b.  Crash Pulse

The comments received on this aspect of the NPRM focused generally on the issues of the sled pulse shape (widening of the corridor) and severity. [9]

1.  On Widening The Corridor

As for widening the corridor of the sled pulse from 80 milliseconds (ms) to approximately 90 ms in duration, all but few of the commenters responding to this issue supported the change. Many agreed with the agency that the change would allow more laboratories to run the compliance test "without decreasing the effectiveness of the testing" (quoting UMTRI).   SafetyBeltSafe (SBS) also agreed with NHTSA's assessment, explained in the preamble to the NPRM, that the pulse would enable tests to be conducted closer to 30 mph.

The JPMA and Graco did not support revising the corridor. JPMA stated that widening the corridor necessarily makes the standard more stringent, because child restraint manufacturers will have to design products that can comply at the new extremes of the compliance corridor. The commenter stated that difficulties experienced by test labs in fitting their pulses within the existing corridor "should be addressed by insisting that the test labs figure out how to meet the existing test corridor."  JPMA and Graco believed that a wider test corridor will necessarily lead to more lab-to-lab variability during certification and compliance testing, which, the commenters stated, increases the compliance burden on manufacturers. JPMA stated that the agency did not provide data on the effect of the different crash pulse with the new bench seat, and believed that the agency must assess the effect of a wider sled pulse corridor on child restraint compliance.

The agency responds by concurring that the revision to the pulse could affect the manufacture of child restraints. Widening the test corridor from 80 ms to approximately 90 ms in duration does enable NHTSA to test child restraints closer to 30 mph than the present. To the extent that the 30 mph tests are more stringent than tests conducted in the past at slightly lower speeds, that result is a desired outcome of the amendment. Widening the corridor improves the effectiveness of the test. Child restraint manufacturers will have to certify that their child restraints meet the requirements of FMVSS No. 213 when tested using the test pulse, possibly at a higher velocity. They may have to conduct some testing to ensure that the restraints can be certified to the requirements when tested in the more effective manner using this pulse. The agency acknowledged in the NPRM the likely need for manufacturers to retest their restraints because of the new seat assembly and, by implication, due to the changes to the crash pulse (67 FR at 21829). However, the agency believed then and continues to do so now that it is unlikely that child restraints must be redesigned because of the change in the assembly and pulse. [10]   Restraints are generally manufactured with enough of a compliance margin that will allow them to meet the requirements of the standard when tested at a slightly higher velocity.

  To illustrate, NHTSA examined some of the work that was performed in support of the development of the child restraint ratings program required under Section 14(g) of the TREAD Act. As part of this effort, the agency examined the margin by which existing child restraints meet the injury limits currently specified in FMVSS No. 213. In model year 2000, the agency tested 50 upright, forward-facing child restraints in accordance under the agency's FMVSS No. 213 compliance test program. Twenty-four (24) seats were tested without a top tether, and 26 seats were tested with a top tether. We secured all seats with only a lap belt (no lower anchorages or shoulder belts). Currently, to pass the FMVSS No. 213 compliance test, a child restraint must achieve dummy injury numbers of a HIC less than 1,000 and a resultant chest acceleration of less than 60 G's. As shown below in Figure 1, regardless of whether we equipped the child restraints with a top tether, all child restraints achieved dummy injury readings below the maximum allowable values. Figures 2 and 3 illustrate the margin of compliance for HIC and chest acceleration, respectively. The margin of compliance is one minus the measured injury reading divided by the injury assessment reference value (IARV) times 100. Higher percentages are better, having less probability of injury. Regarding the HIC, all model year 2000 child restraints tested easily fall within the limits specified by the FMVSS No. 213 compliance tests. Most had a compliance margin of more than 50%. Although the margin is not as large for chest acceleration, all tested child restraints passed this compliance requirement as well.

FMVSS. No. 213 also has a requirement for head and knee excursions. Head excursion is limited to 720 mm (28 in) when a top tether is used, and 813 mm (32 in) without use of a top tether. Knee excursion is limited to 915 mm (36 in). Figures 4 and 5  below illustrate the margin of compliance for head excursion and knee excursion, respectively. Head and knee excursion limits are compliance limits imposed to reduce the chances of a child striking the vehicle interior or submarining (sliding under the belt feet first) in an automotive crash. Head and knee excursions are much closer to the compliance limits than HIC and chest acceleration. This may reflect attention to occupant protection, since increases in distance traveled by the occupant reduces the forces experienced by the occupant.

