This refers to the way leading the redeemed back to Jerusalem, the throne of Messiah, literally and spiritually. Christ Himself is to be the leader on that way, called in Isaiah 40:3, the “way of the Lord.”
Commentary from the MacArthur study Bible, notes for Isaiah 35:8.
-On the narrow road-
You have not come to a mountain that can be touched
and that is burning with fire;
to darkness, gloom, and storm;
to a trumpet blast or to such a voice speaking words
that those who heard it begged that no further
word be spoken to them,
because they could not bear what was commanded;
“If even an animal touches the mountain,
it must be stoned.”
There is a great contrast in the way people approached God under the old covenant and how people approach God under the new covenant. When God established the old covenant on Mount Sinai, the people experienced the terror of God. The Israelites saw God’s awesome power demonstrated; they had to stand at a distance from the mountain. Only Moses was allowed to go up the mountain when he received the Ten Commandments. Under the new covenant, because of what Christ has done, Christians can enter directly into the presence of God without fear. Christ has made it possible to commune intimately with God the Father because he has secured the forgiveness of sin for anyone who chooses to follow Him. Let us come boldly before God on the basis of Christ’s shed blood.
Commentary from: The One Year NIV Devotional New Testament-October 30, Tyndale, 2003.