Category:612 Impact Attenuators: Difference between revisions

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[[image:612 Impact Attenuators 2013.jpg|right|325px|thumb|<center>'''Protective vehicle with a truck-Mounted Attenuator and Flashing Arrow Panel'''</center>]]
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|[http://sp/safety/csp/Pages/Safety-Videos.aspx 2014 TMA Safety Points]
| <big>'''Protective vehicle with a truck-Mounted Attenuator and Flashing Arrow Panel'''</big>
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Impact attenuators are designed to absorb energy of an impacting vehicle and reduce the force on a passenger. Types of impact attenuators include:  truck -mounted attenuators (TMA), freestanding impact attenuators (sand barrels), and work zone crash cushions.


==612.1 Truck-Mounted Attenuators==
Truck-mounted attenuators (TMAs) are energy-absorbing devices attached to the rear of appropriate protective vehicle. The protective vehicle that supports the TMA must meet manufacturer specifications.


The National Cooperative Highway Research Project 350 (NCHRP 350) and the 2016 AASHTO Manual for Assessing Safety Hardware (MASH 2016) set the crash criteria for TMAs. Non MASH 2016 impact attenuators manufactured prior to January 1, 2023 may be used until January 1, 2030. All impact attenuators manufactured after January 1, 2023 shall meet MASH 2016 Test Level 3 crash test requirements.


Damaged TMAs are to be removed from service and either repaired or replaced.


The traffic control guidance of the EPG serves as MoDOT’s Manual on Uniform Traffic Control Devices (MUTCD) and should be used as the primary document when applying traffic control guidance to MoDOT’s roadways.  Per federal regulations, MoDOT’s traffic control policies are required to be in substantial conformance with the MUTCD.  There are occasions in the MUTCD where more than one traffic control device can be applied, but the EPG guidance typically selects the most applicable option for use on MoDOT roadways. The EPG also omits content from the MUTCD which doesn’t apply to Missouri.
===612.1.1 MoDOT TMA Marking, Emergency Alert Lights, and Audible Alert System===
While in the operating position, the rear facing of the TMA shall be marked with alternating 8-inch yellow and 8-inch black retroreflective sheeting forming an inverted “V” at the center and slope downward at an angle of 45 degrees toward each side of the unit or a checkered board pattern consisting of 12- inch square red and 12-inch square white retroreflective sheeting. The TMA may be marked with the same operating pattern or red and white DOT conspicuity tape to simulate the looks of a standard van body trailer when traveling.


Like the MUTCD, the traffic control guidance of the EPG is permissive guidance, meaning this guidance outlines what is permissible in regard to applying traffic control devices to MoDOT roadways.  When specific traffic control guidance is not found in the EPG, contact the Highway Safety and Traffic Division office for assistance.  The Division office can obtain assistance from FHWA, research facilities and other states for possible solutions that are MUTCD compliant.  If these solutions have the possibility of being applicable to more than one site, consideration will be made to include the solution into the standard guidance found in the EPG as a standard.
Emergency Alert Lights (EALs) and Audible Alert systems on MoDOT MASH 2016 tested TMAs are used in accordance with typical applications.  


This article provides information for the use of protective vehicles and three primary types of impact attenuators:  truck-mounted attenuators, trailer-mounted attenuators and freestanding impact attenuators (sand barrels).  Impact attenuators are designed to absorb energy of an impacting vehicle and reduce the force on a passenger to an acceptable level.  
All lighting should be appropriately set, depending on the day or nighttime conditions.


===612.1.2 TMA Operations===
====612.1.2.1 Construction Projects====
For construction projects, the use of a stationary TMA may be required on the temporary traffic control plans above and beyond the requirements of the MUTCD. If the use of a stationary TMA is required, the stationary TMA should be included on the temporary traffic control plans, contain the appropriate bid item as well as the applicable JSP for Truck Mounted Attenuator for Stationary Activities.


==612.1 Protective Vehicles/TMAs (MUTCD "Shadow Vehicle")==
For construction projects, TMAs required for mobile operations, such as striping, are considered incidental per Sec 612.5.


Protective vehicles/TMAs are used to safeguard the workspace from errant vehicles. In some operations, these devices also serve as platforms for signs and other devices used to warn traffic of upcoming conditions or inform them of needed actions. For increased motorist, driver and worker safety, the protective vehicle may be equipped with a truck-mounted attenuator.
====612.1.2.2 MoDOT Operations====
During MoDOT stationary operations, it is recommended the operator not wait inside or near the TMA vehicle.