During the development of the child restraint ratings program, the agency also conducted dynamic testing of a number of child restraints both at 30 and 35 mph to examine what differences, if any, resulted from the increase in the velocity at which the test was conducted. To attain the higher speed, a sled pulse with a similar shape and duration length as that of the FMVSS No. 213 pulse was used, except that the change-of-velocity was elevated from 30 mph (48km/h) to 35 mph (56km/h). All of the child restraints tested produced dummy injury measurements well below the FMVSS No. 208 criteria of 570 HIC and 55g chest acceleration (Hybrid III 3-year-old dummies were used in the tests). Although the injury assessment values were slightly greater in the 35 MPH (56 km/h) sled tests than in 30 mph (48 km/h) sled test, eight of the nine child seats tested rated within the 5 star range, and one fell just marginally below in the 4 star range. This data, in conjunction with the information provided above regarding the compliance margin achieved by existing child restraints, demonstrates that a nominal increase in the test velocity resulting from the crash pulse corridor established as part of this final rule will not necessitate a redesign of existing child restraint designs to meet the injury criteria limits established in the standard.

The agency also does not believe that unusual or unacceptable variability will be introduced into the test results simply because more test labs will be involved in conducting child restraint tests. Any lab-to-lab variability resulting from a properly conducted test will be insignificant, in part because each laboratory must ensure that the pulse it uses in the FMVSS No. 213 sled test falls within the corridor specified in the standard. In addition, it is the responsibility of manufacturers to design and manufacture child restraints to meet the requirements of the standard, taking into account whatever variability occurs from seat-to-seat manufacturing differences and from lab-to-lab testing differences. It should also be noted that child restraint manufacturers are responsible for ensuring that their restraints meet the requirements of the standard when tested by NHTSA in its compliance test. Manufacturers testing their products to the most demanding requirements under the most demanding test conditions increase the likelihood that their products will meet the requirements when tested by NHTSA under the same or less severe conditions. In the same manner, prudent testing by the manufacturer accounts for routine lab-to-lab variability that may occur when testing child restraints. Manufacturers must design and produce products that will pass the compliance test regardless of the laboratory conducting the test.

2. Increase Pulse Severity

ARCCA opposed the NPRM based on concerns that the proposed changes to the crash pulse would "lower, rather than raise, the bar for child restraints." The commenter believed that the Standard No. 213 pulse is actually less severe than all of the 30 mph barrier test pulses from actual vehicles, and that the standard's pulse severity should be increased.   The commenter suggested that the standard specify that the dynamic test will be conducted at velocities of not less than 30 mph. "This will ensure that manufacturers do not take advantage of the wider corridor to conduct testing that is less severe than what is currently required by FMVSS 213."  ARCCA also stated that the standard "should contain a minimum acceptable peak acceleration level that is more than the 19 G's or [sic] the proposed corridor in the NPRM."  ARCCA stated:

This minimum acceleration level should be high enough to ensure that a child restraint will offer acceptable performance and be capable of remaining structurally intact. Testing performed by one auto manufacturer in a minivan demonstrated that various child restraints structurally failed in 30 mile per hour sled testing using the vehicle's barrier crash pulse. By setting a high minimum peak acceleration, confidence can be gained in the ability of a child seat to remain structurally intact and protect a child no matter in what vehicle it is installed.    

ARCCA suggested that the agency specify in Standard No. 213 that the test pulse must fall within a specific corridor and must have a velocity of at least 30 mph and a peak acceleration of at least some predetermined value. ARCCA believed that that acceleration value should be based on the values obtained from barrier crash tests and be greater than the majority of all FMVSS No. 208 tests reported. ARCCA was also concerned about how the values presented in Table 4 of the NPRM were calculated, especially the peak g values. The commenter believed that the values in the NPRM were erroneously based on "average pulses" i.e. point-by-point averaging of the pulse data to form a single curve for a class of vehicles. ARCCA stated that the problem with this method is that when pulses with peaks at different times are combined, the resulting peak is less than either of the pulses averaged. "This is due to the fact that the crash pulses are out of phase. This is similar to the principle used in noise cancellation devices, when two waves are superimposed the magnitude of the resulting pulse is less." 

The agency does not agree with ARCCA that the standard's pulse is deficient and should be increased. The pulse is representative of a severe crash and subjects child restraints to "worst case" testing in a sufficient manner. The severity of a crash pulse is determined through a combination of three factors: the acceleration onset rate, the peak acceleration, and the time duration of the pulse. The data presented in the PAX report are based on FMVSS No. 208 rigid barrier testing at 30 mph impact speed (approximately 32 mph total change in velocity, ∆V) and New Car Assessment Program (NCAP) rigid barrier testing at 35 mph (approximately 37 mph ∆V).

The FMVSS No. 213 pulse was very similar to the pulses generated by sport utility vehicles (SUVs), trucks and small school buses in an FMVSS No. 208 (32 mph ∆V) crash test. NHTSA believes that the pulse should be severe enough to be adequately representative of these vehicles since child restraints are regularly and increasingly used in these types of vehicles. That is, the stringency of the pulse is justified to better ensure that each child restraint will not have structural degradation in a crash and will limit forces to the child's head, neck and torso to tolerable levels, no matter the vehicle the child is in.

