The Type 2 no-back booster seats attached to the vehicle with the lower anchors or UAS, worked well in mild crashes, but increased the risk of the belt injuring the child's abdomen and under the arm in moderate to severe crashes, especially when the child is sitting in a slouched posture. Why?
* The booster seat's rigid attachment does not allow it to move forward. The bigger and heavier a child is, the more likely the front edge of the booster cushion will tip down as he or she slides forward. This increases the likelihood that the lap belt will ride over the abdomen. And since children don't always sit according to manufacturer's instructions, sudden braking or swerving before a crash can place a child further forward on the booster seat. This reduces the distance between the child's bottom and the front edge of the booster seat, which means a child can drop off the front edge of the booster cushion if the collision is severe.
* In one model, the vehicle lap and shoulder belt is routed through two plastic guides on either side of the cushion (Figure 6). As the child moves forward with the belt around his or her hips, the belt stretches forward with the child and can slide out of these guides. This releases the extra length of webbing that was routed through the guide, and can cause the child to move even further forward towards the edge of the seat. This increases the risk of the lap belt injuring the child.