Proper positioning of the protective vehicle/TMA vehicle within the work zone is critical to its effectiveness. It is the operator’s responsibility to make sure that the protective vehicle/TMA is in the proper position to protect the crew, to provide ample roll-ahead distance, and to provide adequate warning to your co-workers and traveling public.  
During MoDOT short duration and mobile operations, TMA operators are allowed to take preventive action by rolling ahead when they perceive possible interaction with an errant vehicle. All TMAs, except TMA#1 are allowed take preventive action.


{| border="1" class="wikitable" style="margin-left:10px" align="right"
===612.1.3 MoDOT TMA Operator’s Training===
|+ '''''[[media:612.xlsx|Pros and Cons of Truck- and Trailer-Mounted TMAs]]'''''
MoDOT operators are required complete MoDOT’s Truck-Trailer Mounted Attenuator (TMA) Protective Vehicle training (both classroom and On The Job training) in advance operating a TMA.
|-
! style="background:#BEBEBE" colspan="2"|Truck-Mounted TMA <br> Host Vehicle 16,000 lbs GVWR !!  style="background:#BEBEBE" width="5"| !! style="background:#BEBEBE" colspan="2"|Trailer-Mounted TMA <br> Host Vehicle 10,000 lbs GVWR
|-
! style="background:#BEBEBE"|Pros !! style="background:#BEBEBE"|Cons !! style="background:#BEBEBE" width=5| !! style="background:#BEBEBE"|Pros !! style="background:#BEBEBE"|Cons
|-
|align="center" colspan="2"|Cost and Maintenance|| style="background:#BEBEBE" width="5"| ||colspan="2" align="center" |Cost and Maintenance
|-
|width=100|After an impact the TMA is easily secured to the host truck, for transport to shop  ''(Ex: no need for a tow truck)''||width=100| After impact, damage to host truck is common and extended down time of truck is possible. This may be an issue if the vehicle fleet in an area is low. ||style="background:#BEBEBE" width="5"| ||width="100"|For the "boat" trailer style, the TMA normally has less parts, maintenance cost, and assembly cost. ||width=100| Tire issues can cause down time
|-
|No tire maintenance or down time due to tire issues || Maintenance can be costly and timely ||style="background:#BEBEBE" width="5"| || align="center"| - || align="center"| -
|-
| align="center"| - || TMA must be removed prior to Arrow Board maintenance or repairs ||style="background:#BEBEBE" width="5"| || align="center"| - || align="center"| -
|-
|align="center" colspan="2"|Operations || style="background:#BEBEBE" width="5"| ||colspan="2" align="center" |Operations
|-
|width=100|Follows directly behind truck ''(Ex: Will not track paint from striping operations)''|| Difficult to remove and install. ''(Ex: Truck cannot be quickly used for other applications)'' || style="background:#BEBEBE" width="5"| || Trailer TMA are usually easier to install and remove from the truck hitches.||The driver may have difficulties backing, a spotter is recommended.  ''(Ex: Backing up to pick up channelizers within a work zone and keeping the trailer out of the travel lane)''
|-
|Easy Backing when spotter not available ''(Ex: During work zone removal)''|| Difficult to access plugs, wires and lighting connections during installation and removal || style="background:#BEBEBE" width="5"| ||After an impact, the trailer is removed from host vehicle and the truck is usually still serviceable. ||After an impact, the trailer needs towing by flat bed back to shop for repairs based on manufacturer recommendations
|-
|Easy Maneuvering||Truck may not have hitch.  This may limit the number of applications of the truck. || style="background:#BEBEBE" width="5"|  || align="center"| - ||During operations such as striping, tires can track paint
|-
| align="center"| - || When in the down position, the TMA will swing in to adjacent lane || style="background:#BEBEBE" width="5"|  || align="center"| - || Finding a location to turn around can be difficult  ''(Ex:  Rural areas with narrow lanes, small entrances, etc.)''
|-
|-
|align="center" colspan="2"|Other Considerations || style="background:#BEBEBE" width="5"| ||colspan="2" align="center" |Other Considerations
|-
|width=100  align="center"| - ||Overhead hazards become an issue when the TMA is in the Up position, which reaches 13'6". || style="background:#BEBEBE" width="5"| ||When the TMA is not deployed as a protective truck, the tailgate can be left attached for cargo/material hauling. ||Depending on the TMA, the unit may rotate into an adjacent lane upon a side impact.
|-
| align="center"| - ||Due to added weight of the TMA on rear of the truck, the ride is rough and is tough on truck suspension. || style="background:#BEBEBE" width="5"| ||The TMA can be used with different host vehicles and is not dedicated to one vehicle. || Some trailer TMAs look similar to "boat" trailers, which give a perception the TMA units are not safe.
|-
| align="center"| - || Due to the over swing and lowering of the TMA, the area behind the truck should  be clear of personnel and vehicles. || style="background:#BEBEBE" width="5"|  || align="center"| -  || align="center"| -
|}
 
