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Vehicle factors affecting tether use and misuse

Jermakian, Jessica S.; Klinich, Kathleen D.; Orton, Nichole R.; Flannagan, Carol A.C.; Manary, Miriam A.; Malik, Laura A.; Narayanaswamy, Prabha
Insurance Institute for Highway Safety
February 2014

Objective: Field studies show that top tethers go unused in half of forward-facing child restraint installations in the United States, despite the presence of tether anchors in passenger vehicles for more than a decade. In the current study, parent volunteers with experience installing child safety seats were asked to use the Lower Anchors and Tethers for Children (LATCH) hardware to install forward-facing child restraints in several vehicles to identify tether anchor characteristics that are associated with tether use.
Methods: Tether anchor characteristics were documented in 57 popular 2012-13 vehicles. These features were used to select 16 vehicles with a range of tether anchor characteristics for volunteer testing. Thirty-seven volunteers were assigned to four groups; each group tested four vehicles and two forward-facing child restraints (one with a single tether strap and one with a v-shaped tether) in a split-plot experimental design. Volunteers were given verbal instruction on LATCH halfway through their trials. Mixed-effects logistic regression models were used to identify predictors of tether use, correct tether attachment, and correct and acceptable tether routing and head restraint position. Tether routing and head restraint position were evaluated independently of correct tether attachment.
Results: Subjects used the tether in 89 percent of the 294 forward-facing child restraint installations and attached the tether correctly in 57 percent of the installations. Tethers were more likely to be used when the tether anchor was located on the rear deck typically found in sedans, which had a use rate of 95 percent, compared with when the anchor was located on the floor, roof, or seatback of other vehicles, which had use rates ranging from 79 to 89 percent. Tethers were less likely to be attached correctly when there was potentially confusing hardware present (OR: 0.21, 95% CI: 0.10-0.44). In addition, tether anchors located on the rear deck or mid seatback had higher rates of correct attachment, 60 and 69 percent, respectively, than those on the floor, roof, or lower seatback, which all had correct attachment rates lower than 50 percent. Subjects had the greatest difficulty in a pickup truck that has a tether anchor constructed of webbing and located on the top of the seatback of an adjacent seating position, requiring the use of a router. The tether was attached correctly in only 11 percent of pickup installations. No vehicle tether hardware characteristics or vehicle manual directions were associated specifically with correct tether routing and head restraint position. Installations involving the single tether strap were 10 times as likely to have the tether attached correctly and 1.7 times as likely to be routed correctly and have the head restraint positioned correctly, compared with installations with the v-shaped tether.
Conclusions: Tether use rates were higher than seen in the field, but parents did not use the top tether for every installation even after being instructed to do so. Tether anchors located on the rear deck of sedans, were associated with increases in tether use and correct tether attachment. Among tether anchor locations that may be found in other vehicle types, tethers were more likely to be attached correctly when the tether anchor was located in the middle of the seatback compared with the floor or lower seatback. The presence of potentially confusing hardware was associated with reduced likelihood of tether use and correct tether attachment, although this finding may reflect the high tether use rates in sedans, which are least likely to have confusing hardware. Child restraint installations using a v-shaped tether were less likely to be attached correctly and less likely to be routed according to vehicle manual instructions, in part because few manuals provide instructions for v-shaped tethers.