Always tied. The one time we didn't l don't to this day know why the back door opened on the highway but it did and if that filly wouldn't have been tied to the wall she would have jumped out. You do feel when the back door of a trailer swings open while driving but l'm not sure you would know about the one that just slides if a horse decided to test that and it wasn't locked. l think they brace themselves more when tied then loose so if something was to happen like a sudden stop they are more alert then one walking around loose that could loose there balance and fall even. lt's like seat belts sometimes it's the answer and helps and sometimes you would have been better without one but who knows ahead of time....