===612.1.1 Truck- and Trailer-Mounted Attenuators===
 
Truck/Trailor-mounted attenuators (TMAs) are energy-absorbing devices attached to the rear of the trucks and used as protective vehicles, thus protecting the motorist and the protective vehicle's driver upon impact.
[[image:612.1 Trailer-type TMAs.jpg|left|275px|thumb|<center>'''Trailer-Mounted Attenuator'''</center>]]
 
The National Cooperative Highway Research Project 350 (NCHRP 350) and the 2016 AASHTO Manual for Assessing Safety Hardware (MASH 2016) set the crash criteria for TMAs. TMAs purchased by MoDOT meet these requirements.
 
Damaged TMAs are to be removed from service and either repaired or replaced.
 
NCHRP 350 and MASH crash-tests  straight-on and offset collisions, not side impacts. TMAs are not designed for side impacts.
 
Articles on MoDOT's [[612.4 Maintenance Planning Guidelines for Impact Attenuators|Maintenance Planning Guidelines for Impact Attenuators]] are available upon request.
 
===612.1.2 MoDOT Protective Vehicle/TMA Marking and Lighting===
'''Lighting.''' All lighting should be appropriately set, depending on the day or nighttime conditions.
 
The Light Bar may be used and the Emergency Alert lights should be used on the Rear Advanced Warning Truck for striping and sweeping operations and are optional on other mobile operations in accordance with typical applications. If used, the rear facing amber/white Light Bar is installed on top of the vehicle and the Emergency Alert lights are installed below the flashing arrow panel.
 
If an approaching vehicle is observed driving in the occupied moving work zone lane, the TMA driver should activate the Emergency Alert lights by pushing the switch for a short duration. It is not recommended to leave the lights on very long because continuous or long term use may reduce the effectiveness of the Emergency Alert lights.
 
'''Rear Face of TMA.''' The rear face of the attenuator device, when in the horizontal or operating position (protective mode), shall be marked with red and white retroreflective sheeting. The marking shall form a checkerboard pattern consisting of 12” by 12” red squares and 12” by 12” white squares spaced symmetrically starting from the top center.
 
'''Vehicle Size.''' On MoDOT’s maintenance operations where the normal posted speed limit is 60 mph or greater, the protective vehicle shall be a Heavy-Duty Single Axle Truck style or heavier, and meet the truck-mounted and trailer-mounted attenuator manufacturer’s recommendations.
 
'''Incident Response.''' For incident response operations, a vehicle with a truck/trailer-mounted attenuator is not required. If used, the host vehicle shall meet the truck/trailer-mounted attenuator manufacturer’s recommendations.
 
===612.1.3 Protective Vehicle/TMA Operations===
The term "protective vehicle" represents the use of the protective vehicle within operations. The term TMA represents a truck or trailer-mounted attenuator.
 
The flashing arrow panel and warning lights shall not be active when the Protective Vehicle/TMA is traveling  from the home domicile to the staging area/desired lane/work location.  Once the Protective Vehicle/TMA has reached the staging area/desired lane/work location, the appropriate operating mode shall be displayed and turned on.  If the Protective Vehicle/TMA leaves the staging area/desired lane/work location and travels to another staging area/desired lane/work location, the flashing arrow panel and warning lights shall not be active.
 
====612.1.3.1 TMA Typical Operations====
 
=====612.1.3.1.1 Stationary Operation=====
During Stationary Operations, all protective vehicle/TMA vehicles shall be parked with the transmission in neutral and the parking brake engaged. The vehicle shall maintain a minimum of 150 feet of roll ahead distance to the next vehicle or work activity, be parallel to traffic, and have its wheels aligned with the striping and lane  to maintain lane discipline and try to stay within the lane if struck. The operator shall not wait inside or near the protective vehicle/TMA vehicle. The employee should proceed to the Work Space to assist other employees and wait for instructions.
 
