Gyeongbu Expressway built 50 years ago. It is saturated with traffic. At peak hours traffic flow drops up to 7km/h.  It is part of Seoul Expressways and important feeder for national road system.

Experts suggest that Seoul section of the Gyeongbu Expressway (Hannam IC~Yangjae IC), which consists of 8~12 lanes, can be taken underground for 6.8km to connect Gangnam and Gangbuk, and Gangnam and Pangyo and other areas faster.

Tunnel can be 40 meters underground. It can have 2 levels. Each level 6 lanes.

https://www.mk.co.kr/news/economy/view/2020/10/1107975/