The route via the Seto Ohashi Bridge was the first of the three routes linking Honshu and Shikoku opened to traffic. In an area studded with many islands, this route hops from one island to another across the Seto Inland Sea. Joining the city of Kurashiki in Okayama Prefecture on the Honshu side with Sakaide City in Kagawa Prefecture on the Shikoku side, the complete route was opened in 1988. Stretching across a total distance of 9.4 kilometers (5.8 miles) there are six bridge sections that span the gaps between islands that lie between the two cities, as well as four viaducts on the islands themselves. The whole route is a double-decker construction, with an expressway running above a railway. In terms of scale, it is the largest combined road and rail bridge system in the world. Among all, the 1100-meter (3,609-foot) central span of the 1,723-meter (5,653-foot) long Minami Bisan Seto Ohashi Bridge at the southernmost end is on its own the world's longest combined road and rail bridge. activities. 

The bridge's history dates back to 1889 when a member of the Prefectural Parliament, Jinnojo Okubo  1849—1891), suggested the idea of a Seto Ohashi Bridge. Okubo Jinnojo was born in a remote village of West Sanuki. At 23, Jinnojo became a village official. He said: "The four provinces of Shikoku are like so many remote islands. If united by roads, they will be much better off, enjoying the benefits of increased transportation and easier communication with each other." He envisioned and worked on several major road projects. He aired his idea for the Bridge in a speech made at the opening ceremony of the first railway in Shikoku between Margame and Kotohira. While it took a century for his vision to become a reality, another of his ideas, mentioned in a drinking song he composed, was accomplished twenty years sooner:

I'll tell you, dear, don't laugh at me,
a hundred years from now, I'll be seeing you
flying to and from the moon in a space ship.
Its port, let me tell you, dear,
will be that mountaintop over there!

The bridge idea lay dormant for about sixty years. In 1955, after 171 lives were lost when a ferry wrecked in dense fog off Takamatsu, a safer crossing was deemed necessary. By 1959, meetings were held to promote building the bridge. Scientists began investigations shortly after, and in 1970, the Honshū-Shikoku Bridge Construction Authority was inaugurated. Work was postponed by the "oil shock" of 1973. In 1978 the Environment Assessment Report was published and construction got underway. The project took ten years to complete at a cost of $7 billion. Although nets, ropes and other safety measures were employed, the lives of 13 workers were lost during the 10 years of construction. The bridge opened for traffic on April 10, 1988.