Here’s my idea:
Getting to Ierochos is tedious, so now you not only get to leave that place faster, but also have a convenient way to travel across the northern Bronze Sea all the way to Palo Town. Most notably, it doesn’t pass near Redwake, so freshies who sail from Dawn to there aren’t picked and carried away by the current.
Going from northern to southern regions takes time, so it makes sense once Highway goes straight south. I put it a away from the center, so it won’t slow down whoever goes north, but I put it in a place that is still well-visited - Harvest Island.
Most camps are placed at Palo or Cernunno, since those are in the middle of the sea. So, take those two islands, as well as both Gravy’s and AS’s spawns, and give them quick access to the western entrance into the Dark Sea - the best side out of the four to start the run from in my opinion.
Spawning at the border with Nimbus Sea is so much drag, you have to sail across a third of the map to get somewhere. Now solved.
You often get near the Vents of Aeolus, and sailing to the other important islands down south could get much easier with a Highway there. Important to place it east from them, and not west, since that is too far for convenience.
The Ice Mines are in the corner of the sea, very far from everything. Now there is a Slipstream to get out of that place faster, and it opens also for those near Shale Reef. It passes far from Tidecliff, since who cares for that isle anyway?
The Highway near Sameria is there to get you to either Forest of Masts, Claw Island, or the Bronze Sea. Simple and comfortable.
Got bored at Claw Island? Well, go visit Drakos now. Also serves as an okay way to get to Sameria if just came to Nimbus Sea from Bronze. This Highway could be put instead at Makrinaos, but the Eastern Reaches nearby completely ruin that idea, so I didn’t place it there.