Difference between revisions of "606.2 Guard Cable"

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|'''[[Key Points 606.2 Guard Cable|Key Points]]'''
 
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[[image:606.2 Guard Cable.JPG|right|425px|thumb|<center>'''Low-tension Guard Cable'''</center>]]
 
 
Guard cable consists of steel cable mounted on weak posts.  It is relatively inexpensive to install and very effective at capturing errant vehicles. 
 
 
 
==606.2.1 Types of Guard Cable==
 
 
 
There are two types of guard cable systems in use:  low-tension and high-tension.  Each system has advantages and disadvantages.  Generally, a high-tension system has a higher initial cost with lower long-term maintenance costs and concerns.  MoDOT is allowing high-tension cable as a no-cost change order whenever a [http://www.modot.mo.gov/business/BecomingaMoDOTSubContractor.htm subcontractor] is able to provide it at the same cost as low-tension.
 
 
 
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|-
 
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|'''For Additional Information'''
 
|'''For Additional Information'''
 
|-
 
|-
|[[For Additional Information 606.2 Guard Cable|Maintenance]]
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|[[media:Guard Cable Program 2007.pdf|"MoDOT's Cable Median Barrier Program"]], a report from 2007.
 
|-
 
|-
|[http://www.savemolives.com/programs/documents/I70GuardCableStateFair--updated.ppt Installing Guard Cable and Safety Information about Guard Cable]
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|[https://savemolives.com/safety-topics/topic/guard-rails Guard Cable Safety Information]
 
|-
 
|-
|'''Video'''
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|'''Videos'''
 
|-
 
|-
 
|[{{SERVER}}/documents/606.2_Cable_Rail_Test.mpg Successful guard cable test]
 
|[{{SERVER}}/documents/606.2_Cable_Rail_Test.mpg Successful guard cable test]
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|-
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|[[media:606.1 Guard cable.wmv|Guard Cable in Action]]
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|-
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|[http://www.youtube.com/modotvideo#p/u/1/IZTtBN7CHxY MoDOT's You Tube Guard Cable video]
 
|}
 
|}
  
'''606.2.1.1 Low-Tension.'''  Since no single producer exclusively manufactures low-tension guard cable, this system has been commonly called the “U.S. generic” systemLow-tension guard cables are tensioned only enough to eliminate sag between posts.  Large springs at either end of the cable run are compressed, according to temperature, to achieve the system’s low tension.
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==606.2.1 Guard Cable Types==
 +
 
 +
Cable median barriers, commonly referred to as guard cable, remain one of the most efficient roadside safety treatment available todayGuard cable consists of twisted wire ropes mounted on weak posts.  It is relatively inexpensive to install, compared to more rigid systems, and has been proven effective at capturing errant vehicles.
 +
There are two types of guard cable systems in use on Missouri roads:  low-tension and high-tension.
  
 +
'''606.2.1.1 Low-Tension.'''  Since no single producer exclusively manufactures low-tension guard cable, this system has been commonly called the “U.S. generic” system.  Low-tension guard cables typically consists of three cables placed at different heights and are tensioned only enough to eliminate sag between posts.  Large springs at either end of the cable run are compressed, according to temperature, to achieve the system’s low tension.  The cable itself is strung on posts that are directly driven into the ground.
 +
[[image:606.2 Guard Cable.JPG|right|400px|thumb|<center>'''Low-tension Guard Cable'''</center>]]
 
When a vehicle impacts the low-tension system under normal conditions, the cable laterally moves as much as 12 ft.  This movement is known as the dynamic deflection.
 
When a vehicle impacts the low-tension system under normal conditions, the cable laterally moves as much as 12 ft.  This movement is known as the dynamic deflection.
 
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Given the lack of tension in the system, individual installations, or “runs”, of cable are limited to 2000 ft. with an anchor assembly at each end.  When a vehicle strikes low-tension cable, the system will not function properly if struck by another vehicle since the cables will be loosened.  It is critical to repair the guard cable promptly.
 
 
 
{| border="1" class="wikitable" style="margin: 1em auto 1em auto"
 
|+ '''''PROs & CONs '''''
 
! colspan="2" style="background:#99ff99"|LOW-TENSION
 
 
|-
 
|-
!width="300" style="background:#99ff99"|Advantages!!width="300" style="background:#99ff99"| Disadvantages
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|'''Median Guard Cable'''
 
|-
 
|-
|• low initial cost||• individual "runs" are limited to 2,000 ft.
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|[http://library.modot.mo.gov/RDT/reports/Ri06014/ss07006.pdf Summary, 2006]
 
|-
 
|-
| rowspan="2"|• employs readily available materials||• entire run is ineffective after a single strike
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|[http://library.modot.mo.gov/RDT/reports/Ri08039/or10016.pdf Report, 2010]
 
|-
 
|-
|• large deflections
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|'''See also:''' [http://www.modot.gov/services/OR/byDate.htm Innovation Library]
|-
 
! colspan="2" style="background:#99ff99"|HIGH-TENSION
 
|-
 
!style="background:#99ff99"|Advantages!!style="background:#99ff99"| Disadvantages
 
|-
 
|• lower maintenance costs||• can have higher initial cost
 
|-
 
|• unlimited length of runs||• all systems are proprietary
 
|-
 
|• cable stays strung after impact allowing the rest of the system to function|| rowspan="2"|• unfamiliarity in most states
 
|-
 
|• lower deflections||
 
 
|}
 
|}
  
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Given the lack of tension in the system, individual installations, or “runs”, of cable are limited to 2000 ft. with an anchor assembly at each end.  When a vehicle strikes low-tension cable, the system becomes disabled and will not function properly if subsequently struck by another vehicle.  As such, it is critical to repair the guard cable promptly.
  
'''606.2.1.2 High-Tension.'''  High-tension cable looks very similar to low-tension cable but the two systems are very different in most other aspectsHigh-tension guard cable consists of three or four pre-stressed cables supported by weak postsCurrently, all high-tension systems are proprietary, that is, marketed under exclusive rights of a specific manufacture.  Five systems are currently marketed in the United States.
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Low-tension systems have been in service for some time and have proven their value by reducing cross-median accidentsHowever, the issues related to down time and the necessity to utilize on-call contracting cause a perpetual drain on MoDOT resourcesFor these reasons, the use of low-tension cable systems should be limited to small-scale installations with special circumstances.
  
