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Summer 1994

Defending Seat Belt Buckle Inertial Unlatch Claims

Defending Seat Belt Buckle Inertial Unlatch Claims

By Tom Branigan


Several months before NBC burned itself by airing its "Dateline NBC" program about fuel tanks on certain General Motors pickup trucks, CBS aired a separate misguided attack on the America Automobile Industry during its September, 1992 "48 Hours" program. This program was about inertial seat buckle unlatch. The program claimed that seat belt buckles of the type used in almost ever American automobile manufactured in nearly the last thirty years could unlatch during an accident if the back side of the buckle was struck by the occupant wearing the seat belt.1 The show's commentator demonstrated this phenomenon on camera by slapping the back of a latched sea belt buckle with the assistance of a plaintiff’s personal injury lawyer and an engineer who is frequently retained by plaintiffs' personal injury lawyers. The buckle released and viewers were led to believe that this could occur in a real world crash.

The natural result of the "48 Hours" show was an onslaught of lawsuits against automobile manufacturers claiming that an occupant of a car involved in an accident was injured because the seat belt "inertially unlatched."

The purpose of this article is to provide an overview of seat belt buckle inertia unlatch and to provide suggestions about dealing with this defensible claim.

What is Inertial Unlatch

The claim is that a seat belt buckle with a spring loaded release button can collide with an occupant's body (usually a leg or the hip) during a crash, accelerating the buckle sufficiently to cause the release button to be activated without human contact. This movement causes the buckle release. All buckles, vehicles and crash sequences may be subject to this claim. Experience has shown that in automobile accident cases where a plaintiff's seat belt use is in question an inertial unlatch case will likely be made.

Plaintiff’s Position

In September, 1992, the Institute for injury Reduction (IR), a plaintiff s advocacy group petitioned the National Highway Transportation Safety Administration (NHTSA) to initiate a defect investigation of side release seat belt buckle designs and it initiate rule making leading to amendment of U.S. Federal Motor Vehicle Safety Standard (FMVSS) 2092 to preclude such designs in the future. Not surprisingly, IR offered no alternative design. Instead, IR, and many plaintiffs, alleged that inertial unlatch occurs due to the belt wearer's impact against the back of the buckle during it crash which can activate the buckle release hardware with unlatching results the same as those occurring while pressing the release button. They also claim that the "defect" appears to worsen with age and use through the repeated operation of the buckle release hardware and the resulting weakening of the spring component of the hardware.

Government Investigations

This claim is not new. The claim was first formally raised by Ralph Nader during testimony before a Congressional Committee in 1966. Nader claimed that side release buckles were dangerously unsafe because they could be inertially unlatched.3 General Motors and Ford responded by presenting the government with a Joint Safety Belt Presentation containing crash test data disputing Nader's claim. It is uncertain at this time whether Congress performed or commissioned any study beyond entertainment Mr. Nader's allegation and General Motors and Ford's joint response.

NHTSA has formally looked at this alleged defect on two occasions during the past sixteen years and concluded both times that there is not a safety problem associated with inertial unlatching of safety belt. In 1978 NHTSA conducted an analysis of side release buckles on 1975 Chevrolet Monzas after receiving one allegation of inertial unlatch of a Monza buckle.

The buckle systems of three Monzas were thoroughly tested by engineers at NHTSA. This analysis also sought to determine if seatbelt buckles on 1971-78 passenger cars could unlatch when impacted on the back side opposite the push button.

Of two-hundred twenty-five buckle systems tested from 1971-78 passenger cars, fifty opened.4 However. NHTSA was critical of these results and its testing techniques and concluded that additional work needed to be performed to develop a "more realistic impact with the buckle." As part of this study, NHTSA also reviewed actual field accident data about the performance of seat belt buckles in automobile accidents and determined there were no claims which might indicate any problems in real world accidents and thus further experiments were not required. Therefore, NHTSA concluded that while some buckles may be capable of inertial opening when accelerations are applied, they are not susceptible to opening with loadings actually experienced by buckles during a crash.5

In 1992 NHTSA conducted its second investigation into inertial unlatch. This investigation was commenced in response to the September, 1992 petition submitted by Institute for Injury Reduction. NHTSA, in an effort to finally put this issue to rest, conducted an extensive evaluation of its crash test data, real-world field accident data and complaints filed with its "Auto Safety Hotline." Information was also requested from auto manufacturers, safety belt manufacturers and auto safety agencies of foreign countries. (Australia and Canada). NHTSA also conducted seat belt buckle component test and full scale automobile crash tests to test the claim.

