My son is 9 years old, does he have to use a booster seat?
We answer the question that all parents ask themselves
The RTR tells us that we must carry a child in a child restraint system up to a height of 135 cm. Have you ever wondered why the limit is 135 cm and not 125 cm, or 140 cm?
The explanation lies in a test that is carried out when the car’s seat belts are approved. This test ensures that the seat belt does not create any slack when a 137.6 cm tall P10 dummy is sitting in the seat. From this, the limit of 135 cm was extrapolated, as it was considered at that time that this measurement ensured that the abdominal part of the belt would tighten correctly and, therefore, the child would be properly restrained.
In reality, this has nothing to do with biomechanics, nor with the position of the belt on the child’s neck at 135 cm, but merely a simple belt tensioning check.
The upshot is that this is clearly not a value we should take into account if we are concerned about the child’s safety.
A child under 135 cm is about 10 years old. At that age, neither the iliac crests nor the iliac fossae of the pelvis are fully formed. These are responsible for retaining the belt and preventing it from moving up to the abdomen, but at this age they are rounded and do not retain the abdominal belt. Instead, they promote the submarining effect.
It is therefore necessary to stop the seat belt moving towards the abdomen. How do we do this?
The only proven way we know of is to raise the height of the pelvis, and therefore of the child, so that the angle of the belt prevents the belt from sliding upwards. In short, use a booster seat.
In addition, this same fact makes the position of the pectoral part of the belt more “centered” on the child’s collarbone, improving the restraint and diverting the energy to a part capable of supporting the loads resulting from a collision.
And until what age should we use a booster seat? It is considered that when the child reaches a height of 150 cm, the pelvis has developed and the pectoral part of the belt has become centered. These two factors mean that it is no longer necessary to “assist” the belt position. It is no longer necessary to use a booster seat.
Technically, these are the reasons that justify the use of the booster seats for children up to 150 cm.
Before we stop using a booster seat for our child, we should ask ourselves why we are taking it away, and whether this is scientifically defensible, and we should consider whether our decision not to use a booster seat is based on non-scientific opinions, or on legal requirements, which may not be scientifically proven.