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'''606.2.1.2 High-Tension.'''  High-tension cable barrier looks very similar to low-tension cable but the two systems are very different in most other aspects.  High-tension guard cable consists of three or four pre-stressed cables supported by weak posts. 
 +
[[image:606.2.1.2 High-Tension.jpg|right|575px|thumb|<center>'''High-tension Guard Cable'''</center>]]
 
During installation, the cables are placed on the posts and then tightened to a specific tension, ranging from approximately 2,000 to 9,000 pounds according to temperature.  Due to this tightening, the cable installations can be of indefinite length.  In fact, the runs are typically only limited by the presence of obstacles such as median openings or bridge columns.
 
During installation, the cables are placed on the posts and then tightened to a specific tension, ranging from approximately 2,000 to 9,000 pounds according to temperature.  Due to this tightening, the cable installations can be of indefinite length.  In fact, the runs are typically only limited by the presence of obstacles such as median openings or bridge columns.
  
Under normal conditions, when a vehicle impacts the high-tension system the cable laterally deflects as much as 8 ft.  The inherent tension within the system also allows the cable to remain strung, even after an impact removes several posts.  This allows the remainder of the run to continue functioning normally.
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Under normal conditions, when a vehicle impacts the high-tension system the cable laterally deflects as much as 8 ft.  The inherent tension within the system also allows the cable to remain at the proper height, even after an impact removes several posts.  While the system is not designed to continue to function in that condition, there is a great deal of anecdotal evidence that it does just that.
 
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{| border="1" class="wikitable" style="margin: 1em auto 1em auto" align="right"
==606.2.2  Testing Criteria==
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|+'''Currently Approved High-Tension Systems and Manufacturers'''
 
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! style="background:#BEBEBE"|High-Tension System !! style="background:#BEBEBE"|Manufacturer 
Within the National Cooperative Highway Research Program Report No. 350 (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 and pickups at 60 mph.  This category of traffic accounts for nearly 90% of all vehicle traffic in Missouri.
 
 
 
The table below summarizes data for the six test levels:
 
 
 
====<center>Table 1 What is TL-3? </center>====
 
 
 
{| border="1" class="wikitable" style="margin: 1em auto 1em auto"
 
|+  
 
! style="background:#BEBEBE"|Test Level !! style="background:#BEBEBE"|Vehicle!! style="background:#BEBEBE"|Angle (degrees)!!style="background:#BEBEBE"|Speed
 
|-
 
|rowspan="2" align="center"|1
 
|1800 lb. car|| align="center"|20||30 mph
 
|-
 
|4400 lb. pickup|| align="center"|25||30 mph
 
|-
 
|rowspan="2" align="center"|2
 
|1800 lb. car|| align="center"|20||45 mph
 
|-
 
|4400 lb. pickup|| align="center"|25||45 mph
 
|-
 
|rowspan="2" align="center"|3
 
|1800 lb. car|| align="center"|20||60 mph
 
|-
 
|4400 lb. pickup|| align="center"|25||60 mph
 
|-
 
|rowspan="3" align="center"|4
 
|1800 lb. car|| align="center"|20||60 mph
 
 
|-
 
|-
|4400 lb. pickup|| align="center"|25||60 mph
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|Brifen|| Brifen USA 
 
|-
 
|-
|17,600 lb. Single-Unit Truck|| align="center"|15||50 mph
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|CASS||Trinity Industries, Inc.
 
|-
 
|-
|rowspan="3" align="center"|5
+
|Gibraltar||Gibraltar 
|1800 lb. car|| align="center"|20||60 mph
 
 
|-
 
|-
|4400 lb. pickup|| align="center"|25||60 mph
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|Safence||Safence, Inc. 
 
|-
 
|-
|80,000 Semi Truck (Cargo)||  align="center"|15||50 mph
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|U.S. High Tension|| Marion Steel Company 
|-
 
|rowspan="3" align="center"|6
 
|1800 lb. car|| align="center"|20||60 mph
 
|-
 
|4400 lb. pickup|| align="center"|25||60 mph
 
|-
 
|80,000 lb. Semi Truck (Tanker)|| align="center"|15||50 mph
 
 
|}
 
|}
  
A roadside safety hardware feature must undergo rigorous safety tasting 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 NCHRP 350.   
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A common installation of high-tension guard cable employs concrete footings into which metal tubes are cast, forming socketsThe socket allows a post to be replaced with relative ease during a repair operation. The damaged post is simply removed from the socket and replaced with a virgin postSocketed systems eliminate the requirement for specialized post driving equipment and subsurface utility location for each repair.
  
NCHRP 350 evaluates safety hardware according to three general factors:
+
A socketed, high-tension system should be chosen for large-scale guard cable installations.  While such a system generally has a higher initial cost, the low cost and high efficiency with which it can be maintained make it a better value over its life cycle.  A high-tension system incorporating socketed posts is easily repaired and maintained with the resources currently available to the district maintenance personnel.  Additionally, high-tension systems can be used on a variety of median inslopes, often eliminating the need for costly slope corrections and drainage modifications.
  
- Structural Adequacy: the system must contain and redirect the vehicle with no under-riding, overriding or penetration.
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As of 2007, all high-tension systems are proprietary, that is, marketed under exclusive rights of a specific manufacturer. Five systems are currently marketed in the United States.
  
- 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.
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==606.2.2 Systematic Application of Median Guard Cable==
  
- 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.
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Median guard cable is most effective when installed as a system-wide solution to address cross-median crash types. The benefits are severely limited if the cable is only used in spot locations in response to crashes at those locations.
  
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Additionally, when determining the most appropriate locations for guard cable application, the designation of a route (interstate, US highway, state route) should not be a primary consideration.
  
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A corridor should have similar geometry and traffic volume and the placement of guard cable on the corridor should have logical termini.  Spot location installation of new median guard cable should be used sparingly only in unique situations.
  
==606.2.3 Design Guidelines==
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==606.2.3 Warrants==
  
A guard cable barrier is to be considered for median applications on freeways where cross-median accidents are occurringGuard cable used at any location other than in the freeway median will require approval through the design exception.
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Analyses of cross-median crash history and traffic volume provide valuable information in determining the likelihood of future severe crashes on these routesIn order to prevent future fatalities and disabling injuries, it is important to focus safety efforts on locations that will benefit the most from safety countermeasures.
  