NHTSA was unable to find any verified case of inertial unlatch and it concluded that there was no evidence of' a safety problem associated with inertial unlatching of safety belts.6 Therefore, IR's petition was denied and NHTSA again told America's motoring public to continue buckling up.

Inertial Unlatch -Science or Fiction

The United States automobile industry has known since the 1960's that seat belt buckles can be unlatched by inertial forces. However, this phenomenon has been known as a "parlor trick" because the unusual combination of forces necessary to cause inertial unlatch would not likely be generated in a real world crash. What does "unusual combination of forces" mean? To unlatch the buckle inertially through the "parlor trick," the buckle must be accelerated or moved due to contact by the wearer or the car itself and the seat belt webbing must experience a tension lever that is not likely to occur in a real world car crash. The parlor trick does not account for the normal amount of tension or the seat belt and the increase in belt tension that occurs during the crash.

Most seat belts are firmly anchored to the car's rigid structure at each end of the belt. During a car crash, the seat belt does not mechanically retract or tighten around the wearer. Instead, decades of automobile crash test data proves that the normal amount of tension on the belt rapidly increases during an accident while the belt wearer physically moves against the belt. This normal but rapidly increasing belt tension causes the seat belt buckle to lock up during the crash. Under typical parlor trick conditions, the buckle opens because there is zero or little tension on the seat belt webbing and the buckle is accelerated with substantial force for a extremely short time duration.

This position is consistent with NHTSA's findings and with findings of independent researchers.

In September, 1992, Failure Analysis Associates, an independent engineering and testing laboratory based in California conducted a series of automobile crash and laboratory tests involving Type I7 buckles in 1983 and 1984 Chevrolet S-10 Blazers. The purpose of these tests was to collect date about the buckles that would be akin to data found in a real world crash setting. No inertial release occurred during: the crash tests.8 The laboratory tests also confirmed that the high buckle accelerations needed to achieve inertial unlatch of this type of buckle are not likely to be seen outside of the laboratory and in a real car crash.

Defending the Inertial Unlatch Claim

If you are confronted with an inertial unlatch claim either as a part of an automobile negligence or product liability case, an early investigation of the accident and the plaintiff can help disprove the claim.

First, develop as early as possible facts regarding the plaintiff's seat belt use history and seat belt use on the date of the accident including belt tension, i.e. how did plaintiff wear the belt, tight, loose, on the hip, etc. Scene witnesses should be consulted on those points, too. Also, the plaintiff's medical records should be analyzed to determine if the plaintiff sustained bruising or other injury in the hip region. Bruising in this area of the plaintiff’s body would be expected if contact with the buckle was sufficient to cause inertial unlatch. This work should be done first through informal discovery and later through formal discovery, if necessary.

Determine how the plaintiff moved inside the car during the accident. i.e. the plaintiff's occupant kinematics. This area of inquiry is critical since the plaintiff’s occupant kinematics during the accident may have the plaintiff moving away from the buckle, thereby ruling out or greatly reducing the chance that the plaintiff made contact with the buckle. You will need to consult with or retain an expert in the field of automobile occupant kinematics to assist you and the fact finder with this issue.

Have the seat belt hardware inspected by an engineer specializing in seatbelt design and performance for evidence of use during the accident. A well qualified seat belt expert should be able to rule out or confirm belt use based on the post-accident condition of the hardware. The existence of such evidence contradicts inertia at unlatch since a belt under enough load to leave load marks during use will likely not inertially unlatch because of the high accelerations of the buckle necessary to overcome this load.