ARCCA was correct that the agency had averaged the pulses for the three classes of vehicles (SUVs, trucks and a small school bus) to develop a composite pulse for each vehicle class, and that the composite pulses had peak acceleration levels that are typically lower than the highest peak accelerations measured in the individual tests. However, the averaged pulses allowed the agency to examine general trends with respect to the crash parameters that determine the performance of vehicles in a crash. As such, they are representative of the pulses of vehicles in which child restraints are likely to be used and provide a reasonable foundation upon which the standard's pulse can be based. Further, the agency is unaware of the testing to which ARCCA referred that allegedly demonstrated "that various child restraints structurally failed in 30 mile per hour sled testing using the vehicle's barrier crash pulse." To the contrary, child restraints have proven very effective in real world crashes and have performed well in the agency's studies of child restraint performance in vehicles tested in NCAP 35-mph frontal crashes.

ARCCA suggested that the standard specify that the dynamic test will be conducted at velocities of not less than 30 mph. This specification is unnecessary, since the standard currently requires the dynamic tests to be frontal barrier impact simulations "at a velocity change of 48 km/h [30 mph] with the acceleration of the test platform entirely within the curve shown in Figure 2…."  Thus, the agency already conducts the dynamic test at velocities as close as possible to 30 mph without exceeding 30 mph or causing the pulse to fall outside of the curve of Figure 2 of the standard.

ARCCA believed that the velocity of the sled test should be increased from 30 mph to 33 mph to replicate the change in velocity typically seen in a 208 barrier test. "For the 213 pulse to be near the 30 mph barrier test the velocity, acceleration and duration would all have to be increased."  The commenter also believed that, since "well-restrained adult occupants are capable of surviving crashes comparable to a 35 mph barrier crash where the change in velocity is closer to 40 mph," tests of child restraints should be performed at the levels specified by the agency in testing vehicles in the New Car Assessment Program.

In contrast, all other commenters except ARCCA commenting on this issue did not want to increase the severity of the crash pulse. SafetyBeltSafe (SBS) believed that the velocity change should not be raised to 33 mph because "the current test is already reflective of the top few percent of crashes." SBS stated that increasing the velocity "will not significantly improve child restraint performance in the real world but will surely make the products more expensive."  Graco stated that if the pulse were increased to 33 mph, it would expect a large number of child restraints needing to be redesigned with "minimal benefit to child passenger safety."  UMTRI stated that the change in velocity for the test should remain at 30 mph, stating that it conducted a recent analysis of National Automotive Sampling System (NASS) data from 1995-2000 which showed that a 30 mph change in velocity is more severe than approximately 98 percent of the frontal impact crashes nationwide. UMTRI further noted that since the NASS database only includes tow-away crashes, "this is a conservative estimate of the percentage of frontal impacts that are less severe than 30 mph."  UMTRI was concerned that increasing the velocity of the test is not likely to increase safety, but will increase consumer cost of child restraints and may lead to child restraint designs that could make the restraints less effective or more easily misused at lower severity crashes, "which occur much more frequently."  The Insurance Institute for Highway Safety (IIHS) stated that its review of NASS cases showed that child restraints designed to pass the current 30 mph sled test are providing very good protection to children in frontal crashes. IIHS also stated, "There was no indication, based on an analysis of injuries, crash description, and photos in these 10 frontal crashes that designing child restraints to withstand higher crash forces could have prevented or mitigated any of the serious or fatal injuries."

NHTSA concurs with these comments that the standard's crash pulse adequately meets a safety need. Increasing the severity could necessitate the redesign of many child restraints and could increase costs of the restraints to manufacturers, without a proportionate safety benefit. Thus, the agency concludes that the pulse should not be made more severe at this time.

3.  Decrease Pulse Severity

While there was almost unanimous agreement among commenters that the crash pulse should not be increased, commenters expressed opposing opinions on whether the severity of the test pulse should be decreased. The crash pulse is more severe than most other pulses, but is similar to crash pulses of large sport utility vehicles and light trucks (passenger vehicles that are becoming more and more popular for use as family vehicles) and very similar to the crash pulse of small school buses. The agency determined in the NPRM that the crash pulse should maintain its level of stringency so as to replicate vehicle crashes involving vehicles that had relatively severe crash pulses. Some commenters disagreed, believing that the crash pulse should be reduced in severity because the most frequent crashes involving children in child restraints are those with lower crash pulse severities than the test pulse, while other agreed that a relatively severe, "worst case" scenario should be replicated.   

In support of reducing the severity of the crash pulse, the Alliance of Automobile Manufacturers (Alliance) stated that the current sled pulse represents—

an extremely rare "worst case" [(e.g., a stiff vehicle hitting a full-width non-deformable wall at high speed)]. As a result the addition of the new dummies/injury criteria coupled with this unrepresentative test pulse may create significantly unintended consequences such as reduced availability and increased costs of compliant restraints as well as the addition of features that may make them more cumbersome and less user friendly. All of which will reduce their use in the real world.

