We always put Puce on the back seat, with a secure dog harness on. To this we attached two special leads, which had seat belt clips on the other end. So Puce was securely kept in her seat, just like a human! That is probably the safest way to keep a dog securely restrained. However, with cats there must be a real problem. I would think that any container would need to be very securely fixed down in the safest part of the car (somewhere near the middle).
Regarding a serious accident (I think totalling is called "write-off" over here), our car had air bags, but luckily they were never needed.
Regarding a serious accident (I think totalling is called "write-off" over here), our car had air bags, but luckily they were never needed.