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FHWA Home / Safety / Roadway Departure / Safety Eligibility Letter

Safety Eligibility Letter CC-118

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U.S. Department of Transportation
Federal Highway Administration

1200 New Jersey Ave. S.E.
Washington, D.C. 20590

August 22, 2011

In Reply Refer To: HSST/CC-118

Mr. Mark R. Morgan
Director of Engineering
Smith & Wesson Security Solutions
277 Mallory Station Road, Suite 112
Franklin, TN 37067

Dear Mr. Morgan:

This letter is in response to your request for the Federal Highway Administration (FHWA) acceptance of a roadside safety device for use on the National Highway System (NHS).

Name of system: Expeditionary Mobile Barrier
Type of system: Crash Cushion / Arrester net
Test Level: NCHRP Report Test Level 3
Testing conducted by: Texas Transportation Institute
Date of request: December 10, 2010
Date of completed package: December 10, 2010
Initially acknowledged: December 14, 2010

You requested that we find this system acceptable for use on the NHS under the American Association of State Highway and Transportation Officials Manual for Assessing Safety Hardware (MASH).

Requirements
Roadside safety devices should meet the guidelines contained in the NCHRP Report 350 or the American Association of State Highway and Transportation Officials’ Manual for Assessing Safety Hardware (MASH).  The FHWA Memorandum “Identifying Acceptable Highway Safety Features” of July 25, 1997 provides further guidance on crash testing requirements of longitudinal barriers.

Decision
The following device was found acceptable:

Description
The Smith & Wesson Security Solutions Expeditionary Mobile Barrier (EMB) is a deployable net system which may be used as a method of lane closure or temporary access control.  The EMB consists of four main components: (1) net; (2) lift arms; (3) textile brakes; and (4) anchors.

The net is constructed of 2 inch polyester webbing woven together. At each joint, the webbing is stitched together.  The net is 48 ft long, 42 inches tall and weighs approximately 29 pounds. It is configurable to provide a barrier width range from 14 ft to 40 ft. Enclosure 1 shows the details of the net.

The net is supported by two lift arms, one on each side of the roadway where the net is to be deployed. Each lift arm is constructed of aluminum tubing and contains an electric powered actuator that lowers and raises the net (Enclosure 2).  Power to each attenuator is provided by a 12 volt battery that is attached at the base of the lift arm. Each assembled lift arms weighs approximately 132 pounds.

Upon impact by a vehicle, the net releases from the lift arms.  Two textile breaks connect the two ends of the net to the anchors.  Once the net is released from the lift arms, it begins to engage the textile brakes.  The textile brake is an energy absorption technology that is based on absorbing kinetic energy through the tearing action of two textile straps woven together.

In this submission, two separate methods for anchoring the textile brakes were used: (1) earth anchors or (2) vehicle anchors.  In the earth anchor method, each end of the net is anchored to two M.K. Rittenhouse & Sons, Ltd. 88DB1 duckbill earth anchors.  Each anchor cable is 42 inches long and ¼ inch in diameter and is rated for 3000 pounds static holding force.  The duckbill earth anchors are driven into the ground leaving steel loop above the ground.  In the vehicle anchor method, each brake is anchored to a standard class III trailer hitch of a vehicle that weighs at least 6000 pounds.

Enclosures 3 and 4 show the earth anchorage system and the vehicle anchorage system respectively.  Connections between the anchor and the textile brake are accomplished using two 10,000 pound rated polyester straps with hooks on each end and three shackles.  A ½ inch shackle attaches the 10,000 pound straps to each earth anchor while a ¾ inch shackle attaches the straps to the textile brake.  When the textile brake is anchored to a trailer hitch receiver, a ¾ inch shackle is used to connect the two 10,000 pound straps to the receiver. 

Crash Testing
Three full crash tests were conducted on the test article described above according to MASH guidelines as shown in the following table:

ID MASH Test Width of the Net (ft) Anchorage Method

USR16 3-40 40 Earth Anchor
USR17 3-41 40 Vehicle Anchor
USR18 3-41 14 Earth Anchor

In test 3-40, the centerline of the vehicle (MASH 1100C small car) was aligned with the right quarter point of the barrier.  In both tests conducted according to test 3-41, the centerline of the vehicle (MASH 2270P pickup truck) was aligned with the centerline of the net.

In all three tests, the test article slowed, captured, and stopped the test vehicle.  The post impact trajectories of the test vehicles were predictable.  In all three tests the Occupant Impact Velocity values are smaller than the maximum limits set by MASH and Occupant Ridedown Acceleration values are below the preferred limits.  Therefore, it is judged that the test article successfully passed the three tests.  Enclosures 5 through 7 summarize the test results for tests USR16 through USR18 respectively.

The stopping distances of vehicles in tests USR16, USR17, and USR 18 were 78 ft, 152.3 ft, and 138.2 ft respectively.

Findings
According to MASH, test designations 3-40 through 3-45 are to be conducted for non-redirective gating crash cushions for Test Level 3 approval.  The test article described above was not crash tested according to MASH tests 3-42 through test 3-45.  You have requested that these tests be waived for the EMB system.

The test article is a symmetric net.  Therefore, the oblique impacts are not perceived to be more critical than straight impacts (the impacts where the longitudinal axis of the impacting vehicle is perpendicular to the net).  Therefore, it is judged that test 3-42 will not be more critical than test 3-40. Also, test 3-43 and 3-44 will not likely to be more critical than test 3-41.

Test 3-45 is intended to examine the performance of crash cushions during impact by mid-sized vehicles.  The main concern for the test is that attenuator staging can be tuned to meet the testing requirements for small cars and heavy pickup trucks without adequately accommodating mid-sized vehicles.  Due to the design of the EMB, it is not expected that the system will perform differently for mid-sized cars than small cars or heavy pickup trucks.

Therefore, we concur with your request that the tests 3-42 through 3-45 be waived.

Because the lift arms are substantial structures, they would generally require shielding in an actual field installation to prevent errant motorists from striking one and further losing control of their vehicles.  The same consideration would apply when other vehicles are used to anchor the ends of the EMB.

In your letter you requested the approval of the EMB for the following conditions:

The FHWA concurs with your first request on the grounds that the EMB performed as expected with both anchorage systems.  We also agree with your second request because the test results associated with test USR 17 and USR 18 are consistent with each other.  The videos of both tests also show a consistent behavior of the system.

According to the results obtained from crash testing, no hazard shall be present within 160 ft behind the nets for the net width of 14 ft and 40 ft respectively.  No information is available for the stopping distance of vehicles for nets with the net width between 40 ft and 14 ft and a conservative approach has to be adopted.

Therefore, the device described in the various requests above and detailed in the enclosed drawings is acceptable for use on the NHS under the range of conditions tested, when acceptable to a highway agency.

Please note the following standard provisions that apply to FHWA letters of acceptance:

 

Sincerely yours,

/* Signature of Michael S. Griffith */
Michael S. Griffith
Director, Office of Safety Technologies
Office of Safety

Enclosures

Page last modified on January 17, 2013.
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