## 8 Hot Spots Around Station  
Hotspots are locations where large number of people converge, for undertaking activities such as work, shopping, recreation, etc. Various methods and definitions have been proposed to identify activity or employment sub-centers by [PERSON] [27], [PERSON] [28], McMillen and Smith [29], [PERSON] [30], and [PERSON] [31]. In this study, using mobile phone data, based on a 50 m \(\times\) 50 m P-cell in each city level allows us to identify hotspots to consider the density of mobile population spread over spaces. Figure 5 shows the hotspots changes caused by HSR in the three cities. We developed a map to identify the hotspot locations, showing the difference of mobile population before and after the opening of the HSR. In the figure, we define the hotspot and semi-hotspot with the increased mobile population over 200 and 100, respectively. As shown in Figure 5a, Iksan city has a population of approximately 300,000 people, with an area of 506 km\({}^{2}\). There are many spots that changed to semi-hotspots and hotspots due to the HSR, and the hotspots are shown to be appear within 10 km of Iksan station. Although the hotspots are spread out over the city boundary, more hotspots are located at the HSR station periphery, within 5 km. The appearance of many hotspots provides general evidence of the positive impact of the HSR, which attracts more people at Iksan city and the station catchment area. Jeongeup has a small population of 114,000 people, with large area of 700 km\({}^{2}\). The area comprises small farms and mountains. Therefore, no significant difference in the hotspots can be seen before and after the opening of the HSR. However, the number of hotspots is slightly increased, and they appear in the northeast and southeast region from Jeongeup station, because the agricultural and industrial complexes are located northeast, and the city hall is located southeast from the station. Figure 5: Maps of hotspot changes after the opening of the HSR. (Note: x and \(y\)-axis unit is km). Mokpo has many hotspots after the opening of the HSR, which spread out farther from the station due to the impact of the HSR. Interestingly, it is found that the hotspots at the intercity bus terminal periphery decreased, which means that a transport mode shift occurred, from intercity bus to HSR. By comparing the number of hot spots in the three cities, we expect that the attractive places such as shopping centers or convention centers may have been built station periphery since the opening of HSR. This may explain the low number of hot spots in Jeongeup and the high number of hotspots in Iksan and Mokpo. According to ticket sale statistics, bus passengers diminished by as much as 20% after the opening of the HSR. This was reflected by the hotspots at the intercity bus terminal located northeast from Mokpo station, as seen in Figure 5c, with a negative value of difference (denoted as \"o\"). It is also interesting to examine the compactness of hotspots from the HSR station, revealing how close their spatial distributions are to the HSR station. The compactness of hotspots can also be explained by how land use is formed and distributed around the station. If hotspots in a city are located spatially close to the HSR station, it would indicate that the hotspots are seen to converge at the station. The spatial compactness of hotspots, \(C_{i}\), is calculated by Equation (3) in this study [32]. \[C_{i}=\frac{H_{hotspot}}{N_{i}\sqrt{A_{i}}}\times 100 \tag{3}\]  
where, \(H_{hotspot}\) is the average distance between station, \(N_{i}\) is the number of hotspots (mobile population greater than 50) in a city i, and \(A_{i}\) is the area of city i. Therefore, \(C_{i}\) is the compactness, the average distance between the HSR station and hotspots, weighted by the number of hotspots and area of the city. A lower \(C_{i}\) means that the spatial structure of hotspots is compact around the station; that is, they are spatially close to station.