Federal Highway Administration
400 Seventh St., S.W.
February 26, 2007
In Reply Refer To: HSSD/CC-95
Mr. Felipe Almanza
Mr. Jan Miller
TrafFix Devices, Inc.
220 Calle Pintoresco
San Clemente, California 92672
Dear Messrs. Almanza and Miller :
In your July 24, 2006, letter, you requested the Federal Highway Administration's (FHWA) acceptance of a new redirective crash cushion called Compressor at the National Cooperative Highway Research Program (NCHRP) Report 350 test level 3 (TL-3). To support your request, you also sent a copy of a KARCO Engineering report dated July 20, 2006, entitled "Crash Test Report for TrafFix Devices Compressor" and digital videos of crash tests you conducted on the Compressor. You also requested the waiver of tests 3-36, 3-37 and 3-39.
Crash cushions should meet the guidelines contained in the NCHRP Report 350, "Recommended Procedures for the Safety Performance Evaluation of Highway Features". The FHWA memorandum, “ACTION: Identifying Acceptable Highway Safety Features” of July 25, 1997, provides further guidance on crash testing requirements.
The TrafFix Devices Inc. Compressor is a re-directive, non-gating crash cushion with a total length of 255.25" (6.5 m). Its effective length is 196" (4.98 m). The Compressor measures 48.66" (1.24 m) wide, and is 53.5" (1.36 m) in height.
The main components include: a steel mounting base, six plastic energy absorbing modules, and twelve steel fender panels. The base deck surface is comprised of a dovetail rail and a structural underside. The smooth deck surface allows the modules to slide as they compress during an impact. The dovetail retains the modules on the dovetail track, preventing lateral or vertical movement upon impact. The modules are made of high density polyethylene (HDPE) and are manufactured by an injection molding process. Each module is comprised of two module halves. The module halves are shaped with a combination of a concave and convex curvature. Two heights of modules are used on the Compressor, Modules # l and 2 are 24" (0.6 m) tall and have a wall thickness of 1-1/2" (38.1 mm). Module # 3 is 48" (1.22 m) tall and has a wall thickness of 1-1/2" (38.1 mm). Modules # 4, 5, and 6 are 48" (1.22 m) tall and have a wall thickness of 1-7/8" (47.6 mm). Modules are attached to their adjoining modules using three 5/8" (15.9 mm) -11 grade 8 bolts between the 48" (1.22 m) tall modules and two 5/8" (15.9 mm) -10 grade 8 bolts between the 24" (0.61 m) tall modules. Each module uses lower and upper clips to fasten each module half together to make one module assembly. A dovetail shape is molded into the bottom of each module half. The dovetail is centered about the spine of the convex surface of each module half. The molded dovetail mates the module to the base dovetail, allowing for modules to compress, and slide longitudinally, but retains the modules from lateral or vertical motion. In addition to the dovetail shape molded into the module halves, the modules are further restrained to the base by retaining plates located between adjoining modules. To alleviate snagging of the test vehicle on a re-direct impact, a 29" (0.74 m) tall HDPE lateral support stiffening rib is installed between modules 1 and 2 and between modules 2 and 3. The module clips on modules 3, 4, 5 and 6 incorporate an anti snag guard.
The nose piece incorporates an expanded contact surface that mates to the dovetail rail on the base. Sliding friction of the nose shoe is reduced by incorporating fiber reinforced nylon slide inserts on the contact surfaces between the nose shoe and the dovetail rail on the base. The nose shoe is attached to the front module and the nose piece by a 1-1/4" (31.8 mm) steel pin.
The side fender panels are made from high strength steel, 0.125" (3.2 mm) thick. The shape of the fender panels allows them to resist damage on impact, slide, and telescope during longitudinal compression of the attenuator. Six sets of side fender panels are used, and are attached at their leading edge to corresponding lower edge clips of each module. The rear set of side fender panels are retained at their trailing end by two 3/8" (9.5 mm) wire rope cables. The wire rope cables allow the panels to telescope, stack, and minimize flaring of the panels during impact.
