ISOFIX was conceived as an alternative to the seat belt for installing integral child restraint systems (e.g. harness seats). However, ISOFIX-like attachments are now offered on some non-integral child restraints (booster seats). In such cases, the booster seat is attached to the car using the ISOFIX anchorages, with the child secured by the seat belt. This is usually provided as a ‘comfort feature’ to assist in the positioning of the booster seat and to ensure it is secure in the event of a crash when unoccupied. ISOFIX is not thought to convey significant safety benefits (or disbenefits) for children in booster seats;​ however, very little research has been carried out. The aim of this study was to investigate the effect of ISOFIX on booster seat performance in front impact. The paper presents a series of sled experiments using fully instrumented Q-Series dummies. The same booster seat was used in all experiments, but it was installed in one of four conditions:​ no ISOFIX attachments;​ ISOFIX fitted, but stowed away;​ ISOFIX used with deformable attachments (that allowed the booster seat to translate, while remaining attached);​ ISOFIX with fixed attachments (with no translation of the booster seat). The frontal impact test procedure specified in United Nations (UN) Regulation No. 129 was followed in all experiments.

ISOFIX generally had marginal effects on the performance of the booster seat in these tests. However, pelvis displacement (with respect to the booster) was greater with fixed ISOFIX attachments. In this condition, the dummy and the booster seat were less well coupled, compared with the other installation modes, but submarining was not observed. Measurements associated with compressive loading of the torso (i.e. chest deflection and abdomen pressure) were expected to be lower when ISOFIX was used (based on previously reported tests), but this was not observed uniformly. Effects of the dummy design, such as head-to-chest contact and diagonal belt slippage may have influenced its sensitivity to differences in booster seat installation modes. Nevertheless, deformable ISOFIX attachments appeared to offer the benefit of ensuring the dummy and booster seat were coupled together throughout the impact, while reducing the potential for compressive loading to the torso.

These findings suggest that the current practice of offering consumers a choice with respect to the provision of ISOFIX on booster seats is appropriate. UN Regulation No. 129 is being amended to include booster seats. They can be approved with or without ISOFIX attachments, but where present, they must be stowable. This study supports this philosophy;​ however, it was based on one booster seat only (installed in different attachment modes). Different trends might be observed in other booster seats. Furthermore, this study focussed on front impact. Other impact directions (such as side impact) might yield different results.