Summary of Institute research on lap belts and booster seats

study conducted with Children's Hospital of Philadelphia

back to keeping children safe in crashes | options for vehicles without shoulder belts

There is convincing evidence that belt-positioning booster seats, coupled with lap/shoulder seat belts, offer the safest way for children to travel in cars after they outgrow their child safety seats, usually around age 4 (Durbin et al., 2003; Elliot et al., 2006; Arbogast et al., 2005). However, evidence has been conflicting about whether to use belt-positioning booster seats with lap belts only. This practice may prevent serious abdominal injuries caused when a child submarines under a poorly fitting lap belt, but in serious frontal crashes significant head and chest excursions still occur because the torso is not restrained by a shoulder belt. Head excursion can be even greater when belt-positioning booster seats are used with lap belts than when lap belts are used alone (Weber and Melvin, 1983; Henderson et al., 1994).

No data have been available to quantify whether the potential for reduced abdominal injury risk while using a belt-positioning booster seat and a lap belt is outweighed by the potential for increased head excursion. In the absence of such data, recommendations typically have followed those of booster seat manufacturers, who have stated that belt-positioning booster seats should be used only with lap/shoulder belts to provide optimal protection. Based on this guidance, parents faced with the scenario of restraining their booster-age child safely in a seating position with only a lap belt must either pursue other, more expensive forward-facing child restraint options or, more likely, use a poorly fitting lap belt and risk serious abdominal injuries. According to the federal government (Glassbrenner and Ye, 2007), 16 percent of vehicles in the current fleet have only lap belts in rear seats.

To address this gap in research, data from the National Automotive Sampling System — Crashworthiness Data System (NASS-CDS) and the Partners for Child Passenger Safety (PCPS) child crash surveillance system were examined. Analyses focused on the relative risk of moderate or greater injury (MAIS ≥ 2) among crash-involved booster-age children who were restrained in belt-positioning booster seats and lap belts, lap belts only, and belt-positioning booster seats plus lap/shoulder belts. The NASS-CDS analysis was performed by researchers at the Insurance Institute for Highway Safety, and the PCPS analysis was performed by researchers at the Children's Hospital of Philadelphia.

NASS-CDS is a nationally representative sample of crashes involving passenger vehicles in which at least one vehicle was towed. The focus of these analyses was crashes occurring during 1997-2006 and involving restrained children ages 3-8 years seated in 2nd or 3rd rows. Among 374 cases of children restrained with lap belts alone, 29 (1.21 percent as computed with NASS-CDS sampling weights) received moderate or greater injuries (MAIS ≥ 2). Of the 240 children seated in belt-positioning booster seats, only 21 were using the lap belts only. One of these 21 children (0.12 percent as computed with NASS-CDS sampling weights) received an AIS 3 injury. None of the remaining 20 children was moderately or more severely injured. Of the 219 children riding in a belt-positioning booster seat with lap/shoulder belts, 14 (1.22 percent as computed with NASS-CDS sampling weights) sustained moderate or greater injuries. None of these differences was statistically significant.

PCPS data included crashes occurring from December 1998 to December 2006 and involving children ages 3-7 years seated in 2nd or 3rd rows of passenger vehicles. These cases are part of a large-scale child crash surveillance system created by linking electronic insurance claims data at State Farm Insurance Company (Bloomington, IL) to telephone survey and crash investigation data. The rate of MAIS ≥ 2 injuries was 1.74 percent among children restrained by lap belts only in center rear-seat positions (unweighted n of 616), 0.96 percent among those in lap belts with belt-positioning booster seats in center rear-seat positions (unweighted n of 31), and 0.68 percent among those with lap/shoulder belts with belt-positioning booster seats in rear rows (unweighted n of 1279). To compare these injury risks, odds ratios were calculated, both unadjusted and then adjusted for vehicle type, age of child in years, crash direction, and intrusion. After adjustment, the data indicated a 53 percent decrease in the relative risk of injury among the children in belt-positioning booster seats with lap belts, compared with children restrained in lap belts alone (not statistically significant, given small sample). The risk of injury among children in belt-positioning booster seats with lap/shoulder belts was reduced by 57 percent compared with children using lap belts alone (this difference was statistically significant).

The current research focuses on the potential tradeoff among lap belt and booster seated children of increased head excursion and lower abdominal injury. The NASS-CDS data were examined to determine the specific body region injured among children ages 3-8 who received a moderate or greater (MAIS ≥ 2) injury. Injuries were classified as head and trunk (defined as injuries to the head, face, neck, abdomen, spine, chest, or pelvis) or extremity (defined as injuries to the legs, ankles, knees, arms, wrist, or elbows). A substantial proportion of injuries were to the extremities. Among the 374 children restrained with a lap belt only, 20 (0.86 percent as computed with NASS-CDS sampling weights) received a head or trunk injury. None of the children seated in belt-positioning booster seats with lap belts only received moderate or greater head or trunk injuries. Of the 219 children riding in a belt-positioning booster seat with lap/shoulder belts, 11 (0.39 percent as computed with NASS-CDS sampling weights) sustained moderate or greater head or trunk injuries. None of these differences was statistically significant.

In summary, results of both the NASS and PCPS analyses suggest lower injury risks among children restrained in belt-positioning booster seats with lap belts compared with children restrained in lap belts alone. In both datasets, the percentage of children using belt-positioning booster seats with lap belts was very low. Although the sample sizes were very small and most of the differences in injury risk were not statistically significant, this study is, to our knowledge, the only examination of the injury risk associated with using belt-positioning boosters with lap belts in real-world crashes. Clearly, a lap belt only position is not optimal for booster-age children due to the lack of torso restraint. However, faced with the need to restrain a booster-age child in a lap belt only seat position, real-world crash outcomes from two large crash surveillance systems suggest that the current recommendation against using a belt-positioning booster seat is inappropriate.

References

Arbogast, K.B.; Kallan, M.J.; and Durbin, D.R. 2005. Effectiveness of high back and backless belt-positioning booster seats in side impact crashes. Association for the Advancement of Automotive Medicine 49th Annual Proceedings: 201-213.

Durbin, D.R.; Elliott, M.R.; and Winston, F.K. 2003. Belt-positioning booster seats and reduction in risk of injury among children in vehicle crashes. Journal of the American Medical Association 289:2835-40.

Elliott, M.R.; Kallan, M.J.; Durbin, D.R.; and Winston, F.K. 2006. Effectiveness of child safety seats vs. seat belts in reducing risk for death in children in passenger vehicle crashes. Archives of Pediatrics & Adolescent Medicine 160: 617-21.

Glassbrenner D. and Ye, J. (2007) Rear-seat belt use in 2006. Traffic Safety Facts Research Note. Report no. DOT HS-810-765. Washington, DC: National Highway Traffic Safety Administration.

Henderson, M.; Brown, J.; and Paine, M. 1994. Injuries to restrained children. Association for the Advancement of Automotive Medicine 38th Annual Proceedings: 75-87.

Insurance Institute for Highway Safety. July 2007. Child restraint laws. Available: http://www.iihs.org/laws/ChildRestraint.aspx. Accessed July 31, 2007.
Weber, K. 2000. Crash protection for child passengers: a review of best practice. University of Michigan Transportation Research Institute Research Review 31.

Weber K. and Melvin J.W. 1983. Injury potential with misused child restraining systems. 27th Stapp Car Crash Conference. Warrendale, PA: Society of Automotive Engineers; 1983:53-59. SAE Publ. No. 831604.