For additional guidance for protective vehicle/TMA use within work zones, refer to the typical applications in [[616.8 Typical Applications (MUTCD 6H)|EPG 616.8 Typical Applications]].
 
=====612.1.3.1.2 Short Duration/Mobile Operations=====
During short duration/mobile operations, protective vehicle/TMA operators are allowed to take preventive action when they perceive possible interaction with an errant vehicle. Preventive action includes moving the protective vehicle/TMA forward to lessen impact. If the operator takes preventive action, they should be observant of all crew workers and equipment to maintain a minimum of 150 feet roll-ahead distance and stay in the closed lane. In instances where you do not have the recommended 150 ft. of roll-ahead distance, DO NOT roll forward to lessen the impact and, if possible, engage the parking brake.
 
During short duration/mobile operations, if any employees exit their vehicles in the work activity, the protective vehicle/TMA#1 operator closest to work activity shall engage the parking brake when exiting their vehicle. The vehicle should maintain a minimum of 150 feet roll ahead distance to the work activity, be parallel to traffic, and have its wheels aligned to stay within the lane if struck. If the operator of the protective vehicle/TMA closest to the work activity feels he/she is about to be struck by an oncoming vehicle, the operator may take preventive action if the roll-ahead distance is greater than 150 feet.  
 
For protective vehicle/TMA#2 during short duration/mobile operations, if the operator feels he/she is about to be struck by an oncoming vehicle, the operators may take preventive action to lessen the impact. However, the operator shall maintain a minimum of 150 feet of roll ahead distance to the next protective vehicle/TMA, be parallel to traffic, and have its wheels aligned to stay within the lane if struck.
 
For additional guidance for protective vehicle/TMA use within work zones, refer to the typical applications in [[616.8 Typical Applications (MUTCD 6H)|EPG 616.8 Typical Applications]].
 
===612.1.4 MoDOT Equipment/Materials Stored in Bed of Protective Vehicle Guidelines===
 
Loads or cargo, such as sign posts or tools, may be moved to and from work areas in the bed of the protective vehicle/TMA, but must be removed when protective vehicle/TMA is deployed for protection or has the possibility of deployment prior to getting to the work area.  Truck beds may be secured to the truck frame to maximize stability during impact.  Steel plates and/or containers, secured by approved methods, may be used for ballast or weight to keep the dump bed against the host vehicle frame and may remain in the vehicle while the protective vehicle/TMA is deployed.  Loads or cargo are not allowed to be carried on trailer type TMAs.
 
[[image:612.1.4.jpg|center|725px|thumb|<center>'''This bead container is securely fastened to the protective vehicle. The container is bolted through the bed of the truck. The container door is securely latched down.'''</center>]]
 
 
===612.1.5 MoDOT Protective Vehicle/TMA Operator’s Training===
 
Training is conducted in two stages:


:Stage 1 – Classroom General Knowledge Training
==612.2 Impact Attenuator Array (Sand Barrels)==
:Stage 2 – OJT (On the Job) Training and Skills Assessment.
[[image:612.2 Sand Barrels.jpg|right|225px|<center>'''Sand Barrels'''</center>]]


==612.2 Sand-Filled Impact Attenuators (Sand Barrels)==
An Impact Attenuator Array (Sand Barrels) is most often used to shield fixed objects that cannot be removed or relocated, when posted speeds are greater than 35 mph. These devices are recommended for temporary usage such as in work zones. A benefit/cost analysis is to be conducted before sand barrels are used in a permanent application.


[[image:612.2 Sand Barrels.jpg|left|225px|thumb|<center>'''Sand Barrels'''</center>]]
An Impact Attenuator Array (Sand Barrels) consists of a group of freestanding plastic barrels configured in an array of increasing weights from the impact point toward the object. Such an array transfers the vehicle’s momentum to the increasing masses of sand in the barrels and provides a gradual deceleration. Each barrel is designed with a specific weight of sand to absorb the energy of an errant vehicle. The sand barrel array's "footprint" length and width and the number of barrels will change based upon the permanent posted speed limit of the roadway. For the correct setup and array of sand barrel impact attenuators, refer to the manufacturer’s recommendations. For more information about manufacturer’s recommendations, see [http://www.modot.org/business/standards_and_specs/endterminals.htm End Terminals, Crash Cushions and Barrier Systems].


