Yes, only a dashed white line would work. This is not really a use case for decals from how you describe it, not the kind of thing they are intended for, and other solutions (make a spline renderable; create a texture for it, either generated from the scene and saved out or painted manually) would be the way to go. Decals are more about smaller, local features that follow their own UV (e.g. to add potholes onto this road, or to add singular STOP markings here and there) and not about having textures follow along the shape of a more complex geometry.