If you are defending the manufacturer of the car or seat belt, or both, obtain all data available on the buckle involved in your case. This data should include test reports and FMVSS compliance testing for FMVSS 209-210.9 Also, try to obtain any actual full scale vehicle sled testing or seat belt hardware testing. Once obtained, this data must be carefully analyzed to determine if there is any possible test event resulting in a buckle opening. If an open buckle is found, all test back-up data should be analyzed by an engineer to determine the reasons for opening.

Consider using the 1978 NHTSA engineering analysis and 1992 NHTSA information request affirmatively in your case in chief. These studies, which concluded no defect, arguably validate the side release seat belt buckle design. MRE 803 (8) permits federal government reports, such as these studies, to be admitted as evidence.10

A motion in limine under MRE 401, 403 and 802 should be filed to preclude admission of the "48 Hours" story on inertial unlatch.


Despite the emotional and unscientific allegations raised by the September, 1992 CBS "48 Hours" story and by the Institute for Injury Reduction, inertial unlatch claims are clearly defensible. Simple scientific principles supported by the facts of the accident developed through an early and thorough investigation will prove the "parlor trick" is not likely to occur in a real world automobile crash.

1. This buckle is commonly known in the automobile industry as a Side Release Buckle.

2. FMVSS 209 can be found at 49 CFR 571.209. This regulation controls seat belt buckle design and performance and states, in pertinent part:

(g) Buckle latch. The buckle latch of a seat belt assembly shall not fail, nor gall or wear to an extent that normal latching and unlatching is impaired, and a metal-to-metal buckle shall separate when in any position of partial engagement by a force of not more than 5 pounds or 2.3 kilograms.

3. Nader, R., Testimony of Traffic Safety, Hearings Before the Committee on Interstate and Foreign Commerce, House of Representatives, On 11.R. 132228 and other bills relating to Traffic Safety, U.S. Government Printing Office, Washington. 1966,

4. Bayer, R.B. Jr. and Kirkbride R. L., Survey of Seat Belt Latching Mechanisms Used on 1971-78 Passenger Cars, Final Report, National Highway Traffic Safety Administration, March 1978.

5. Chiang G. and Ostrosky E.. Alleged failure of seat belt buckle on 1975 Chevrolet Monza 2+2: EA7-040, Memorandum, U.S. DOT. National Highway Traffic Safety Administration, October 24, 1979.

6. Bochly, W.A. and FeIrice B., Letter To Benjamin Kelley, President, Institute for Injury Reduction: DOT National Highway Traffic Safety Administration, Denial of Petition, Nov. 18, 1992.

7. The Type I buckle is the seatbelt buckle most commonly found on motor vehicles in the United States. Approximately 500 million Type I buckles have been manufactured and used to date.

8. Thomas. T.M.. Limbert D.A.. and Lange R.C., An Investigation of Seat Belt Buckle Dynamic Responses to Inertial Loading Conditions, FaAA-AZ,R-92-11-09, Failure Analysis Associates. Inc., November 12, 1992.

9. 49 CFR571.208,49, CFR 572.109 and 49 CFR 571.210.

10. In Bradbury v Ford Motor Co., 123 Mich. App 179, 183-86, 333 NW2d 214 1983), a product liability action. The Court of Appeals held the trial court erred in refusing to admit a National Highway Transportation Safety Administration report containing data compilations concerning automobile transmissions. The report in Bradbury, like NHTSA’s inertial unlatch reports, was prepared pursuant to duty imposed by law, specifically 15 USC 1411 et seq. (1976). and was admissible under MRE 803(8) (B). The report could be used to show the truth of the matter asserted.

TOM BRANIGAN graduated Cum Laude from the Detroit College of Law in 1988. While at DCL he was Editor-In-Chief of the Detroit College of Law Review. He is currently a senior associate in the Detroit office of the national law firm of Bowman and Brooke LLP where his practice is devoted defending product manufacturers in complex product liability trials and litigation.


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