There is a real danger that we are so familiar with John the Baptist fulfilling Isaiah 40:3 that we lose sight of the staggering implication this has for Jesus.
So France comments:
Taken in the wider setting of the wilderness theme in Deutero-Isaiah, the text in its original context announces God’s coming to lead his people in their ‘new exodus’ through the wilderness from Babylon back to Palestine. It is God himself who is to come and will use the processional way. There is no hint of any other person (a Messiah) intermediate between the ‘voice’ and God himself. Christian interpretation has for so long taken it for granted that John’s role was to prepare for the Messiah that it is easy to miss the radical significance of Matthew’s choice of text: the coming one of Isaiah 40:3 is not the Messiah, but God himself. … There is a remarkable christological claim involved in applying Isaiah’s depiction of God’s forerunning to the man who prepared the way for the coming of Jesus. (Page 105)