Part of the "Megastructures: The Visual Encyclopedia" book project, on sale now at http://www.neilblevins.com/projects/megastructures/megastructures.htm
An Interstellar Highway would be a set of relays that would fire powerful lasers to launch spaceships from solar system to solar system at near light speed. This system would consist of two elements - relay stations to project the lasers, and spaceships designed to be pushed by the lasers.
Each relay station would consist of solar panels to provide power (or fusion power) and a laser or array of lasers. The laser would be focused toward the next relay station in a neighboring solar system, or toward the next station in a chain of stations headed toward your final destination.
Your spaceship would not need to use most of its mass on fuel. Instead, it would have solar sails that would reflect and absorb light. When the laser light hit the sail, it would transfer the light’s momentum to the craft, causing the ship to be pushed in the direction of the light beam. The sail would ideally be thin, to reduce mass, but have a wide wingspan, to account for the laser widening over long distances.
Think of the highway system like wind on an ocean. You would open your sail on your ship, catch the wind, and go off in the direction the wind was blowing. Once your ship got up to speed, pushed by the laser, it would be struck by the next relay station’s laser, which would either push you toward the next relay station in the chain or, if you had arrived at your destination, slow you back down for arrival.