WITH his maverick political legacy secure, Junichiro Koizumi is poised for one last act of extravagant showmanship before stepping down as Japan’s Prime Minister next month: a promise to return daylight to Tokyo’s most important landmark.

For more than 40 years the elegant Nihombashi bridge has cowered in chilly darkness beneath the capital’s busiest motorway flyover — a nine-lane steel and concrete monstrosity built just before the 1964 Olympic Games as a symbol of Japan’s emerging modernity.

Under the flyover’s grim shadow Nihombashi has lost all attraction. The traffic above it is deafening; the river beneath it is stinking and filthy. Professor Shigeru Itou, an adviser to Mr Koizumi, recently labelled it the ugliest spot in Japan.

Within the next few weeks Mr Koizumi is expected to approve a £5 billion plan to restore the bridge to its former glory. The enormous task involves moving the entire flyover underground.

If the plan succeeds everything that is currently above the bridge will be hidden beneath the river.

The bridge, constructed in 1603 and rebuilt a century ago, is the reason Tokyo is Japan’s capital. The trade that passed over it made the city the country’s commercial and cultural hub in the 17th century. All distances from Tokyo are still measured from a small brass plaque on the middle of the bridge.

From almost the day that he took power, in 2001, Mr Koizumi has believed that the bridge deserves to see the sun again. He assembled the finest minds in engineering and architecture to work out how to “open the skies” and turn the bridge into a tourist attraction, but, while the the intention is simple, turning the plan into reality is one of the most expensive and complicated civil engineering projects imaginable.

Nihombashi is in Tokyo’s oldest financial and commercial district, and the buildings that surround it are the headquarters of Japan’s biggest companies. Working around them is just one of many puzzles faced by Koizumi’s engineers.

The committee that drafted the plan has identified three other factors that make the scheme fiendishly complicated. The first is that the roads blotting out Nihombashi are indispensable to the city — they cannot be out of action for even a single day, meaning that the new road will have to be completed and in use before the old one is demolished.

The second challenge is a decision on how much of the flyover to make subterranean — the offending stretch over Nihombashi is currently about 2km (1.2 miles) long, but the new underground bypass would have to be much longer.

The third problem is that the space beneath the river has already been grabbed. Two underground railway lines and a big part of the city sewage system have been below the riverbed for decades, meaning that the new expressway will have to weave around them.

The project by which Mr Koizumi will wish to be remembered will make Baroness Thatcher’s physical legacy — the Channel Tunnel — look simple by comparison.

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