They are safe if you use the right kind of charger and you treat them with respect. They are no more dangerous to be around than a prop spinning at 10000rpms. The key is only charging them at a 1amp/hr. They pack quite a bit of output and they are VERY light for the power they provide which is the main reason they are great for model airplanes. However each cell is sealed in a very thin foil wrapper. What is inside is somewhat pyrophoric and burns VERY hot if it ignites. As long as you dont bang the battery and expose the lithium inside AND you charge them corectly they are pretty doggone safe. You just have to folow the safety recommendations they come with the battery like charging in a container with a lod that can contain the fire IF it should ignite. I have several of them for my foamies and I have had no problems at all. Thats the gist of it, Im sure you will get some more scientific explainations.

I dont know about a car. I would pack it in foam and in a manner that there is no way the foil can be torn open during a rough ride. The gain is the lighter weight, I cant see them being practical for a car unless the weight is critical...