Isaiah 19:23 probably does not refer to a literal highway. This fourth proclamation also includes Assyria. The highway signifies a close and peaceful connection between two nations who had often been at war in the past but would worship God together.
In the time of the Messiah, the countries that used to be called Egypt and Assyria will serve the Lord. As these two nations represent world power, they symbolize the conversion of all the heathens. It is as if Babel is reversed: where once nations were dispersed in chaos, the Lord will establish a true unity.
23 In that day there will be a highway from Egypt to Assyria, and Assyria will come into Egypt, and Egypt into Assyria, and the Egyptians will worship with the Assyrians.