This system consists of a group of freestanding plastic barrels configured in an array of increasing weights from the impact point toward the object. Such an array transfers the vehicle’s momentum to the increasing masses of sand in the barrels and provides a gradual deceleration. Each barrel is designed with a specific weight of sand to absorb the energy of an errant vehicle. The sand barrel array's "footprint" length and width and the number of barrels will change based upon the permanent posted speed limit of the roadway. For the correct setup and array of sand barrel impact attenuators, refer to the manufacturer’s recommendations. For more information about manufacturer’s recommendations, see [http://www.modot.org/business/standards_and_specs/endterminals.htm End Terminals, Crash Cushions and Barrier Systems]. The pay item for one (1) sand barrel impact attenuator array will include the number of sand barrels the manufacturer requires for the posted speed limit and will be paid for each array.  If it is anticipated that the sand barrel array will be relocated during staged construction, the entire array will be relocated and paid for by the pay item Impact Attenuator (Relocation) each time the sand barrel array is relocated. An estimate for replacement barrels needs to be included as a separate pay item, typically calculated as one for each sand barrel array. No direct payment is made for the Type 1 object marker on the lead sand barrel in the array.
The pay item will be based on the posted speed limit for the location the sand barrels will be used. Each Impact Attenuator XX mph (Sand Barrel Array) will be paid for per each, as a unit including the number of barrels and weight of sand in each barrel, as required by the manufacturer.  If it is anticipated that the sand barrel array will be relocated during staged construction, the entire array will be relocated and paid for by the pay item Impact Attenuator (Relocation) each time the sand barrel array is relocated. The relocated array should be used in an area with the same posted speed limit, or another pay item will need to be used. An estimate for replacement barrels needs to be included as a separate pay item, typically calculated as one for each sand barrel array. No direct payment is made for the Type 1 object marker on the lead sand barrel in the array.


[[image:612.2 array.jpg|center|<center>'''Typical Array for Sand-Filled Impact Attenuators
[[image:612.2 array.jpg|center|<center>'''Typical Array for Sand-Filled Impact Attenuators
May Change Based on Manufacturer’s Recommendations'''</center>]]
May Change Based on Manufacturer’s Recommendations'''</center>]]


==612.3 Work Zone Crash Cushion==


Sand barrels are most often used to shield fixed objects that cannot be removed or relocated. Sand barrels are recommended for temporary usage such as in work zones. A benefit/cost analysis is to be conducted before sand barrels are used in a permanent application.
Work Zone Crash Cushions are used to protect traffic from the blunt end of temporary barrier curb.  A crash cushion will be required on the upstream end of barrier curb for divided facilities, and on both ends for all two-way facilities. When space allows, sand barrel impact attenuators are the preferred choice for temporary protection. However, in the event that sand barrels cannot be used (for example, insufficient width), work zone crash cushions may be used instead. Work zone crash cushions provide a narrower option than sand barrels, but still perform the same function. Work zone crash cushions are discussed in [[617.1_Temporary_Traffic_Barriers#617.1.3.3_Crash_Cushion|EPG 617.1.3.3 Crash Cushion]]. Applicable pay items are included in the plans.


An approved sand-filled impact attenuator may be installed on the exposed end of the barrier where the posted speed prior to construction on an existing facility or the anticipated posted speed of a temporary facility is greater than 35 mph.
==612.4 Construction Inspection Guidelines==
<div id="A crash cushion will be required"></div>
'''Material (for Sec 612.2)''' Certifications are to be collected on retroreflective sheeting and any other product specified in Sec 1063.  
A crash cushion will be required on the upstream end for divided facilities, and on both ends for all two-way facilities. When space allows, sand barrel impact attenuators are the preferred choice for temporary protection. However, in the event that sand barrels cannot be used (for example, insufficient width), work zone crash cushions may be used instead. Work zone crash cushions provide a narrower option than sand barrels, but still perform the same function. Work zone crash cushions  are discussed in [[617.1 Temporary Traffic Barriers#617.1.3.3 Crash Cushion|EPG 617.1.3.3 Crash Cushion]].  Applicable pay items are included in the plans.


