## 6 Conclusions  
Demand for UURs has increased steadily in recent years around the world to reduce traffic congestion in urban areas. It is important to introduce network-type underground roads in UUSs to make efficient underground roads. In this study, VR driving experiments were conducted to compare driving behavior in terms of the direction of the ramp and the length of the acceleration lane in the merging sections of the UUR junction based on double-decker tunnels. As a result, it was found that driving behavior was different between the right-side and left-side merging sections. Particularly, the vehicle's speed on ramps conspicuously decreased in the left-side merging sections. Driving behavior became more stable as the length of the acceleration lane increased. Furthermore, it was also found that acceleration behavior was statistically lower in left-side merging sections when the acceleration lane had additional distance. Therefore, from this perspective, to ensure the safe design of left-side merging sections, a separate acceleration lane length standard is required, in addition to an extra distance longer than the existing standard. However, if the acceleration lane is extended, the large-section tunnel will become longer, resulting in increased construction costs. As such, experts must evaluate it in terms of its technical and economic feasibility. Finally, due to the increasing interest in UURs, much research into related design factors is necessary to ensure users' safety on underground roads. In the future, we hope that the optimal design technology for UURs will reflect various driver behavior studies. Conceptualization, D.L. : methodology, D.L. and S.J. : formal analysis, S.J. : data collection, S.J. : writing--original draft preparation, S.J. : writing--review and editing, D.L. : project administration, D.L. : funding acquisition, D.L. All authors have read and agreed to the published version of the manuscript. This work was supported by a Korea Agency for Infrastructure Technology Advancement (KALA) grant funded by the Ministry of Land, Infrastructure and Transport (grant number: RS-2020-KA157786). Ethical review and approval were waived for this study because personal identification information was not collected or used for any analysis in this study. * _[PERSON] et al. (2021)_ [PERSON]; [PERSON] Strategic Planning for the Creation and Development of Livable Underground Spaces in Hot and Dry Climates (Case Study: Zahedan). _Creat. City Des._**2021**, \(4\), 69-78. * [PERSON] and [PERSON] (2016) [PERSON]; [PERSON] [PERSON] An Introduction to Connectivity Concept and an Example of Physical Connectivity Evaluation for Underground Space. _Tunn. Undergr. Space Technol._**2016**, _55_, 205-213. [CrossRef]
* [PERSON] and [PERSON] (2018) [PERSON]; [PERSON] Some Aspects on the Planning of Complex Underground Roads for Motor Vehicles in Chinese Cities. _Tunn. Undergr. Space Technol._**2018**, _82_, 592-612. [CrossRef]
* PIARC (2016) PIARC. _Road Tunnels: Complex Underground Road Networks_: Report 2016.R19.EN: World Road Association: Nanterre, France, 2016. * National Research Council (2016) National Research Council: Division on Earth: Life Studies: Board on Earth Sciences: Committee on Geological: Geotechnical Engineering: Committee on Underground Engineering for Sustainable Development. _Underground Engineering for Sustainable Urban Development_: National Academies Press: Washington, DC, USA, 2013. * [PERSON] et al. (2022) [PERSON]; [PERSON]; [PERSON]; [PERSON]; [PERSON] An Investigation for Driving Behavior on the exit-ramp Terminal in urban Underground Roads using a Driving Simulator. _J. Korea Inst. Intell. Transp. Syst._**2022**, _21_, 123-140. [CrossRef]
* [PERSON] et al. (1995) [PERSON]; [PERSON]; [PERSON] [PERSON] A Simulation Study of Driving Performance in Long Tunnel. In Proceedings of the Pacific Rim TransTech Conference. 1995 Vehicle Navigation and Information Systems Conference Proceedings. 6 th International VINIS. A Ride into the Future, Seattle, WA, USA, 30 July-2 August 1995; pp. 96-103.