Legally, the capsule can be installed in any position... as long as it is correctly installed! I am not aware of any research that says the middle is safer than either side, although it is a common belief. Perhaps someone will come in with some factual research??

We have the capsule behind the passenger seat in my car as there is not enough room behind the drivers seat (so also in the middle). The passenger seat is pushed quite far forward to accommodate the capsule. As I have a back injury, I would have a lot of trouble getting the capsule out of the middle. I have three kids, so all positions in the back are filled with a capsule, a Maxirider and a Hi-Liner. You know, someone has to sit on the side and I value all my kids the same!! In fact, the capsule offers the best side impact protection of all the seats so it's probably the best one to have on a side!