For medians wider than 36 ft. (11 m) as measured between the edges of travelway, the installation of guard cable as a median barrier on freeways is to be considered when one or more of the following conditions exist:
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'''606.2.3.1 Crash Data.'''  Analysis of crashes on a candidate corridor should focus on cross-median crashes on that route and, even more so, on those crashes resulting in fatalities and disabling injuries.
  
* On a horizontal curve with radius less than 2000 ft. (609.6 m)
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It is important this data analysis is robust, particularly on expressways.  Due to at-grade intersection crashes on these routes, a simple query of cross-median crashes may include unwanted events and exclude necessary ones. Accuracy of these data is vital in decision-making.
  
* On “stepped” medians (opposing directions of traffic are at different levations and median slopes are steeper than 1V:6H)
+
The data should be reviewed each year to validate priorities and identify any emerging cross-median safety concerns.  A regular review of divided highway traffic volume and crashes will provide information to proactively address severe cross-median crashes.
[[image:606.2.3 Accident Cleanup.jpg|right|175px|thumb|<center>'''Accident Cleanup'''</center>]]
 
* Accident history (in particular, areas with a cross- median accident rate exceeding 0.8 per 100 million vehicle miles)
 
  
* In the vicinity of traffic conflict points including interchange ramps
+
'''606.2.3.2 Traffic Volume.'''  Recent research has connected traffic volume growth directly to cross-median crash events.  As volume increases, the probability of a motorist crossing the median and hitting an oncoming vehicle also increases.  Instead of relying solely on crash history, there is an opportunity to proactively address this crash type before the crashes occur by studying traffic volume patterns and installing a system of median guard cable on routes with sharply increasing volumes.
  
* Rapidly increasing volumes of traffic
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'''606.2.3.3 [[231.1 Median Width|Median Width]].''' Recent national experience has shown that cross-median crashes can occur on highways with median widths above MoDOT's initial 60 ft. threshold.  Although this width has largely proven to be effective in detering such crashes, no route will be excluded from analysis solely on the basis of median width.  Divided highways with very wide medians are expected to have little or no cross-median crash history, effectively removing them from consideration for barrier installation.
  
* In areas where the level of service of the freeway is “D” or less
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==606.2.4 Design and Installation Guidelines==
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===606.2.4.1 Lateral Placement in the Median===
  
The designer is to evaluate the need to provide guard cable in these situations and document the results of the evaluation.
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'''Dynamics of Cross-Median Crashes.''' When a vehicle leaves the roadway and enters the median, certain predictable dynamics occur.  Vehicles may enter the median at a variety of speeds and angles but for the purposes of roadside safety research and testing, a 60 mph departure at a 20° or 25° angle is generally used.
  
Sheet 3 of [[Media:606.1 Warrant for Median Barriers.pdf|Warrant for Median Barriers]] presents three basic median sections for which placement of guard cable is identified: a depressed “Standard” median with a ditch section, a stepped median with significant differences in elevation and a raised median with a median bermIn all situations, the slopes and the ditch section are to first be checked to determine if the guidelines previously stated suggest installation of a cable barrier.
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Upon departure, a vehicle will initially continue along its vertical trajectory. As the inslope falls away along the 25° vehicle path, the vehicle effectively becomes briefly airborneWhen the vehicle's inertia can no longer overcome gravity, it lands and its suspension is deeply compressedAs the vehicle continues to travel through the median, the suspension rebounds and the bumper of the vehicle stays at a relatively constant height throughout the remainder of the errant journey.
  
* If both slopes are equal to or flatter than 1V:6H ([[Media:606.1 Warrant for Median Barriers.pdf|Illustration 1, Sheet 3 of 5]]), a barrier is to be placed at the center of the median.
+
Every guard cable crash is slightly different because of a host of site-specific factors.  In general, however, the front of the vehicle must engage at least two of the three or four cables present in order to be contained by the system.  Given the dynamics described above, lateral placement of the cable can be grouped into two main categories:  medians wider than 30 ft. and those narrower than 30 ft.
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[[image:606.2.4.1 Placement.jpg|600px|right]]
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'''Medians 30 ft. or wider.'''  The guard cable should be installed no more than 4 ft. downslope of the edge of the shoulder.  With wider shoulders, the downslope location could be less than 4 ft., but in any case, there must be 8 ft. between the barrier and the edge of traveled way.  There are several advantages to this location but chief among them is the performance of the system in a crash.  At the 4 ft. downslope location, the errant vehicle adjacent to the barrier, while airborne, is not at a great enough altitude to override the cable during a front side encounter.  From the opposing direction, or backside, the suspension of the errant vehicle will have recovered enough to allow an impact to occur under relatively normal impact conditions.  
  
* If an embankment slope is steeper than 1V:6H ([[Media:606.1 Warrant for Median Barriers.pdf|Illustration 2, Sheet 3 of 5]]), a barrier is to be placed on the 1V:6H or flatter slope at least 14 ft. from the shoulder point of the opposing lanes of travel.
+
If the 8 ft. separation cannot be obtained, the designer must work with the Central Office Design Division to assess the potential safety impacts of a decreased deflection distance. If the arrangement cannot be proven reasonably safe, a different barrier system must be used.
  
* Placement criteria for barriers in raised medians, or median berms ([[Media:606.1 Warrant for Median Barriers.pdf|Illustration 3, Sheet 3 of 5]]) have not been clearly defined.  Research has shown that a cross-section of sufficient height can redirect vehicles impacting it at relatively shallow anglesGenerally, if the cross-section itself is not adequate to redirect errant vehicles (i.e. the slopes are relatively flat), a guard cable barrier is to be placed at the apex of the cross-section.
+
'''Medians narrower than 30 ft.'''  In medians narrower than 30 ft., the guard cable should be installed within 1 ft. of the vertex of either a V or flat-bottomed ditchAs previously discussed, this location performs the most advantageously.  The 4 ft. downslope location starts to fail in narrower medians as the suspension of the vehicle impacting from the back side (i.e. the opposite direction) is the most tightly compressed around that location.  Again, a fully compressed suspension has proven to be the principal reason for vehicles underriding the system.
  
