Albert Barnes Commentary Isaiah 40:4

"Every valley shall be exalted, and every mountain and hill shall be made low; and the uneven shall be made level, and the rough places a plain:" — Isaiah 40:4 (ASV)

Every valley shall be exalted - That is, every valley, or low piece of ground, shall be filled up to make a level highway, as was done to facilitate the march of armies. This verse is evidently designed to explain what is intended in Isaiah 40:3, by preparing the way for Yahweh. Applied to the return of the Jews from Babylon, it means simply that the impassable valleys were to be filled up to make a level road for their journey.

If applied to the work of John, the forerunner of the Messiah, it means that the nation was to be called on to put itself in a state of preparation for his coming and for the success of his labors among them. Vitringa and others have endeavored to specify what particular moral qualities in the nation are meant by the valley, by the mountain and hill, and by the crooked and rough places. But the illustrations are such that it cannot be demonstrated that the prophet was referring to them. The general sense is plain. The language, as we have seen, is taken from the march of a monarch at the head of his army.

The general idea is that all obstructions were to be removed, so that the march would be without hindrance. As applicable to the work of John, the language also generally means that whatever in the opinions, habits, and conduct, and in the pride, self-confidence, and irreligion of the nation would prevent his cordial reception, was to be removed.

Every mountain and hill - They shall be dug down to make the journey easy. All obstructions were to be removed.

And the crooked - The word used here (עקב ‛âqob) is usually rendered "crooked"; but perhaps not by any good authority. The verb עקב ‛âqab usually denotes to be behind, to come from behind, or, as Gesenius supposes, to be elevated like a mound, arched like a hill or tumulus; it is therefore applied to the heel due to its shape (Hosea 12:4). According to this, the word would properly denote a hill, mound, or acclivity, which would push back those who attempted to ascend.

Shall be made straight - Margin, "A straight place". The Hebrew word (מישׁור mîyshôr) properly denotes "evenness," a level region, a plain. The hilly places would be reduced to a level.

And the rough places - These are places that are hard, bound up, stony, and difficult to pass. These include places abounding with rocks and precipices, which presented obstructions to a journey. Such places were common in the region lying between Palestine and Babylon.

Plain - Margin, "A plain place". This means a smooth, level plain.