## 3 Methodology
### Driving Simulator Experiments  
We conducted a VR driving experiment using a driving simulator (DS) owned by the University of Seoul in Korea. A DS is an experimental device designed to record driving data in a VR driving environment that allows drivers to feel as if they are driving on a real road so they can conduct research that would otherwise be dif cult to conduct in a \"eld\" experiment. To create the VR driving environment, we used UC-win/Road ver. 15, made by Forum 8 in Japan, a 3D virtual reality software that can implement the road driving environment professionally. This software, synchronized with a DS, can extract driving behavior data, including speed, lateral placement, brake power, and steering angle. Figure 3 shows a VR driving experiment using a DS on underground roads.  
A VR driving environment for a double-deck tunnel was created using Korea's UUR design standards. So, we created a VR driving environment consisting of a simple network that includes ramp terminals that connect the two double-deck tunnels. Considering that this road environment is a UUR, it was assumed that only small cars would be permitted, and the facility limit accordingly was 3.3 m. The design speed of the main lines was set at 80 km/h, the design speed of the ramps to 50 km/h, and the lane width to 3.25 m. As the design level of service for Korea's UURs is generally set at LOS D, the main line of the VR driving environment was maintained at 1200 vehicles per hour per lane.  
This study used experimental scenarios in which the length of the acceleration lane was varied as an experimental factor to compare driving behavior at the merging section based on the acceleration lane. According to the direction of merging into the main line, we divided it into right-side and left-side merging scenarios. Design special cations for the left-side and right-side on-ramp terminals were applied equally. The length of the acceleration lane was set at 100 m, which is the minimum design standard in Korea when the design speed of the main line is 80 km/h and the design speed of the ramp is 50 km/h. Also, to compare driving behavior, acceleration lanes with lengths of 120 m and 140 m were set as additional scenarios. The 20 m interval is determined by applying a larger design margin to 13.89 m per second converted from a ramp design speed of 50 km/h. In all scenarios, the taper at 60 m was applied equally. As a result, six experimental scenarios were set up based on the length of the acceleration lane and the direction of merging into the main line. Each scenario was named RS100, LS100, RS120, LS120, RS140, and RS140.  
The main experiment for the VR driving experiment was conducted from April 17 to April 27 in 2022. Participants were recruited from among people with a driver's license who drove regularly and had at least two years' experience driving, and those working in the transportation industry were excluded. A total of 35 participants were recruited in the VR driving experiment, with 7 in their 20s (3 males/4 females), 9 in their 30s (5 males/4 females), 9 in their 40s (5 males/4 females), and 10 in their 50s (5 males/5 females). The participants were instructed to drive while observing road signs and speed signs based on their own driving experience. In addition, to ensure that the participants adapted fully to the DS device, they were asked to run a pre-test for approximately \"ve minutes before  
Figure 3: Driving simulator with UC-win/Road.  
the main experiment. After that, the main experiment was conducted for approximately 25 min. All these processes were conducted under the supervision of the experimenter.