A minimum of ten and a maximum of fourteen 3/4" (19 mm) studs were used to secure the base to the concrete test pad. The 3/4" (19 mm) studs used in the testing were 5" to 6" (127 mm to 152.4 mm) ACME thread length below grade with a 1" (25.4 mm) shoulder and 3/4" (19 mm) -10 x 1-1/2" (38.1 mm) above grade to secure the base to the studs. A high strength two part catalyzed resin anchoring material is used to secure the below grade portion of the 3/4" (19 mm) studs.
Enclosure 1 shows the general layout of the TrafFix Devices Inc. Compressor and selected components.
It should be noted that the described design differs from the initial designs used in test 3-33 that was successfully passed and first unsuccessful test 3-38. The described design incorporates a change to alleviate snagging of the test vehicle on a re-direct impact observed in first unsuccessful test 3-38 by using 29" (0.74 m) tall HDPE lateral support stiffening rib between modules 1 and 2 and between modules 2 and 3 and also by addition of an anti snag guard on module clips on modules 3, 4, 5 and 6.
Full-scale crash testing to evaluate the impact performance of the TrafFix Devices Compressor included tests 3-30, 3-31, 3-32, 3-33 and 3-38. The results of these tests are summarized in Enclosure 2. All evaluation criteria as per NCHRP 350 requirements in these tests were met.
Initially, test 3-38 failed due to the observed severe damage to the right A-pillar sustained by the test vehicle. The design of the TrafFix Devices Compressor was changed to alleviate snagging of the test vehicle on a re-direct impact and re-testing was successful. I agree that these design changes should not affect the results of test 3-33 conducted previously and therefore, no retesting to NCHRP 350 TL-3-33 conditions was required.
You requested the waiver of tests 3-36 and 3-37 and acceptance of the TrafFix Devices Compressor based only on tests 3-30, 3-31, 3-32, 3-33 and 3-38.
In test 3-38 the C.I.P was selected to maximize the potential for pocketing or snagging, and was located 39" (1 meter) back from the leading face of the triangular nose piece, between the first and second short modules. In test 3-37 the test vehicle would be impacting with its front right corner the beginning of the first side fender panel, within the first short module. Since the crash cushion design remains essentially unchanged along its sides at these two impact points, I agree that test 3-37 would be very similar to the test 3-38 which was successfully passed. But if the truck’s front right corner were impacting at the break of some sort between the actual nose and the first side panel, the test would be similar to 3-33 which was also successfully passed. Based on these considerations I agree that test 3-37 can be waived. Similarly, with tests 3-38 and 3-32 (which is generally the most demanding for a small car in terms of occupant impact velocity and ridedown acceleration) successfully passed, I agree that the test 3-36 would be less severe and can be waived.
Even though this is not a pass/fail criterion, it is preferable that the vehicle’s post impact trajectory not intrude into adjacent traffic lanes. In the tests impacting the nose of the Compressor, it absorbs the impact energy by compressing and then strives to restore its initial shape by pushing back the impacting vehicle, which may result in vehicle intrusion in the adjacent traffic lanes in rearward/sideward manner. E.g. in tests 3-30 the impacting car came to rest approximately 36.7 ft (11.2 m) to the left and approximately 24 ft (7.3 m) rearward of the point of impact. The observed post-impact performance should be taken into account when determining suitability of the device to the specific highway applications
In summary, I agree that the TrafFix Devices Compressor, as described above, meets the appropriate evaluation criteria for a NCHRP 350 TL-3 devices. Since no transition design has yet been developed for a reverse direction hit and the NCHRP Report 350 Test 3-39 was not conducted, the TrafFix Devices Compressor should not be used in locations where opposite-direction impacts are probable without further development and testing. However, the TrafFix Devices Compressor may be used at all appropriate locations on the NHS when selected by the contracting authority, subject to the provisions of Title 23, Code of Federal Regulations, Section 635.411 as they pertain to proprietary products.
Please note the following standard provisions that apply to the FHWA letters of acceptance:
John R. Baxter, P.E.
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