Accessory child safety harnesses: do the risks outweigh the benefits?

Accessory child safety harnesses are available in some countries as alternative restraints for young children or as an accessory restraint used with booster seats. Their use, in Australia at least, is becoming more common. There have been concerns that the risk of misuse of these restraints outweighs any potential benefit this system might have over a retractable lap-shoulder belt system used with a booster seat. However to date there is no evidence to confirm or deny this. This study used laboratory simulated frontal crash tests to examine the performance of accessory child safety harness systems compared to the lap-shoulder belt when used alone and when used with two common designs of Australian booster seat. The performance of the child safety harness system when misused was also investigated. The results demonstrate that the correctly used child safety harness system performed no better than the lap-shoulder system, and in fact allows for a greater risk of submarining. Furthermore, one common form of child safety harness misuse, where the harness is over-tightened causing the lap belt to be positioned high over the abdomen, allowed extremely undesirable dummy motion. This involved gross submarining and direct contact between the harness system and the dummy's neck. These findings suggest that the risks associated with accessory child safety harness systems most likely outweigh any potential benefits, in frontal impacts at least.

The need for enhanced protocols for assessing the dynamic performance of booster seats in frontal impacts.

OBJECTIVE: The primary objective of this work was to examine variations in the level of crash protection provided by different models of high-back booster seats in frontal impact. Secondary objectives included examination of the influence that specific belt positioning features have on the ability of a booster to achieve and maintain good seat belt fit; and the relationship between dummy loads, motion, and belt fit, both statically (pre-impact) and dynamically (during impact). METHODS: Seventeen different models of high-back booster seats were subjected to simulated frontal impacts on a rebound crash sled. The TNO P10 dummy, instrumented to measure head and chest accelerations and targeted to allow head motion tracking, was used in these tests. Three high-speed cameras were used to record dummy motion. Associations between pre-impact seat belt geometry, the dynamic seat belt fit, and dummy response were examined. RESULTS: Clear variations were observed in the level of protection provided by the booster seats tested. Specifically, there were variations in the ability to provide and maintain good seat belt fit. Only three of the seventeen booster seats provided good sash (shoulder) and lap belt fit during dynamic testing. All seventeen boosters had a "sash guide." Sash guide type did not appear to influence the dynamic belt fit. However, the location of the guide and ultimately where on the shoulder the sash was positioned pre-impact did influence the dynamic sash fit. Anti-submarine clips (ASCs) that work to position the lap belt low on the abdomen were also found to maintain good lap belt fit during the dynamic tests. However, two booster seats without ASCs were also able to maintain good dynamic lap belt fit, although the mechanism of this behavior is less clear. Though there was a relationship between head excursion, head acceleration and the pre-impact static position of the sash belt (shoulder belt), there was no relationship between dummy response and the overall ability of a booster seat to provide and maintain both good sash and lap belt fit. CONCLUSIONS: Booster seats aim to achieve a good seat belt fit for children too small to use the adult seat belt. Variations in dynamic seat belt fit observed among these seventeen commercially available booster seats demonstrate the need for regulatory protocols that incorporate assessment of dynamic seat belt fit. With current technologies, visual examination of the seat belt during dynamic testing is the best method for assessing this performance.