The only thing I could think of was to place some routing paint, plant a large rock or create a lake in between the two roads to discourage sims from taking a shortcut.
Or...you could move that road farther away to keep it from becoming a magnet for sims.
I think you'll still have issues, though. The problem with routing is that sims are designed to be "lazy." They will route down the path of least resistance. So, the more creative you get with road layouts, the harder it is to make them route the way you want them to without resorting to routing paints, fencing, etc.
If I'm reading the screenshot and your comments correctly, that big circular road that the paths run to leads to the military base, science facility, junkyard, and fishing spot. Unfortunately, that's what's causing the shortcutting. The sims living at the dead end are too lazy to use the other road to get to those locations. It would mean going down that long sweeping curve to the right, turning at the intersection, then going back up the road they keep shortcutting to. So, they are just cutting across instead.