## 1 Introduction  
There is chronic traffic congestion in metropolitan areas throughout the world due to high population density and an increase in car ownership. Considering complex land use and over-saturated above-ground spaces, the current focus has been on providing roads to underground spaces to solve urban transportation issues. Underground roads are already being constructed and operated in many major cities around the world. Examples include the Central Tunnel in Boston, the A86 West Tunnel in Paris, the M30 Tunnel in Madrid, the Cross City Tunnel in Sydney, Airport Link Tunnel and the Clem Jones Tunnel in Brisbane, the Yamate Tunnel in Tokyo, and the Bund Tunnel in Shanghai [1]. Previous studies have shown that the use of urban underground space (UUS) can promote sustainable urban development [2, 3]. UUS can improve the resilience of urban spaces [4] and can contribute to the development of livable cities [5, 6]. Utilizing underground space for the purpose of dealing with urban and transportation problems is a continuing trend in urban development over the past few decades [7]. Ref. [8] found that underground roads reduce traffic aboveground, improve road networks, free up land use changes for redevelopment, provide opportunities for landscape preservation, reduce greenhouse gas emissions, and protect against incentent weather. Thus, the construction of underground roads in UUS contributes to the sustainable development of urban transportation. In this context, a deep urban underground road (UUR) that extends more than 40 m beneath ground level has recently come to be one of Korea's most important road issues. It was planned for the Shinwol-Yeouil UUR to begin operations in 2021, as well as the Seobu UUR, which were constructed sequentially to alleviate the ongoing traffic congestion in large cities and disconnection of urban areas due to existing freeways or expressways, and to provide open space for users. Additionally, UUR construction projects, especially in the metropolitan area of Korea and Busan, the second largest city in Korea, are constantly being planned and promoted. Unlike in the past, tunnel technology for UUS construction has advanced rapidly. As the demand for underground road infrastructure in cities increases, more complex underground road infrastructure will be developed in the future to improve some of the urban traffic problems. According to [9], complex underground roads for automobiles have multiple connections to the ground road network, and there must be the possibility of interconnectivity between adjacent UURs. It is necessary to implement a network-based underground traffic system within the UUS, rather than a link-based system. In contrast to this, the underground roads that we still experience around the world include those that flow into and out of the ground only at the starting and ending points, as well as those that diverge or merge in the middle of the main line and connect to the ground road network [10]. Urban road construction must evolve to underground routes due to land use restrictions in urban areas. Consequently, urban interchanges and underground roads are now being constructed and utilized. In this context, it is expected that two underground roads will intersect in the UUS, forming a junction that will allow traffic from all directions in the long run [11]. However, there is almost no technical review or research on UUR junction design worldwide at the moment. The underground road design guidelines that we can confirm apply the design standards established for existing roads to underground roads for many design factors and refer to considering additional safety margins for major design factors such as radius and sight distance. It has been noted that unstable driving behavior exhibited by drivers under constraints such as spatial closures is not sufficiently considered when designing underground roads. indicators for underground roads, and designers do not have access to separate design standards to refer to when designing [16].