testing.

<span class="mw-page-title-main">Transportation Research Center</span>

References

  1. 1 2 3 American Association of State Highway and Transportation Officials. Roadside Design Guide. Third Edition, 2006. Task Force on Roadside Safety, 2006.
  2. Daniello, Allison; Gabler, Hampton C. (2011). "Effect of Barrier Type on Injury Severity in Motorcycle-to-Barrier Collisions in North Carolina, Texas, and New Jersey". Transportation Research Record: Journal of the Transportation Research Board. 2262 (2262): 144–151. doi:10.3141/2262-14. S2CID   110660381.
  3. Missouri Department of Transportation. Missouri Standard Plans for Highway Construction. Design Division, Engineering Policy Section, 2006.
  4. 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.
  5. 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.
  6. 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.
  7. 5 Investigates Freeway Cable Barriers Archived 2011-06-14 at the Wayback Machine ; August 7, 2008; KPHO CBS 5; retrieved October 24, 2008.
  8. 5 Investigates Cable Barrier Contradictions. Archived 2011-06-14 at the Wayback Machine ; September 25, 2007; KPHO CBS 5; retrieved October 24, 2008.
  9. 1 2 5 Finds Freeway Barrier Hazards Archived 2011-06-14 at the Wayback Machine ; KPHO CBS 5; Retrieved on October 24, 2008.
  10. Significant Cases Archived 2008-10-30 at the Wayback Machine ; Torres.
  11. Appendix C Archived 2010-06-16 at the Wayback Machine ; WS Dot; PDF file