The Highway is an area that is accessible from interacting with the taxi in Onyx Tile World.
Features
It is a mainly a long, yellow road with a starry sky in the background. If you travel to the right, you will come across a tunnel with portals to the Purple Neon Maze (by chainsawing the red face on the wall) and Concrete World B (through the doorway). Traveling left leads to more highway, and at the end an entrance to the Gray Road.
If you continue to travel right from the tunnel, it will take you to either Pokemon World or a blocked off area (depending on which version of the game you're playing).
In version 0.94a (and beyond), the layout has changed slightly. The screen you are transported to from the taxi is now a small intersection with blocked-off buildings at the edge of the area (which suggests that there may be more areas accessible from there). In this version, the Fairy effect can be obtained here by going into the small hut in the main screen with a Robot Guard standing next to it.