The Alliance stated that an attachment it submitted with its comment contains an analysis comparing the severity (acceleration pulses) of full frontal barrier crashes with vehicle-to-vehicle crash tests. "In this analysis a 30 mph full frontal barrier test is found equivalent to a 41 mph vehicle-to-vehicle crash. A reduced speed of 22 mph for full frontal rigid barrier test is found to represent vehicle-to-vehicle crashes with 50%-100% overlap, with each vehicle traveling at 30 mph."

Along the same lines, General Motors (GM) believed that the crash pulse should represent the most frequent collision event. The commenter urged research to define the real world collision speeds and deceleration pulses at which the majority of the harm to children occurs. GM believed that increasing the pulse duration and widening the corridor increases the pulse severity somewhat, and coupling this increase with the use of the new test dummies and injury criteria "could make compliance more difficult." GM suggested that NHTSA consider using the FMVSS No. 208 generic sled pulse if the final rule adopts the Hybrid III test dummies and injury measures proposed in the NPRM.

The Children's Hospital of Philadelphia (TraumaLink) supported altering the pulse to be more representative of the passenger car environment to "make it more relevant to a larger proportion of the real-world crash-involved population."  The commenter stated that out of the 59,968 children studied in TraumaLink's Partners for Child Passenger Safety study, only 24.1 percent of children were riding in SUV's and light trucks.

In contrast, in support of the agency's decision not to reduce the severity of the crash pulse, Advocates for Highway and Auto Safety (Advocates) believed that although cars remain more numerous in the vehicle fleet, use of an LTV crash pulse is representative of real-world crash experience given that increasing numbers of LTVs have entered the fleet and are frequently used as passenger and family vehicles. The commenter also discussed why it believed the crash pulse should replicate the "worst case" scenario over the "most frequent" or "average" crash:

Although Advocates has urged the agency to update its test procedures in certain respects to ensure that they are representative of the modern vehicle fleet, this does not mean that critical test procedures should mirror the attributes or test the performance of only the "average" vehicle. While test procedures should be representative of the vehicle fleet in many respects, not all tests or test procedures should be based on the most common or average vehicle in the fleet. To ensure safety protection for all vehicle occupants, critical aspects of test procedures should replicate more stringent conditions than would be experienced in the average vehicle. This is especially true when only one test and a single set of test conditions are used as the basis for compliance. Thus, although there are still more cars than LTVs on U.S. highways, and even though more children are injured while riding in cars than are injured while riding in LTVs, the FMVSS 213 sled test should replicate the faster acceleration onset rate and higher peak acceleration exerted in an SUV crash pulse. Of the two, the LTV crash pulse presents the more stringent test condition. Using the LTV-like crash pulse ensures that children exposed to such a severe force, as well as children exposed to less severe conditions in cars, will be afforded protection. The reverse, however, is not true. If FMVSS 213 adopted a car-like sled test crash pulse, children in cars may be protected but that same degree of safety would not necessarily be provided to children in LTVs with "stiffer" frames that transfer more of the crash generated energy to the occupants. As a result, Advocates concurs in the agency's judgment that the existing FMVSS 213 crash pulse be retained.

After reviewing all the comments on this issue, NHTSA has decided to retain the current severity of the pulse and not reduce it. The agency concurs with Advocates that to ensure safety protection for as many child occupants as possible, "critical aspects of test procedures should replicate more stringent conditions than would be experienced in the average vehicle," and that, given that child restraints are used with a wide range of vehicle types and are involved in crashes of varying degrees of severity, such a critical aspect is the sled pulse. Accordingly, the agency declines to replicate the crash conditions of the most frequent collision event.

GM suggested that NHTSA consider using the FMVSS No. 208 generic sled pulse if this final rule adopts the Hybrid III test dummies and injury measures proposed in the NPRM. As discussed later in this preamble, this final rule adopts the Hybrid III test dummies but does not adopt the majority of the injury measures proposed in the NPRM. Nonetheless, the agency makes the following observations about the suggestion to use the FMVSS No. 208 generic sled pulse. The generic sled pulse is less severe than the FMVSS No. 213 pulse. As shown in the following overlay of the existing FMVSS No. 213 pulse with the FMVSS No. 208 generic sled pulse, the former has a greater onset rate, higher peak acceleration and shorter time duration. Further, the FMVSS No. 208 sled pulse, with a peak acceleration of about 17 g's, is less stringent than most 30 mph passenger vehicle crashes. Because the FMVSS No. 208 sled pulse is less severe than the FMVSS No. 213 pulse, this final rule declines the suggestion to adopt it.