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Setonaikai NP, Okayama / Kagawa
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Six bridges—not one—make up the 9.4-km sea crossing called Seto Ohashi. With road above rail, its full 12.3-km shared section was recognized by Guinness in 1998 as the world’s largest combined road-and-rail bridge.
In 1889, Kagawa politician Ōkubo Jinnojo proposed bridging the Seto Inland Sea. Seto Ohashi opened 99 years later, on April 10, 1988—nearly a century after his idea.
On Iwakurojima, the original plan relied on an elevator to reach the highway bus stop. After strong requests from local residents, Kagawa Prefecture and Sakaide helped fund a loop bridge instead.
Where the paired South and North Bisan spans cross an international shipping lane, their decks sit at least 65 m above the sea—high enough for 500,000-tonne-class tankers to pass.
The bridge idea was older, but the 1955 Shiun Maru ferry disaster gave the campaign new urgency. On July 4 that year, Kagawa’s assembly petitioned for a rail link across the strait; Seto Ohashi opened in 1988.
The Hitsuishijima Bridge was first planned as a truss bridge. To better preserve the Seto Inland Sea scenery, designers chose a cable-stayed form, whose elegant outline is likened to a swan spreading its wings.
On the Honshu approach, Washuzan Tunnel stacks 2 road bores above 2 railway bores through brittle rock. Kagawa Prefecture describes it as the world’s first double-deck tunnel.
Strong currents could sweep away detonation wires, so engineers used ultrasonic signals to trigger seabed explosives wirelessly. Sakaide City describes the method as a world first in bridge construction.
At depths reaching 50 m, plankton made underwater cameras unreliable. Engineers descended into near-darkness, where visibility was about 1 m, and inspected the seabed with their foreheads pressed close to the rock.
One anchorage for the South Bisan Seto Bridge weighs 1 million tonnes, countering cable forces of 90,000 tonnes. The North and South spans also share a central anchorage that holds both bridges’ cables.
To reduce confusing radar echoes for passing ships, Seto Ohashi’s anchorage surfaces slope upward to send reflections skyward. Radar-absorbing material on the towers weakens reflections there, too.
