PURPOSES : This paper is to deliver an engineering design guide currently unavailable that highway engineers can refer to when they need to determine the safe distance between the starting point of the underground highway in a metropolitan area and the access points of the adjacent highway. METHODS : The distance between two different highway points should be sufficiently far to support the number of lane changes that drivers may take comfortably at the cruising speed in critical traffic conditions. RESULTS : The proper distance for the selected case study is calculated to be 160 and 200 m at the cruising speeds of 40 km/h and 50 km/h, respectively, without significant increase in potential conflicts estimated by the combination of the VISSIM and SSAM analysis. CONCLUSIONS : The proposed design method can be referred in urban metropolitan areas where no specific design guide is yet available, and it would prevent the unnecessary increase in construction costs and encourage private-public partnership highway businesses.