If you have a road made up of tiles with a single path ("pipeline" as you called it), then you don't need boids, avoidance, AI, or really anything super special. Just have each balloon move in the natural direction of the road tile. For example, given a starting tile with only one non-sand tile to touch, you can figure out which direction to move -- it is the direction that is "not here, not where I was, and not sand". Since your road has only two possible directions of flow, once the initial direction and tile are established, the algorithm will solve the problem.
The balloons will appear to follow each other simply because there isn't anywhere else to go. If they all move at a constant speed, then no collisions are possible. Even if they don't move at a constant speed, the "not here, not where I was, and not sand" can be have "and make sure there is enough space for me" appended.
You could generalize a little and use the tile's image as a method of extracting direction. For example, an L shaped road stripe means either "if you're going south, then turn east" or "if you're going west, turn north".
This system breaks down when you decision making to do, i.e. more than a single path that can be taken, but from your screenshot and description of the problem, that doesn't seem to be an issue. If it is a requirement for your application, then definitely invest in a more AI-centric approach.