Albert Barnes Commentary Joshua 3:4

"Yet there shall be a space between you and it, about two thousand cubits by measure: come not near unto it, that ye may know the way by which ye must go; for ye have not passed this way heretofore." — Joshua 3:4 (ASV)

The ark, which had been the special shrine and seat of God’s presence since the covenant was made, went ahead to show the people that God, through it, was their leader. They were to follow at a distance so that they could better observe and mark how the miracle was accomplished. They would do this to the greatest advantage while coming down the heights, the ark going on ahead of them into the ravine.