That is a good point. I am aware of the risks and tend to only use 1 or 2 containers max, as I am still mainly trad. (There are exceptions to this and I have had 8 in the car for a short journey.) I ensure that I put them in the car on the passenger side and up against the back of the seat. It will not stop them, but I can only hope it will slow them down and, if they go straight on, they won't hit me.
This is one of the problems with wfp. You take the dangers of ladders and replace it with the dangers of transportation. And I don't wish to start a wfp vs trad discussion. Even the crash testing of top end van mounts is only done at 30mph, but how many people never go above that? You can have the best system in the world in your van, but I bet it won't save your life in the event of a major shunt.