In situations where median slopes are steeper than 1V:6H the median is to be reshaped, if at all possible, to attain cross-slopes that are 1V:6H or flatterIf utilities are present in the center of the median, the guard cable alignment may be offset as much as 3 ft.
+
'''Alternating Sides.'''  The designer may choose to alternate the sides of the median where the barrier is placed for the purpose of reducing any shy line issues or discomfort for motoristsThe change should occur at natural breaks in the barrier such as emergency crossovers or median bridge columns.
  
To be effective, a cable barrier must be mounted on a moderate slope (1V:6H or flatter)The approach to the cable barrier from the travelway must not have a curb or a ditch.  
+
'''Lateral Placement of Low-tension Guard Cable.'''  New installations of low-tension guard cable should be installed within 1 ft. of the vertex of either a V or flat-bottomed ditchRetrofits should be located at the existing offset, provided the system is functioning well.
  
Following installation of guard cable or access control cable, controlling vegetation beneath and immediately adjacent to the cable may not be practical by mowing or hand trimming methods. Typically, herbicide will be applied under the cable to control vegetation and minimize hand trimming. Placement of aggregate bedding material as a rock ditch liner 4 in. deep and 4 ft. wide and a vegetative barrier (weed block) beneath guard cable or access restraint cable during installation is recommended to minimize washout of the median due to lack of vegetationSee [http://www.modot.mo.gov/business/standards_and_specs/documents/60641.pdf Standard Plan 606.41] for details of aggregate bedding material used as ditch liner beneath cable.
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===606.2.4.2 Parallel Installations===
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In-service experience with parallel installations has shown less than desirable results. The close proximity of each installation to traffic has caused an inordinately high incidence of nuisance hits resulting in higher than acceptable long-term maintenance costsVegetative maintenance is also a concern.
  
Although cable system installation is relatively inexpensive compared to a concrete barrier or Type B guardrail system and performs well when hit, it must be repaired after each hit to maintain its effectivenessThis repair must be done as quickly as possible after a hit to ensure the effectiveness of the barrier.  Consequently, its use in areas where frequently hits are likely is not recommended and other types of median barrier are to be considered.
+
Parallel installations of guard cable should not be usedInstead, designers should rely upon guard cable designed for the situation as a single run or consider a barrier system other than guard cable.
  
==606.2.4 Installation==
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===606.2.4.3 Post Spacing===
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While guard cable has been tested and approved with post spacing ranging from 6.5 to 32.5 ft., it is widely believed that the wider post spacing leads to greater deflections and an increased likelihood of vehicle penetration due to underride or traveling between the cables.  For this reason, post spacing should not exceed the conventional limit of 20 ft.  Additionally, increasing post spacing through horizontal curves increases the opportunity for the cable to assume a chord length if the posts are damaged.  If enough posts are damaged, the cable could project into the travelway on the inside of the curve.
  
[http://www.modot.mo.gov/business/standards_and_specs/documents/60641.pdf Standard Plan 606.41] provides the installation criteria for three-strand, low-tension guard cableHigh-tension guard cable is to be installed according to each system’s    specifications, information that is usually available online.   
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===606.2.4.4 Slopes===
 +
'''1V:6H (6:1) or Flatter Slopes.''' Guard cable, like most roadside hardware, is intended for use on slopes that are 1V:6H (6:1) or flatter.  This requirement is based on both computer modeling and full-scale crash testing and represents sound theoryIn practice, however, slopes as flat at 1V:6H are often the exception.
  
'''606.2.4.1 Length of Need.'''  Like guardrail, all guard cable systems approved for use in Missouri have passed NCHRP 350 TL-3 testingTherefore, they represent a legitimate roadside safety device and may substitute for the W-beam guardrail length of need (LON) absent from many median bridge endsThis practice is only financially expedient if median guard cable is present or if guard cable is being designed for the median.
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'''Steeper Slopes.'''  All of the [[131.2 Proprietary Items and Public Interest Findings#131.2.1.1 Proprietary Items|proprietary]] high-tension systems are now approved for use on slopes with gradients between 1V:6H (6:1) and 1V:4H (4:1).  Their use, while generally more expensive, represents the most cost-effective solution for shielding steeper slopesFurther, since more than three equivalent sources exist, there is no need to obtain a [[131.2 Proprietary Items and Public Interest Findings#131.2.1.2 Public Interest Findings|material certification]] for their use.
  
[[image:606.2.4 Barrier Length of Need at Median Bridge Ends.jpg|right|400px]]
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===606.2.4.5 Vegetative Barrier===
 +
[[:Category:822 Roadside Vegetation Management|Vegetation control]] in the area between the cable and the passing lane must be addressed.  Failure to provide some positive form of vegetation control will hinder the future maintenance of the system. Positive vegetation control measures may include [[:Category:821 Herbicides and Roadsides|herbicides]], a geotextile-aggregate strip or asphalt apron.  The core team must consult with the local maintenance personnel to arrive at a vegetative control measure that is mutually agreeable.
  
In locations where no cable is present and safety concerns warrant a LON correction to the W-beam rail, the correction should be made with the guardrailStandard Plan 606.41 provides the details for transitioning guard cable to W-beam guardrail.
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A district's decision to mow around the barrier must be approved by [http://wwwi/maintenance/ Central Office Maintenance]Such mowing operations must be accomplished without impeding through traffic in any manner.
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[[image:606.2.4.6.jpg|right|275px|thumb|'''<center>Anchor Assembly</center>''']]
 +
Vegetation control may not be omitted from a project as a practical design or value engineering measure.
  
'''606.2.4.2 Slopes.'''  Guard cable, like most roadside safety hardware, is intended for use on slopes that are 1V:6H or flatter.  In practice, however, slopes as flat as 1V:6H are often the exception.  MoDOT and other DOTs have observed virtually no difference between the success rates of guard cable installed on slopes as steep as 1V:5H and those on 1V:6H.  The Roadside Design Guide states, “…a barrier may be considered operational if it has been used for an extended period and has demonstrated satisfactory field performance in terms of construction, maintenance, and crash experience.”  Therefore, on new construction projects having guard cable installed, the inslopes are to be no steeper than 1V:6H.  For rehabilitations, slopes as steep as 1V:5H are permitted.
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===606.2.4.6 Termination at [[:Category:240 Maintenance and Emergency Crossovers|Emergency Crossovers]]===
  
'''606.2.4.3 Parallel Installations.''' The preferred method to achieve 1V:5H or flatter slopes is to re-grade the median. However, in certain situations such as areas of critical drainage gradient or differential profile grades, additional earthwork within the median is not an option.  In these cases, parallel installations of guard cable are to be specified.
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The design for guard cable termination as well as the grading for the crossover should be in accordance with [https://www.modot.org/media/16863 Standard Plan 606.41, Sheet 7 of 7]Refer to [[:Category:240 Maintenance and Emergency Crossovers#240.4 Guard Cable Termination at Emergency Crossovers|EPG 240.4 Guard Cable Termination at Emergency Crossovers]] for additional information.
  
