Incorrect restraint use is widespread among child passengers and increases the risk of injury in a crash. Based on previous laboratory testing, individual modes of incorrect use are often rated as serious or minor. However, in a recent populationbased observational study in NSW Australia, almost one third of incorrect use was found to involve multiple minor errors. In this test series we hypothesized that the cumulative effect of multiple minor errors would result in injury protection reductions of the same order as individual, more serious forms of incorrect use.

To test this hypothesis, four relatively minor forms of incorrect use in a forward facing child restraint were tested individually and in various combinations and the relative reductions in injury protection were compared to that observed in more serious forms of incorrect use. Data from 40 frontal crash sled tests (32km/hr, 16g) using an instrumented CRABI six-month dummy were used in this analysis. Estimations of reduction in injury protection were made on the basis of relative increases in head and restraint motion as assessed using high speed digital photography. The minor errors studied were selected on their high observed frequency in the NSW population and included single and double twists or slack (25mm) in the internal harness and 25mm of slack in the lower anchorage. The serious incorrect use included positioning of the harness below the shoulder level, an incorrectly routed seat belt, 50mm of slack in the top tether, excessive slack (75mm) in the anchorage system, non-use of lower or upper anchorage and non-buckling of the belt used as the lower anchorage.

Combinations of two minor errors increased the relative motion of the dummy by up to 8% compared to when there were single errors only, with the greatest relative increase occurring when two twists were added to a moderately loose (approximately 25mm slack) internal harness of the restraint. However, much greater relative increases occurred when there were combinations of three errors (average increase of 15%, range 10-20%). Compared to when there is no incorrect use, the combination of three errors increased dummy motion in the same order of magnitude (approximately 15%) as serious forms of incorrect use such as excessive slack, partial and non-use of lower anchorages.

The results demonstrate the cumulative effect of minor incorrect use on dummy head excursion and restraint motion. Minor forms of misuse can be serious when they occur in combinations. This extends previous laboratory work demonstrating the effect of incorrect use. Based on these results, similar cumulative effects in combinations of serious errors, and in combinations of more than three errors might be expected. These findings, together with field work indicating that almost 16% of children travelling in cars have three or more errors in the way their restraint is being used, highlight the need for countermeasures to reduce the prevalence of even minor errors.

Limitations include the fact that only a single model of restraint and a standard test bench and set up was used. The absolute effect of the errors studied may vary depending on these factors.