July 10, 2003
Barry D. Stephens, P.E.
Senior Vice President of Engineering
ENERGY ABSORPTION Systems, Inc.
3617 Cincinnati Avenue
Rocklin, California 95765
Dear Mr. Stephens:
Your April 8 and April 23 letters were recently delivered to Mr. Richard Powers of my staff by Mr. Douglas Bernard. In those letters, you requested the Federal Highway Administration's (FHWA) acknowledgement that your modified Safe Stop 180 truck mounted attenuator (TMA) successfully met the evaluation criteria for the supplemental TMA tests recommended in the National Cooperative Highway Research Program (NCHRP) Report 350. To support your request, you provided copies of an April 2003 report by E-TECH Testing Services, Inc., entitled "NCHRP Report 350 Crash Test Results for the Improved Safe-Stop 180 TMA" which contained data on both of the required TMA tests (i.e., test 3-50 and a "modified" test 3-51 as described below), and on the optional tests (tests 3-52 and 3-53). With the April 23 letter, you submitted a supplemental report that contained the results of the standard TMA test 3-51. However, you requested that the FHWA withhold issuance of its formal acceptance letter until July when the new Safe-Stop 180 was expected to be ready for distribution.
Like the original Safe-Stop 180, the improved version is 4190-mm long and 2360-mm wide at the impact face, but weighs approximately 5 kilograms less (940 kg). Although there were some minor external changes made, the most significant change was in the internal arrangement of the individual energy-absorbing cells to accommodate off-center and/or angle impacts into the end of the unit. Enclosure 1 is a drawing of the improved Safe Stop 180.
Enclosure 2 consists of the summary sheets for Report 350 tests 3-50, 3-51, 3-52, and 3-53. All evaluation criteria were met, but in each test the actual impact speeds were below the nominal speed of 100 km/h. In fact, for modified test 3-51 (see Enclosure 3) where the support vehicle was physically blocked to minimize forward movement and in test 3-52, the reported impact speeds were slightly below the speed tolerance recommended for these tests. Since the impact speed was within tolerance for the standard test 3-51 and ridedown acceleration reported in test 3-52 was 17.8 g's (2.2 g's below the allowable limit), I will accept the results of these two tests in spite of the reported low Impact Severity (IS). Please note section 3.3.5 in Report 350 recommends that "…the actual IS be equal to or greater than the target value" for a TMA. In none of the tests you conducted was the target value obtained. However, in the two tests noted above none of the evaluation criteria were at the upper limit. I am reasonably confident that the improved Safe Stop 180 would meet all evaluation criteria if the target IS had been met, but at some point in the future the FHWA may not accept the results of new tests for TMAs (or crash cushions) in which the target impact severities are not substantially met and one or more of the evaluation criteria are at or near the upper limits.
In light of the above discussion, your improved Safe Stop 180 is acceptable for use on the National Highway System as a TL-3 TMA when such use is acceptable to the contracting authority. Since the improved version is similar in outward appearance to the original design, the new unit should be clearly marked in some manner so the two designs can be readily differentiated in the field.
As noted above, your modified test 3-51 consisted of a blocked support vehicle and all evaluation criteria were met except the ridedown acceleration, which was reported to be 20.7 g's. Since the support vehicle could not roll forward when blocked, a truck of any weight over 9,000 kg could be used as a support vehicle and similar accelerations would be likely. I agree that a heavier support vehicle can be used with the improved Safe Stop 180 at the discretion of the contracting authority and with its understanding that ridedown accelerations will approach and may surpass the 20 g limit under some impact conditions when the support vehicle is significantly heavier than 9,000 kg.
(original signed by Michael S. Griffith)
Michael S. Griffith
Acting Director, Office of Safety Design
Office of Safety