==612.3 Construction Inspection Guidelines==
'''Safety Requirements (for Sec 612.3)''' The inspector is to request a copy of the manufacturer’s certification that states the units comply with the crash test requirements of NCHRP 350 or MASH 2016, Test Level 3. This information is to be kept in the project files.


'''Material (for Sec 612.2)''' Certifications are to be collected on both the sand and retroreflective sheeting used in or on the sand-filled impact attenuators.  
'''Truck-Mounted Attenuator (for Sec 612.4.1)''' TMAs are to be inspected to make sure they are structurally sound, the frames are not bent and that they appear to be in good working order. In some cases, the contractor may elect to add TMAs when TMAs are not required. Elective TMAs need to be NCHRP 350 or MASH 2016, Test Level 3 compliant and therefore the certification still needs to be collected.  


'''Safety Requirements (for Sec 612.3)''' The inspector is to request a copy of the manufacturer’s certification that states the units comply with the crash test requirements of NCHRP 350 or MASH 2016, Test Level 3. This information is to be kept in the project files.  
'''Sand-Filled Impact Attenuator Array (for Sec 612.4.2)''' The inspector is to request a copy of the manufacturer’s installation instructions for the particular brand of sand-filled impact attenuator the contractor is using. The use of more than one manufacturer’s sand barrels in an array is not allowed. When inspecting the sand-filled impact attenuator arrays, make sure that the array is in the location as shown in the temporary traffic control plans, and set up and filled in accordance with the manufacturer’s recommendations. All lids are to be on and secured. In order to prevent the sand inside the barrels from freezing in cold weather, the sand shall have a maximum moisture content and rock salt added, according to Sec 612.4.2.1.  The contractor should provide the calculations for moisture content and rock salt content for each barrel, so the specification requirements can be verified. When checking the contents of the barrels, rock salt should be visible in the sand mix, to verify uniform dispersion in the sand. During periods of extended cold weather, the sand should be checked periodically to make sure it has not frozen because the salt content has been exhausted. If this condition is found, the contractor will need to add more salt or replace the sand/salt mixture.


'''Truck-Mounted Attenuator (for Sec 612.4.1)'''  TMAs are to be inspected to make sure they are structurally sound, the frames are not bent and that they appear to be in good working order. In some cases, the contractor may elect to add TMAs when TMAs are not required. Elective TMAs need to be NCHRP 350 or MASH 2016, Test Level 3 compliant and therefore the certification still needs to be collected. Typically, TMAs are only required and paid for under conditions where the contractor is operating without a lane drop set up (cones, channelizers, etc.). TMAs the contractor voluntarily adds to an operation are typically not paid for.
<div id="Sand-Filled Impact Attenuator Array"></div>
'''Sand-Filled Impact Attenuator Array (for Sec 612.4.2)'''  The inspector is to request a copy of the manufacturer’s installation instructions for the particular brand of sand-filled impact attenuator the contractor is using. The use of more than one manufacturer’s sand barrels in an array is not allowed.  When inspecting the sand-filled impact attenuator arrays, make sure that the array is in the location as shown in the temporary traffic control plans, and set up and filled in accordance with the manufacturer’s recommendations. All lids are to be on and secured. MoDOT requires rock salt intermixed with the sand so that any water that gets into the barrels will not freeze and create a safety hazard. When checking the contents of the barrels, rock salt should be visible in the sand mix. During periods of extended cold weather, the sand should be checked periodically to make sure it has not frozen because the salt content has been exhausted. If this condition is found, the contractor will need to add more salt or replace the sand/salt mixture.
<div id="Work Zone Crash Cushions"></div>
'''Work Zone Crash Cushions (for Sec 612.4.3).''' The inspector is to request a copy of the manufacturer’s installation instructions for the particular brand of crash cushion the contractor is using. When inspecting the work zone crash cushion, make sure that the crash cushion is in the location as shown on the temporary traffic control plans and set up in accordance with the manufacturer’s recommendations. If the crash cushion is water-filled, MoDOT requires a mixture content per manufacturer’s recommendations so that the crash cushion will not freeze and create a safety hazard. During periods of extended cold weather, the crash cushion(s) should be checked periodically to make sure it has not frozen. If this condition is found, the contractor will need to correct and/or replace the mixture. In the event the work zone crash cushion is damaged and needs to be replaced, it is considered incidental and replaced at no cost to the Commission.
'''Work Zone Crash Cushions (for Sec 612.4.3).''' The inspector is to request a copy of the manufacturer’s installation instructions for the particular brand of crash cushion the contractor is using. When inspecting the work zone crash cushion, make sure that the crash cushion is in the location as shown on the temporary traffic control plans and set up in accordance with the manufacturer’s recommendations. If the crash cushion is water-filled, MoDOT requires a mixture content per manufacturer’s recommendations so that the crash cushion will not freeze and create a safety hazard. During periods of extended cold weather, the crash cushion(s) should be checked periodically to make sure it has not frozen. If this condition is found, the contractor will need to correct and/or replace the mixture. In the event the work zone crash cushion is damaged and needs to be replaced, it is considered incidental and replaced at no cost to the Commission.

