Complex sentence: Until completion of the bridges, the region was isolated from the rest of Japan, and the freer movement between Honshu and Shikoku was expected to promote economic development on both sides of the bridges, which has not materialized yet.
Split into simpler sentences: Until completion of the bridges, the region was isolated from the rest of Japan. The freer movement between Honshu and Shikoku was expected to promote economic development on both sides of the bridges, which has not materialized yet.