The question about balancing the number and usefulness of Highways is not easy, as it is connected with the question of general pathing.
On Hershfield the highways end near terminal, but due to labyrinth layout you will need roughly 2 min between Terminal and highway with larger bots. This way is needed to move far enough away from terminal to use mobile tp while on Tellesis and Shin you can drive quite fast away from terminal to use mobile tp.
So when balancing highway you need also to balance general island layout especially the part from alpha tp to terminal, but also to the beta tps.
For instance I consider the way hershfield initia and hershfield TMB as quite fast compared to all other routes from alpha2 terminal to beta terminal or outpost.
On the other side, don't balance too much, there should always be some differences to give people reason to choose one staging point over the other.
It should just be balanced enough for light bots as well as HM, to have no reason to change island color just due to path length.