Let's get a really tiny car.

Car seats and booster seats do not make my life easier. Without them children are small and squash into the car easily, without them children are people who can walk or catch a train, a bus, or a plane to somewhere to be met by a loved one and a car as easily as I can, this is a wonderful freedom.

But death and internal injuries have only disadvantages.

Dr. Liz Segedin's powerpoint summary of quite a few years of Starship's statistics shows how very much less likely your car is to ruin or end your life if you sit in a booster seat until you're 1.5m tall (Hazel is 1.25m, Iris 1.15m). Booster seat research, Dr Liz Segedin.ppt

The NZ Child Restraints' site's Booster Seats section includes this little gem:

What is "sub-marining"?

When a small, undersized child, (a child who is unable to pass the 5 step test) is placed into an adult belt there are a number of risks that the child is exposed to. Such risks include injuries to the neck, spine and internal organs, as well as injuries caused when the child slides out the lap portion of the seat belt in an accident. The effect of this is known as "sub-marining" and is shown in the image below.

BCSI

A child will scoot forward in an adult belt simply because they cannot comfortably sit with their back and bottom against the back of the vehicle seat, nor can they sit with their knees comfortably bent over the seat edge.

When a child is uncomfortable, they will find ways to make sure they are comfortable, this means they will pull their lower body forward to rest their knees on the seat edge, leaving a gap behind their back. The child is also likely to place the sash portion of their seat belt behind their neck, since their shoulder is now lower down and the seat belt cuts into their neck causing discomfort.

Useful info for parents.pdf (a pdf summary of how to tell if a child is sitting safely).

Safekids' position statement.pdf They say "It's booster seat time for Kiwi kids" and provide 17 pdf pages of argument for that.

Oh, botheration! Another inconvenient truth.