The two curved lines (the central bulge) indicate the extents of likely path individuals will take when traveling through the room. An individual passing through this room will have no reason to travel outside of this boundary and will only travel through the space assuming there is some indication that something interesting exists beyond the exit.
Several assumptions are made regarding the pedestrians themselves, such as, individuals are neither walking fast or slow, and pedestrian traffic through the space is uncrowded.
The bulge exists simply because individuals are not being confined in the space, and as natural walking patterns dictate, people will not travel in a mechanically straight line if not confined to do so.
Although the width of bulge is somewhat arbitrary, I aligned the outermost, or center, of the curve's tangents with the exit jambs. My assumption is that the jambs will act as a visual cue orienting the traveler towards the exit.
