The term highway is much used as symbolic for a specific of way of life based on given doctrines.
It is a unique way of life that characterises a people who are a direct work of the God of Israel, which way transcends cultural backgrounds.
To understand the whole picture, let's pull up a couple of verses here for reference.
This highway is the same in;
And an highway shall be there, and a way, and it shall be called The way of holiness; the unclean shall not pass over it; but it shall be for those: the wayfaring men, though fools, shall not err therein. ~
In that day shall there be a highway out of Egypt to Assyria, and the Assyrian shall come into Egypt, and the Egyptian into Assyria, and the Egyptians shall serve with the Assyrians.
As well as in
And there shall be an highway for the remnant of his people, which shall be left, from Assyria; like as it was to Israel in the day that he came up out of the land of Egypt.
In this latter, it's obvious that rarely a path is for only a specific people, even less so for only returnee exiles. It refers to a lifestyle relative to certain principles, a distinct culture.
Therefore in the context here, crucial points not to miss are in the terms; remnant, highway, a blessing and My people.
In that day shall Israel be the third with Egypt and with Assyria, even a blessing in the midst of the land: .
Whom the LORD of hosts shall bless, saying, Blessed be Egypt my people, and Assyria the work of my hands, and Israel mine inheritance.
A remnant is quite a familiar term, the seed said by John to be born not of the will of man, a people who are righteous but whose righteousness is largely the work of God and less of personal effort. They are a direct devine work.
This was the ''7000'' in 1 Kings 19:18, Isaiah 10:20-22, etc.
Of this category Messiah's followers are majority.
Now concerning Egypt, Assyria and Israel being a blessing in the land;
First of all Assyria and Egypt or any other nation are never referred to as 'My people,' it's ever Israel, because the 'My people' is the 'remnant' whom even some of Israel's errant generations werent counted a part of.
These are the seed in Abraham's blessing;
17 indeed I will greatly bless you, and I will greatly multiply your seed as the stars of the heavens and as the sand which is on the seashore; and your seed shall possess the gate of their enemies.
18 In your seed all the nations of the earth shall be blessed, because you have obeyed My voice." .
For though thy people Israel be as the sand of the sea, yet a remnant of them shall return: the consumption decreed shall overflow with righteousness.
Note that while in Genesis 22 God says Abraham's seed will be as the sand of the sea, by Isaiah 10:22 He says only a few will be saved though they are as the sand of the sea, meaning that the multitudes promised to Abraham but diminished by the decreed consumption in Isaiah 10:22, are made up by those not of the stock of Israel, but themselves a blessing in the land, even Egypt and Assyria in Isaiah 19, themselves also symbolic for all nations.
So named because of their former characteristic lifesytles from spirit princes who were rulers over them in wickedness before they saw the light of Messiah.
Then they shall bring all your brethren from all the nations as a grain offering to the LORD, on horses, in chariots, in litters, on mules and on camels, to My holy mountain Jerusalem," says the LORD, "just as the sons of Israel bring their grain offering in a clean vessel to the house of the LORD.
21 I will also take some of them for priests and for Levites," says the LORD.
Levites and priestood were hereditary, exile didn't strip one of their tribe or priesthood
He says, "It is too small a thing that You should be My Servant To raise up the tribes of Jacob and to restore the preserved ones of Israel; I will also make You a light of the nations So that My salvation may reach to the end of the earth."
That's why in Isaiah 19:25 even Egypt and Assyria are blessings in the land alongside Israel the chosen people.
The phrase 'in that day' largely refers to messianic times and the fulfilment of the blessing on Abraham in saying ''all nations shall be blessed in your seed,'' a seed starting in Messiah.
The highway is their unique lifestyle based on God's power in them to reject apostasy. That's why the unclean cannot walk on it, not that they can't see it, but that they can't stand it.
Therefore that highway is the common denominator that characterises this category of people that God calls 'My people' and 'My inheritance' but who are as diverse in their roots as Egypt with Israel.