Singapore's Central Business District, new Marina Bay Downtown and its future southern waterfront district may be linked by an extensive underground road network beyond 2030.
The plan being studied by the Land Transport Authority (LTA) will see traffic zipping about unobtrusively beneath the surface in a series of subterranean ring roads.
Such roads, which free up surface space and improve the liveability of urban areas, are found in cities such as Brussels, Stockholm, Madrid, Paris, Hamburg and Boston.
Singapore's plan is seen as part of a larger one to accommodate a growing population, and it dates back to the 1980s.
Then in 1996, the LTA envisioned 30km of two- to four-lane roads forming a pair of concentric rings under the city centre.
It revisited the idea in the recently released 2013 Land Transport Masterplan, but added that the so-called Singapore Underground Road System (Surs) will now be more extensive.
"We are now studying how Surs can serve new developments in the Marina Bay area and the new southern waterfront city that will extend from Keppel Channel to Pasir Panjang Terminal," a spokesman said.
But until exact development plans for these two districts are clearer, he said, the scale and alignment of the underground roads remain conceptual.
Experts said going underground is inevitable.
Dr Park Byung Joon, head of the urban transport management programme at SIM University, said intense development is expected for the new downtown areas. Thus, building roads on the surface "may not be desirable due to the limited supply of land".
Elevated roads may also mar the visual appeal and perceived prestige of a district, he said. Noise is another consideration.
"The only option left is an underground road network," he said.
He noted that it will be very expensive to build, but the benefits may be justifiable.
Observers said the long gestation of such a network - at least 50 years from concept to implementation - held a high cost, as many areas in the city had to be "safeguarded". The term refers to reserving space for a major infrastructure project to avoid conflicting demands in the future.
But retired traffic engineer Joseph Yee, 68, who was involved in early Surs studies, said: "The cost of not safeguarding is higher."
Safeguarding ensures that property acquisition is kept to a minimum, for instance.
Going underground is not entirely new to Singapore. The 12km Kallang-Paya Lebar Expressway, which opened in 2008, is largely underground. The Marina Coastal Expressway, slated to open by the year end, is the first to have a stretch going under the seabed.