The ideal location for each parallel run is 2 ft. from the edge of the shoulder.  This location maximizes the distance from the through traffic while still taking into account the bumper height of the errant vehicle.  Generally, the guard cable is to be located as far from the travelway as possible to avoid non-critical or nuisance impacts.
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==606.2.5 Maintenance and Repair==
  
The bumper height of the impacting vehicle is critical to the function of the system.  If the vehicle is slightly airborne after leaving the roadway, it may vault over the entire systemIf the vehicle’s suspension is completely compressed at impact, the vehicle may go under the lowest cable and pass through the system.
+
Irrespective of routes treated, proper placement or system used, cable median barrier is only as functional as its ongoing maintenance and repairProper maintenance and incident repair will ensure that the system is always in a state of functionality to provide motorists a greater level of safety on Missouri highways.
  
'''606.2.4.4 Cross-Sectional Details.'''  Because the widespread use of median guard cable is a relatively recent occurrence, specific placement geometrics are being developed and tested, so there is very little written guidanceCertain conclusions can be drawn, however, from the AASHTO and FHWA research, other states’ experience and MoDOT’s own in-service performance.   
+
'''Routine Maintenance. '''  Outside of vegetation control, there is little routine maintenance required for a guard cable systemIf pre-stressed cables are used for high-tension systems and compensators are properly compressed for low-tension systems, the tension in the cable should properly acclimate to any weather conditionThe tension monitoring stage occurs during and shortly after construction.
  
[[image:606.3 Median Guard Cable Location Page 1.jpg|right|700px]]
+
'''Cable Height. '''  The importance of cable height to properly capture and redirect errant vehicles has been demonstrated.  Although cable height is relatively static in all systems, erosion under the barrier can sometimes cause a localized increase in height, resulting in possible underride.
  
Due to the importance of the bumper height, it is clear that the barrier performs better in certain locations than in others.  Generally, the barrier should not be placed from 1 to 10 ft. from the vertex of a V-ditch.
+
Maintenance personnel should be educated on the necessity of proper cable height and encouraged to identify and repair locations where erosion or the accumulation of silt have altered the relative cable height.  
  
MoDOT’s practice of placing cable at the centerline of a flat-bottomed ditch should neither be discontinued nor retrofittedOngoing research may yield results that recommend placement at either vertex of the ditch.
+
'''Median Condition. '''  A secondary issue, closely related to incident repair, is the post-entry condition of the median.  In addition to the repair of the roadside hardware, the median condition with respect to rutting, loss of vegetation and accident debris should be remedied following each accidentThese incidental concerns could cause instability in the trajectory of future errant vehicles and could, at worst, result in a failure of the system.
  
Cable placed at these points should be oriented with the two-cable side facing the nearest traffic.
+
'''Low-Tension Cable Barrier Repair:  On-Call Contract.'''  Maintenance of low-tension cable barrier is vastly more complicated than that of a high-tension system.  In fact, the complexity of the system coupled with the frequency of crash incidents, have traditionally resulted in the system’s maintenance being outsourced through on-call contracts.
  
 +
'''High-Tension Cable Barrier Repair:  In-House.'''  Equipment and hardware needs for the repair of high-tension, socketed guard cable are minimal and repairs can generally be accomplished in under an hour with two workers, some hand tools and a pickup truck.
  
{|border="1" align="center"
+
'''Response Time.'''  Due to the importance of the median guard cable performing when needed, it is vital to quickly respond to repair needsThis will often necessitate an effort to identify cable hits as soon as possible after the incident and then respond with repair as quickly as possible.
|+ style="background:#99ff99"| '''Currently Approved High-Tension Systems and Manufacturers'''
 
|-
 
|style="background:#ccccff" border="3"| '''Brifen''':  Brifen USA
 
|-
 
|style="background:#ccccff" border="3"| '''CASS''': Trinity Industries, Inc.
 
|-
 
|style="background:#ccccff" border="3"|'''Gibraltar''': Gibraltar
 
|-
 
|style="background:#ccccff" border="3"| '''Safence''':  Safence, Inc.
 
|-
 
|style="background:#ccccff" border="3"| '''U.S. High Tension''':  Marion Steel Company
 
|}
 
  
==606.2.5 Maintenance Planning Guidelines for Guardcable==
+
==606.2.6 Maintenance Planning Guidelines for Guard Cable==
  
These Maintenance planning guidelines apply to both [http://epg.modot.mo.gov/files/7/72/232_Major_Highways_Map.pdf major] and minor roads.
+
See [[:Category:170 Maintenance Activity Planning Guidelines#R227 Roadway & Bridge Safety Features|Maintenance Planning Guideline for Guard Cable]].
{|style="padding: 0.3em; margin-left:15px; border:1px solid #a9a9a9; text-align:center; font-size: 95%; background:#f5f5f5" width="160px" align="right"
 
|- 
 
|'''Code:''' R227
 
|}
 
  
'''Definition'''
+
Index of all [[:Category:170 Maintenance Activity Planning Guidelines#Index of Printable Planning Guides|Maintenance Planning Guidelines]].
The time and expenses incurred for installing and maintaining fences along the roadway, guardrails and end treatments, guardcable and impact attenuator devices for roadside obstacles as well as all costs to maintain concrete traffic barriers, sidewalks and bicycle paths and raised pavement markers.
 
  
'''Purpose'''
+
==606.2.7 Construction Inspection Guidelines for Guard Cable==
To restore guardcable.
 
