Mr. Agus The
Amsafe Commercial Products
240-C North 48th Avenue
Phoenix, AZ 85043
Dear Mr. The:
This is in response to your letter asking whether the Locktec child restraint buckle release meets the "two or more finger" requirement of Federal Motor Vehicle Safety Standard (FMVSS) No. 209, Seat belt assemblies, as incorporated into FMVSS No. 213, Child restraint systems. As explained below, the two-finger standard is a width requirement, which is not satisfied simply by the use of two fingers in actuating the buckle release.
S18.104.22.168(c) of FMVSS No. 213 requires any buckle in a child restraint system to "[m]eet the requirements of S4.3(d)(2) of FMVSS No. 209, except that the minimum surface area for child restraint buckles designed for push button application shall be 0.6 square inch [387 mm2]." You state in your letter that: "The Locktec buckle has [a sliding mechanism for release] and not a push button or a lever application." Since your buckle release is not designed for push button application, the 0.6 square inch minimum surface area requirement in S22.214.171.124(c) does not apply.
S4.3(d)(2) of Standard No. 209 reads:
A buckle designed for pushbutton application of buckle release force shall have a minimum area of 452 mm with a minimum linear dimension of 10 mm for applying the release force, or a buckle designed for lever application of buckle release force shall permit the insertion of a cylinder 10 mm in diameter and 38 mm in length to at least the midpoint of the cylinder along the cylinder's entire length in the actuation portion of the buckle release. A buckle having other design for release shall have adequate access for two or more fingers to actuate release.(Emphasis added.)
Because your buckle release is designed for slide application rather than push button or lever application, your buckle release falls under the "other design for release" category.
Under the last sentence of S4.3(d)(2), your buckle must have adequate access for two or more fingers to actuate the release. In the photos you provided of the release being actuated, one finger is in the slide action release button and an additional finger is on the buckle base opposite of the sliding mechanism. The placement of the finger on the base merely provides support to the buckle while the release is actuated. Only one finger is accessing the slide action release button.
FMVSS No. 209 requires that a slide action release button be large enough to be accessed by a minimum of two fingers, placed side-by-side. While there is no clear indication of what is meant by "two fingers" in terms of a minimum width, the two-finger requirement of FMVSS No. 209 was included in FMVSS No. 213 to ensure that child restraint buckles are easy to operate.(See, 50 Federal Register 33722.)The buckle release mechanism must be sufficiently large enough to reduce the force to surface ratio required to actuate release. The need to conveniently unbuckle a child restraint system (CRS) is of particular importance in emergency situations when there is a need to quickly remove a child from a CRS.
The release mechanism on your buckle does not accommodate two fingers of a majority of adults. The width of the index finger of a small, 5th percentile adult female at the knuckle nearest the hand is approximately 16 mm, and the width of the index finger of a 50th percentile male at the same position is approximately 21 mm.  Given the normal reduction in finger width at the tip as opposed to the knuckle nearest the hand and approximating the combined width of the index and middle finger, "two or more fingers" for a 5th percentile female is approximately 28 mm. The contactable surface for actuating the release on the Locktec buckle is less than 25 mm. As such, the vast majority of adults would be unable to place two fingers side-by-side to actuate the Locktec buckle.
We recognize that there is some ambiguity in the two-finger specification and that a more objective criteria, specifying a minimum linear width would be appropriate. We plan to address this requirement in future rulemaking. If you have any further questions, please feel free to contact Mr. Chris Calamita of this office at (202) 366-2992.
 Stephen Pheasant, "Bodyspace: Anthropometry, ergonomics, and the design of work" 49 (Taylor & Francis) (1996).