Rusty & Stickers were use to riding in the breeders SUV in a crate. We have 2 regular cars and no room for a crate. Now, they just lay on the backseat. I am concerned that with them switching places during the ride, that someone will end up falling on the floor. Also, if we happen to stop sudden, boom!! Does anyone have any experience with harnesses?? Every picture I've seen shows the dog sitting up. I'm assuming they can also comfortably lay down??? Any opinions???