  
'''Scheduling'''
+
'''For [http://www.modot.org/business/standards_and_specs/SpecbookEPG.pdf#page=9 Sec 606.50.2]'''.  The embankment slope between the shoulder and the guard cable should be 1V:6H (6:1) or flatter, unless the system is approved for use on slopes as steep as 1V:4H (4:1). If only one run of three-strand guard cable is installed in the median, the slope on both sides of the guard cable should be 1V:6H (6:1) or flatter, unless the system is approved for use on slopes as steep as 1V:4H (4:1). No exceptions should be allowed unless approved by the Central Office. This is essential for the guard cable to perform as designed.  
As needed
 
 
 
'''Recommended Equipment'''
 
:* Truck
 
:* Post drive
 
:* Traffic Control Equipment, refer to [[616.23 Traffic Control for Field Operations|Traffic Control for Field Operations]].
 
 
 
'''Recommended Material'''
 
:* cable  
 
:* post
 
:* anchors
 
 
 
'''Recommended Procedure'''
 
 
 
1. Place traffic control devices as needed.  
 
 
 
2. Remove damaged cable and posts.
 
 
 
3. Install new posts and cable.
 
 
 
4. Remove traffic control devices.
 
 
 
'''Safety'''
 
Wear all appropriate Personal Protection Equip (PPE). Refer to [http://lnapp1/RI/RIManual.NSF/SHToC?OpenView Safety Policies, Rules & Regulations-Employee Handbook] .
 
 
 
'''Other Considerations'''
 
None.
 
 
 
'''Reference'''
 
[http://www.modot.mo.gov/business/standards_and_specs/Sec0606.pdf Sec 606]
 
  
 +
The embankment slope behind the guard cable is not critical (may be as steep as 1V:2H (2:1)) if another run of three-strand guard cable is installed on the other side of the median to protect crossovers from that direction of traffic or if adequate clear zone is provided in the other direction of traffic. Such "double runs" are discouraged, however, since both the initial and lifetime costs are doubled.
  
 +
{| border="1" class="wikitable" style="margin: 1em auto 1em auto" align="right"
 +
|+
 +
! style="background:#BEBEBE"|Sieve Size !! style="background:#BEBEBE"|Percent Passing by Weight (mass)
 +
|-
 +
|align="center"|3 in. (75mm)||align="center"| 100
 +
|-
 +
|align="center"|1 in. (25mm)|| align="center"|80
 +
|-
 +
|align="center"|No. 4 (4.75mm)|| align="center"|0-35
 +
|}
 +
'''Aggregate Bedding (for [http://www.modot.org/business/standards_and_specs/SpecbookEPG.pdf#page=9 Sec. 606.50.2.4])'''. Having a predominantly one-sized stone as a bedding material for guard cable, as currently specified in Sec 606.50.4, will act as marbles when a vehicle impacts the bedding material and will likely result in an impacting vehicle to dive under the cable system and continue across the median into the opposing traffic, thereby defeating the purpose of the guard cable system. This is elevated to even a larger safety issue where contractors have provided sand or gravel as the bedding material, which have a greater tendency to roll like marbles when impacted and increases the probability for a vehicle to dive beneath the barrier system. In the interim of getting a specification revision, existing jobs should be change ordered to a bedding material consisting of a uniform, angular graded material of a gradation similar to that shown below. Verification of the gradation should be accomplished by visual inspection, and when in suspect, a sieve analysis should be conducted.
  
 +
'''Delineators (for Sec. 606.50.2.5).''' All three-strand guard cable, regardless of the location of the guard cable, should be delineated, with delineator spacing, reflective sheeting and reflector colors in accordance with Sec 606.10.2.3.
  
 
[[Category:606 Guardrail and Guard Cable]]
 
[[Category:606 Guardrail and Guard Cable]]

Revision as of 09:26, 11 June 2019

For Additional Information
"MoDOT's Cable Median Barrier Program", a report from 2007.
Guard Cable Safety Information
Videos
Successful guard cable test
Guard Cable in Action
MoDOT's You Tube Guard Cable video

606.2.1 Guard Cable Types

Cable median barriers, commonly referred to as guard cable, remain one of the most efficient roadside safety treatment available today. Guard cable consists of twisted wire ropes mounted on weak posts. It is relatively inexpensive to install, compared to more rigid systems, and has been proven effective at capturing errant vehicles. There are two types of guard cable systems in use on Missouri roads: low-tension and high-tension.

606.2.1.1 Low-Tension. Since no single producer exclusively manufactures low-tension guard cable, this system has been commonly called the “U.S. generic” system. Low-tension guard cables typically consists of three cables placed at different heights and are tensioned only enough to eliminate sag between posts. Large springs at either end of the cable run are compressed, according to temperature, to achieve the system’s low tension. The cable itself is strung on posts that are directly driven into the ground.

Low-tension Guard Cable

When a vehicle impacts the low-tension system under normal conditions, the cable laterally moves as much as 12 ft. This movement is known as the dynamic deflection.

Median Guard Cable
Summary, 2006
Report, 2010
See also: Innovation Library

Given the lack of tension in the system, individual installations, or “runs”, of cable are limited to 2000 ft. with an anchor assembly at each end. When a vehicle strikes low-tension cable, the system becomes disabled and will not function properly if subsequently struck by another vehicle. As such, it is critical to repair the guard cable promptly.

Low-tension systems have been in service for some time and have proven their value by reducing cross-median accidents. However, the issues related to down time and the necessity to utilize on-call contracting cause a perpetual drain on MoDOT resources. For these reasons, the use of low-tension cable systems should be limited to small-scale installations with special circumstances.

606.2.1.2 High-Tension. High-tension cable barrier looks very similar to low-tension cable but the two systems are very different in most other aspects. High-tension guard cable consists of three or four pre-stressed cables supported by weak posts.

High-tension Guard Cable

During installation, the cables are placed on the posts and then tightened to a specific tension, ranging from approximately 2,000 to 9,000 pounds according to temperature. Due to this tightening, the cable installations can be of indefinite length. In fact, the runs are typically only limited by the presence of obstacles such as median openings or bridge columns.

Under normal conditions, when a vehicle impacts the high-tension system the cable laterally deflects as much as 8 ft. The inherent tension within the system also allows the cable to remain at the proper height, even after an impact removes several posts. While the system is not designed to continue to function in that condition, there is a great deal of anecdotal evidence that it does just that.

