Luke 3.5: “Every valley shall be filled, and every mountain and hill shall be made low, and the crooked shall become straight, and the rough places shall become level ways,” (ESV)
The coming of Jesus was the coming of the King. And as was a tradition in ancient times, whenever a king would plan to come into some district of his kingdom, he would sometimes send a messenger in advance to announce his soon arrival. This would give the local population the opportunity to get everything in good and proper order, fit for the king. Clean up the city. Smooth out the roads. A public works project to get everything in top condition. The International Critical Commentary of St. Luke by Alfred Plummer speaks to this point.
So this is how it appears that the above verses should be symbolically understood. Isaiah is not talking about roads literally. He is talking about the inner condition of our lives. In other words, get ourselves cleaned up spiritually on the inside. Repent of sin. Turn away from thoughts and actions that displease God. Get our personal lives put in right order with God so we can be ready for the coming of the King. And as the verses above are describing, get ready for the coming of the LORD.
These verses from Isaiah had their historical meaning in the Old Testament times to the return from exile, and their application in the New Testament to the first coming of Jesus as Luke is proclaiming here. But these verses also can apply to us personally as we await the second coming of Jesus. And the application is for us to repent of sin and get ourselves cleaned up and ready for the second coming of the King.