This study aims to develop an underground expressway design for an exit area to mitigate traffic congestion. Thus, we explain the necessity of underground expressways and three reasons for persistent congestion on underground expressways despite an increase in supply. We focus on the first reason, which is a complicated traffic-flow conflict in the exit area, and analyze the traffic flow based on various conditions, such as the exit rate to a nearby interchange and the exit location for underground and ground roads. Consequently, we identify three factors that affect congestion in the exit area. The first factor is the exit rate, where a higher exit rate corresponds to a more severe congestion. The second is the exit location of two roads. When the exit of a road that exhibits a higher exit rate is placed on a curb side, the average delay is reduced. The final factor is the length of the lane-change section, where a longer lane-change section correspond to less congestion. However, after a certain length, the change in congestion is negligible. Based on these results, we suggest revised design guidelines for underground expressways in terms of exit location and the length of lane-change sections.

PURPOSES : This study is to evaluate environmental improvement effects of underground road project, as a case study of ‘Seobu’underground expressway project in Korea. METHODS : The environmental improvement effects of underground road construction were classified into tree sector in this study – air quality, noise, and thermal environment. To evaluate these effects, this study collected and divided environment dataset into before, during, and after ‘Seobu’underground expressway construction, and compared the change of values with entire city. In addition, the quantification method of environmental improvement effects was suggested based on the literature review. RESULTS : The result of this study indicates that there exist significant improvement effects of urban environment after the construction of ‘Seobu’underground expressway. Specifically, the concentration of particulate matters (PM10) and daytime/nighttime noise, and land surface temperature (LST) were improved along the expressway. In addition, the overall environmental benefit of ‘Seobu’ underground expressway was estimated to 1.15 billion won per year. CONCLUSIONS : To promote systematic and consistent underground road project, it is required to establish legislative system for evalauting and supporting urban improvement effects.