Currently Approved High-Tension Systems and Manufacturers
High-Tension System Manufacturer
Brifen Brifen USA
CASS Trinity Industries, Inc.
Gibraltar Gibraltar
Safence Safence, Inc.
U.S. High Tension Marion Steel Company

A common installation of high-tension guard cable employs concrete footings into which metal tubes are cast, forming sockets. The socket allows a post to be replaced with relative ease during a repair operation. The damaged post is simply removed from the socket and replaced with a virgin post. Socketed systems eliminate the requirement for specialized post driving equipment and subsurface utility location for each repair.

A socketed, high-tension system should be chosen for large-scale guard cable installations. While such a system generally has a higher initial cost, the low cost and high efficiency with which it can be maintained make it a better value over its life cycle. A high-tension system incorporating socketed posts is easily repaired and maintained with the resources currently available to the district maintenance personnel. Additionally, high-tension systems can be used on a variety of median inslopes, often eliminating the need for costly slope corrections and drainage modifications.

As of 2007, all high-tension systems are proprietary, that is, marketed under exclusive rights of a specific manufacturer. Five systems are currently marketed in the United States.

606.2.2 Systematic Application of Median Guard Cable

Median guard cable is most effective when installed as a system-wide solution to address cross-median crash types. The benefits are severely limited if the cable is only used in spot locations in response to crashes at those locations.

Additionally, when determining the most appropriate locations for guard cable application, the designation of a route (interstate, US highway, state route) should not be a primary consideration.

A corridor should have similar geometry and traffic volume and the placement of guard cable on the corridor should have logical termini. Spot location installation of new median guard cable should be used sparingly only in unique situations.

606.2.3 Warrants

Analyses of cross-median crash history and traffic volume provide valuable information in determining the likelihood of future severe crashes on these routes. In order to prevent future fatalities and disabling injuries, it is important to focus safety efforts on locations that will benefit the most from safety countermeasures.

606.2.3.1 Crash Data. Analysis of crashes on a candidate corridor should focus on cross-median crashes on that route and, even more so, on those crashes resulting in fatalities and disabling injuries.

It is important this data analysis is robust, particularly on expressways. Due to at-grade intersection crashes on these routes, a simple query of cross-median crashes may include unwanted events and exclude necessary ones. Accuracy of these data is vital in decision-making.

The data should be reviewed each year to validate priorities and identify any emerging cross-median safety concerns. A regular review of divided highway traffic volume and crashes will provide information to proactively address severe cross-median crashes.

606.2.3.2 Traffic Volume. Recent research has connected traffic volume growth directly to cross-median crash events. As volume increases, the probability of a motorist crossing the median and hitting an oncoming vehicle also increases. Instead of relying solely on crash history, there is an opportunity to proactively address this crash type before the crashes occur by studying traffic volume patterns and installing a system of median guard cable on routes with sharply increasing volumes.

606.2.3.3 Median Width. Recent national experience has shown that cross-median crashes can occur on highways with median widths above MoDOT's initial 60 ft. threshold. Although this width has largely proven to be effective in detering such crashes, no route will be excluded from analysis solely on the basis of median width. Divided highways with very wide medians are expected to have little or no cross-median crash history, effectively removing them from consideration for barrier installation.

606.2.4 Design and Installation Guidelines

606.2.4.1 Lateral Placement in the Median

Dynamics of Cross-Median Crashes. When a vehicle leaves the roadway and enters the median, certain predictable dynamics occur. Vehicles may enter the median at a variety of speeds and angles but for the purposes of roadside safety research and testing, a 60 mph departure at a 20° or 25° angle is generally used.

Upon departure, a vehicle will initially continue along its vertical trajectory. As the inslope falls away along the 25° vehicle path, the vehicle effectively becomes briefly airborne. When the vehicle's inertia can no longer overcome gravity, it lands and its suspension is deeply compressed. As the vehicle continues to travel through the median, the suspension rebounds and the bumper of the vehicle stays at a relatively constant height throughout the remainder of the errant journey.

Every guard cable crash is slightly different because of a host of site-specific factors. In general, however, the front of the vehicle must engage at least two of the three or four cables present in order to be contained by the system. Given the dynamics described above, lateral placement of the cable can be grouped into two main categories: medians wider than 30 ft. and those narrower than 30 ft.

606.2.4.1 Placement.jpg

Medians 30 ft. or wider. The guard cable should be installed no more than 4 ft. downslope of the edge of the shoulder. With wider shoulders, the downslope location could be less than 4 ft., but in any case, there must be 8 ft. between the barrier and the edge of traveled way. There are several advantages to this location but chief among them is the performance of the system in a crash. At the 4 ft. downslope location, the errant vehicle adjacent to the barrier, while airborne, is not at a great enough altitude to override the cable during a front side encounter. From the opposing direction, or backside, the suspension of the errant vehicle will have recovered enough to allow an impact to occur under relatively normal impact conditions.

If the 8 ft. separation cannot be obtained, the designer must work with the Central Office Design Division to assess the potential safety impacts of a decreased deflection distance. If the arrangement cannot be proven reasonably safe, a different barrier system must be used.

Medians narrower than 30 ft. In medians narrower than 30 ft., the guard cable should be installed within 1 ft. of the vertex of either a V or flat-bottomed ditch. As previously discussed, this location performs the most advantageously. The 4 ft. downslope location starts to fail in narrower medians as the suspension of the vehicle impacting from the back side (i.e. the opposite direction) is the most tightly compressed around that location. Again, a fully compressed suspension has proven to be the principal reason for vehicles underriding the system.

Alternating Sides. The designer may choose to alternate the sides of the median where the barrier is placed for the purpose of reducing any shy line issues or discomfort for motorists. The change should occur at natural breaks in the barrier such as emergency crossovers or median bridge columns.

Lateral Placement of Low-tension Guard Cable. New installations of low-tension guard cable should be installed within 1 ft. of the vertex of either a V or flat-bottomed ditch. Retrofits should be located at the existing offset, provided the system is functioning well.

606.2.4.2 Parallel Installations

In-service experience with parallel installations has shown less than desirable results. The close proximity of each installation to traffic has caused an inordinately high incidence of nuisance hits resulting in higher than acceptable long-term maintenance costs. Vegetative maintenance is also a concern.

