The Highway is an area that is accessible from interacting with the taxi in Onyx Tile World.
Features
The area you are transported to from the taxi is 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, however this seems to be a temporary location.
If you continue to travel right from the tunnel, it will take you to a blocked off area (which seems to be where the Fairy effect will be located in the future).
When you travel horizonatally in any direction from the intersection, you will end up on a long, yellow road with a starry sky in the background. If you travel to the right, you will come across a tunnel with a portal to the Purple Neon Maze (by chainsawing the red face on the wall). Traveling left leads to more highway, and at the end an entrance to the Gray Road.