Latest revision as of 06:53, 3 May 2024

Protective vehicle with a truck-Mounted Attenuator and Flashing Arrow Panel

Impact attenuators are designed to absorb energy of an impacting vehicle and reduce the force on a passenger. Types of impact attenuators include: truck -mounted attenuators (TMA), freestanding impact attenuators (sand barrels), and work zone crash cushions.

612.1 Truck-Mounted Attenuators

Truck-mounted attenuators (TMAs) are energy-absorbing devices attached to the rear of appropriate protective vehicle. The protective vehicle that supports the TMA must meet manufacturer specifications.

The National Cooperative Highway Research Project 350 (NCHRP 350) and the 2016 AASHTO Manual for Assessing Safety Hardware (MASH 2016) set the crash criteria for TMAs. Non MASH 2016 impact attenuators manufactured prior to January 1, 2023 may be used until January 1, 2030. All impact attenuators manufactured after January 1, 2023 shall meet MASH 2016 Test Level 3 crash test requirements.

Damaged TMAs are to be removed from service and either repaired or replaced.

612.1.1 MoDOT TMA Marking, Emergency Alert Lights, and Audible Alert System

While in the operating position, the rear facing of the TMA shall be marked with alternating 8-inch yellow and 8-inch black retroreflective sheeting forming an inverted “V” at the center and slope downward at an angle of 45 degrees toward each side of the unit or a checkered board pattern consisting of 12- inch square red and 12-inch square white retroreflective sheeting. The TMA may be marked with the same operating pattern or red and white DOT conspicuity tape to simulate the looks of a standard van body trailer when traveling.

Emergency Alert Lights (EALs) and Audible Alert systems on MoDOT MASH 2016 tested TMAs are used in accordance with typical applications.

All lighting should be appropriately set, depending on the day or nighttime conditions.

612.1.2 TMA Operations

612.1.2.1 Construction Projects

For construction projects, the use of a stationary TMA may be required on the temporary traffic control plans above and beyond the requirements of the MUTCD. If the use of a stationary TMA is required, the stationary TMA should be included on the temporary traffic control plans, contain the appropriate bid item as well as the applicable JSP for Truck Mounted Attenuator for Stationary Activities.

For construction projects, TMAs required for mobile operations, such as striping, are considered incidental per Sec 612.5.

612.1.2.2 MoDOT Operations

During MoDOT stationary operations, it is recommended the operator not wait inside or near the TMA vehicle.

During MoDOT short duration and mobile operations, TMA operators are allowed to take preventive action by rolling ahead when they perceive possible interaction with an errant vehicle. All TMAs, except TMA#1 are allowed take preventive action.

612.1.3 MoDOT TMA Operator’s Training

MoDOT operators are required complete MoDOT’s Truck-Trailer Mounted Attenuator (TMA) Protective Vehicle training (both classroom and On The Job training) in advance operating a TMA.

612.2 Impact Attenuator Array (Sand Barrels)

Sand Barrels
Sand Barrels

An Impact Attenuator Array (Sand Barrels) is most often used to shield fixed objects that cannot be removed or relocated, when posted speeds are greater than 35 mph. These devices are recommended for temporary usage such as in work zones. A benefit/cost analysis is to be conducted before sand barrels are used in a permanent application.

