Seoul City said Thursday it will build an underground urban expressway beneath sections of the Naebu Expressway and the Bukbu Northern Expressway, with the existing elevated roads to be dismantled over the long term.

Mayor Oh Se-hoon unveiled the plan to construct a six-lane underground highway stretching about 20-point-five kilometers from Seongsan Interchange to Sinnae Interchange, cutting across northern Seoul.

City officials said the elevated roads, built in the mid-1990s, have caused neighborhood division, noise, and environmental degradation while losing their function as major arterial routes.

The project will be carried out in phases, with the Seongsan–Hawolgok–Sinnae section prioritized first and the remaining Hawolgok–Seongdong section considered later, depending on traffic demand and fiscal conditions.

Seoul estimates the total project cost at around three-point-four trillion won or around two-point-three billion US dollars, and noted that current peak-hour traffic speeds on these roads have fallen to about 34-point-five kilometers per hour.

Once completed, the underground expressway is expected to nearly double average peak-hour speeds to around 67 kilometers per hour, significantly easing congestion in northern Seoul.