Parallel installations of guard cable should not be used. Instead, designers should rely upon guard cable designed for the situation as a single run or consider a barrier system other than guard cable.

606.2.4.3 Post Spacing

While guard cable has been tested and approved with post spacing ranging from 6.5 to 32.5 ft., it is widely believed that the wider post spacing leads to greater deflections and an increased likelihood of vehicle penetration due to underride or traveling between the cables. For this reason, post spacing should not exceed the conventional limit of 20 ft. Additionally, increasing post spacing through horizontal curves increases the opportunity for the cable to assume a chord length if the posts are damaged. If enough posts are damaged, the cable could project into the travelway on the inside of the curve.

606.2.4.4 Slopes

1V:6H (6:1) or Flatter Slopes. Guard cable, like most roadside hardware, is intended for use on slopes that are 1V:6H (6:1) or flatter. This requirement is based on both computer modeling and full-scale crash testing and represents sound theory. In practice, however, slopes as flat at 1V:6H are often the exception.

Steeper Slopes. All of the proprietary high-tension systems are now approved for use on slopes with gradients between 1V:6H (6:1) and 1V:4H (4:1). Their use, while generally more expensive, represents the most cost-effective solution for shielding steeper slopes. Further, since more than three equivalent sources exist, there is no need to obtain a material certification for their use.

606.2.4.5 Vegetative Barrier

Vegetation control in the area between the cable and the passing lane must be addressed. Failure to provide some positive form of vegetation control will hinder the future maintenance of the system. Positive vegetation control measures may include herbicides, a geotextile-aggregate strip or asphalt apron. The core team must consult with the local maintenance personnel to arrive at a vegetative control measure that is mutually agreeable.

A district's decision to mow around the barrier must be approved by Central Office Maintenance. Such mowing operations must be accomplished without impeding through traffic in any manner.

Anchor Assembly

Vegetation control may not be omitted from a project as a practical design or value engineering measure.

606.2.4.6 Termination at Emergency Crossovers

The design for guard cable termination as well as the grading for the crossover should be in accordance with Standard Plan 606.41, Sheet 7 of 7. Refer to EPG 240.4 Guard Cable Termination at Emergency Crossovers for additional information.

606.2.5 Maintenance and Repair

Irrespective of routes treated, proper placement or system used, cable median barrier is only as functional as its ongoing maintenance and repair. Proper maintenance and incident repair will ensure that the system is always in a state of functionality to provide motorists a greater level of safety on Missouri highways.

Routine Maintenance. Outside of vegetation control, there is little routine maintenance required for a guard cable system. If pre-stressed cables are used for high-tension systems and compensators are properly compressed for low-tension systems, the tension in the cable should properly acclimate to any weather condition. The tension monitoring stage occurs during and shortly after construction.

Cable Height. The importance of cable height to properly capture and redirect errant vehicles has been demonstrated. Although cable height is relatively static in all systems, erosion under the barrier can sometimes cause a localized increase in height, resulting in possible underride.

Maintenance personnel should be educated on the necessity of proper cable height and encouraged to identify and repair locations where erosion or the accumulation of silt have altered the relative cable height.

Median Condition. A secondary issue, closely related to incident repair, is the post-entry condition of the median. In addition to the repair of the roadside hardware, the median condition with respect to rutting, loss of vegetation and accident debris should be remedied following each accident. These incidental concerns could cause instability in the trajectory of future errant vehicles and could, at worst, result in a failure of the system.

Low-Tension Cable Barrier Repair: On-Call Contract. Maintenance of low-tension cable barrier is vastly more complicated than that of a high-tension system. In fact, the complexity of the system coupled with the frequency of crash incidents, have traditionally resulted in the system’s maintenance being outsourced through on-call contracts.

High-Tension Cable Barrier Repair: In-House. Equipment and hardware needs for the repair of high-tension, socketed guard cable are minimal and repairs can generally be accomplished in under an hour with two workers, some hand tools and a pickup truck.

Response Time. Due to the importance of the median guard cable performing when needed, it is vital to quickly respond to repair needs. This will often necessitate an effort to identify cable hits as soon as possible after the incident and then respond with repair as quickly as possible.

606.2.6 Maintenance Planning Guidelines for Guard Cable

See Maintenance Planning Guideline for Guard Cable.

Index of all Maintenance Planning Guidelines.

606.2.7 Construction Inspection Guidelines for Guard Cable

For Sec 606.50.2. The embankment slope between the shoulder and the guard cable should be 1V:6H (6:1) or flatter, unless the system is approved for use on slopes as steep as 1V:4H (4:1). If only one run of three-strand guard cable is installed in the median, the slope on both sides of the guard cable should be 1V:6H (6:1) or flatter, unless the system is approved for use on slopes as steep as 1V:4H (4:1). No exceptions should be allowed unless approved by the Central Office. This is essential for the guard cable to perform as designed.

The embankment slope behind the guard cable is not critical (may be as steep as 1V:2H (2:1)) if another run of three-strand guard cable is installed on the other side of the median to protect crossovers from that direction of traffic or if adequate clear zone is provided in the other direction of traffic. Such "double runs" are discouraged, however, since both the initial and lifetime costs are doubled.

Sieve Size Percent Passing by Weight (mass)
3 in. (75mm) 100
1 in. (25mm) 80
No. 4 (4.75mm) 0-35

Aggregate Bedding (for Sec. 606.50.2.4). Having a predominantly one-sized stone as a bedding material for guard cable, as currently specified in Sec 606.50.4, will act as marbles when a vehicle impacts the bedding material and will likely result in an impacting vehicle to dive under the cable system and continue across the median into the opposing traffic, thereby defeating the purpose of the guard cable system. This is elevated to even a larger safety issue where contractors have provided sand or gravel as the bedding material, which have a greater tendency to roll like marbles when impacted and increases the probability for a vehicle to dive beneath the barrier system. In the interim of getting a specification revision, existing jobs should be change ordered to a bedding material consisting of a uniform, angular graded material of a gradation similar to that shown below. Verification of the gradation should be accomplished by visual inspection, and when in suspect, a sieve analysis should be conducted.

Delineators (for Sec. 606.50.2.5). All three-strand guard cable, regardless of the location of the guard cable, should be delineated, with delineator spacing, reflective sheeting and reflector colors in accordance with Sec 606.10.2.3.