## 3 Methodology  
### UUR Junction Based on Double-Deck Tunnels and Left-Side Merging Sections  
A UUR junction formed by two underground roads intersecting three-dimensionally in a UUS can result in very high construction costs if the junction is designed inefficiently. In Korea, most UURs, which are already in operation or being planned, are parallel tunnels placed side by side with a minimum gap between each tunnel. Traffic flows are separated in each direction. As a result, it eliminates the risk of accidents caused by crossing the center line on roads. It also has the advantage of relatively low construction costs compared to one large-section tunnel for two-way traffic flows. The parallel tunnel does not pose a major obstacle when it simply connects the ground and the underground: however, it is not suitable for forming a junction that would allow traffic to flow in all directions between two parallel tunnels in the UUS. This is because the length of a semi-direct ramp between two parallel tunnels may be quite long. Thus, a UUR junction should be designed with double-decker tunnels instead of parallel tunnels to allow traffic to move in all directions. A double-decker tunnel is defined as single-tube tunnel where one carriageway is located above the other [9]. On the upper and lower carriageways, traffic flows in opposite directions. A schematic shape of a UUR junction is shown in Figure 1, when two double-decker tunnels intersect, connecting four right direct ramps and four left direct ramps each. Here, it is possible to design it as a single-tube tunnel by connecting ramps 1 and 8 between the upper floors, and ramps 4 and 5 between the lower floors. In contrast, ramps 2 and 6 connect from the upper to the lower floors, while ramps 3 and 7 connect from the lower to the upper floors, so each tunnel must be designed independently. In this way, separate sections with four diverging sections of the off-ramp terminals and four merging sections of the on-ramp terminals are formed. Figure 1: Schematic shape of a UUR junction. UUR junctions based on a double-decker tunnel inevitably form left-side merging sections. In many countries around the world, such as Korea, the United States, France, Spain, etc., vehicles drive on the right side of the road. The driver's seat is positioned on the left side of the vehicle. Generally, these countries stipulate or recommend that when entering or exiting a major arterial, such as a freeway or expressway, the ramp must be connected on the right side. When changing lanes in a left-side merging section, drivers must turn their heads more to the right-side mirror to keep an eye out for approaching vehicles to the right. The difference in physical behavior required between left-side and right-side merging sections is a major factor affecting a driver's safety. It is anticipated that this factor will have a greater impact on driver behavior and safety in merging sections of tunnel environments. Ref. [25] found that accidents occurred about 7.88 times more often in the left-side merge than in the right-side merge based on the model results, assuming a negative binomial distribution on generalized linear models, built from a case study on the crash frequency of 1-75 through downtown Dayton, Ohio. Also, as noted in Korea's road design guidelines, the left-side ramp terminals have a higher accident rate than the right-side ramp terminals, and the left-side ramp terminals should be avoided whenever possible. However, major countries around the world that have underground infrastructure are already operating underground roads designed with a left-side direct ramp, such as the A86 in Paris, the M30 in Madrid, and the Big Dig in Boston. To create a safe traffic environment at UUR junctions in the future, a safer design of the left-side on-ramp terminals is necessary. To do so, many studies reviewing driving behavior and design factors are required.