An Impact Attenuator Array (Sand Barrels) consists of a group of freestanding plastic barrels configured in an array of increasing weights from the impact point toward the object. Such an array transfers the vehicle’s momentum to the increasing masses of sand in the barrels and provides a gradual deceleration. Each barrel is designed with a specific weight of sand to absorb the energy of an errant vehicle. The sand barrel array's "footprint" length and width and the number of barrels will change based upon the permanent posted speed limit of the roadway. For the correct setup and array of sand barrel impact attenuators, refer to the manufacturer’s recommendations. For more information about manufacturer’s recommendations, see End Terminals, Crash Cushions and Barrier Systems.

The pay item will be based on the posted speed limit for the location the sand barrels will be used. Each Impact Attenuator XX mph (Sand Barrel Array) will be paid for per each, as a unit including the number of barrels and weight of sand in each barrel, as required by the manufacturer. If it is anticipated that the sand barrel array will be relocated during staged construction, the entire array will be relocated and paid for by the pay item Impact Attenuator (Relocation) each time the sand barrel array is relocated. The relocated array should be used in an area with the same posted speed limit, or another pay item will need to be used. An estimate for replacement barrels needs to be included as a separate pay item, typically calculated as one for each sand barrel array. No direct payment is made for the Type 1 object marker on the lead sand barrel in the array.

Typical Array for Sand-Filled Impact Attenuators May Change Based on Manufacturer’s Recommendations
Typical Array for Sand-Filled Impact Attenuators May Change Based on Manufacturer’s Recommendations

612.3 Work Zone Crash Cushion

Work Zone Crash Cushions are used to protect traffic from the blunt end of temporary barrier curb. A crash cushion will be required on the upstream end of barrier curb for divided facilities, and on both ends for all two-way facilities. When space allows, sand barrel impact attenuators are the preferred choice for temporary protection. However, in the event that sand barrels cannot be used (for example, insufficient width), work zone crash cushions may be used instead. Work zone crash cushions provide a narrower option than sand barrels, but still perform the same function. Work zone crash cushions are discussed in EPG 617.1.3.3 Crash Cushion. Applicable pay items are included in the plans.

612.4 Construction Inspection Guidelines

Material (for Sec 612.2) Certifications are to be collected on retroreflective sheeting and any other product specified in Sec 1063.

Safety Requirements (for Sec 612.3) The inspector is to request a copy of the manufacturer’s certification that states the units comply with the crash test requirements of NCHRP 350 or MASH 2016, Test Level 3. This information is to be kept in the project files.

Truck-Mounted Attenuator (for Sec 612.4.1) TMAs are to be inspected to make sure they are structurally sound, the frames are not bent and that they appear to be in good working order. In some cases, the contractor may elect to add TMAs when TMAs are not required. Elective TMAs need to be NCHRP 350 or MASH 2016, Test Level 3 compliant and therefore the certification still needs to be collected.

Sand-Filled Impact Attenuator Array (for Sec 612.4.2) The inspector is to request a copy of the manufacturer’s installation instructions for the particular brand of sand-filled impact attenuator the contractor is using. The use of more than one manufacturer’s sand barrels in an array is not allowed. When inspecting the sand-filled impact attenuator arrays, make sure that the array is in the location as shown in the temporary traffic control plans, and set up and filled in accordance with the manufacturer’s recommendations. All lids are to be on and secured. In order to prevent the sand inside the barrels from freezing in cold weather, the sand shall have a maximum moisture content and rock salt added, according to Sec 612.4.2.1. The contractor should provide the calculations for moisture content and rock salt content for each barrel, so the specification requirements can be verified. When checking the contents of the barrels, rock salt should be visible in the sand mix, to verify uniform dispersion in the sand. During periods of extended cold weather, the sand should be checked periodically to make sure it has not frozen because the salt content has been exhausted. If this condition is found, the contractor will need to add more salt or replace the sand/salt mixture.

Work Zone Crash Cushions (for Sec 612.4.3). The inspector is to request a copy of the manufacturer’s installation instructions for the particular brand of crash cushion the contractor is using. When inspecting the work zone crash cushion, make sure that the crash cushion is in the location as shown on the temporary traffic control plans and set up in accordance with the manufacturer’s recommendations. If the crash cushion is water-filled, MoDOT requires a mixture content per manufacturer’s recommendations so that the crash cushion will not freeze and create a safety hazard. During periods of extended cold weather, the crash cushion(s) should be checked periodically to make sure it has not frozen. If this condition is found, the contractor will need to correct and/or replace the mixture. In the event the work zone crash cushion is damaged and needs to be replaced, it is considered incidental and replaced at no cost to the Commission.