## 1 Introduction  
Opening a new high-speed railway (HSR), as a transport-mobility enhancement strategy, has been approved by policymakers in many countries. Since the introduction of the first HSR in the world (in Japan, 1964), several HSRs have been built, and are still proposed worldwide. The HSR network is expected to reach 25,000 km by 2020, even though the cost of construction, maintenance, and operation of HSRs is high [1]. Because of the remarkable increase in accessibility owing to the HSR, cities connected to the railway line have realized new opportunities and changes of all aspects in our society. For example, urban regeneration opportunities, increasing diverse individual activities owing to the reduced travel time while increasing personal time, providing higher quality of service than competing modes, and/or influencing local economics through individuals changing homes and jobs. Many countries have conducted extensive studies on the impact of HSR from various perspectives, such as regional socioeconomics, changes in transport mode shares, and travel patterns [2, 3]. The results of the studies had a positive effect on the regional economic growth [4, 5, 6]. On the other hand, a recent study in Japan showed that the \"straw effect,\" defined as a negative economic externality of new transport accessibility was observed, wherein a large city absorbs the commercial and industrial activities from smaller cities connected by the HSR. Studies on the impact of the new HSR largely focused on the changes in travel demand, mode share, and travel patterns using travel survey data obtained at sites, such as stations and terminals. The results showed that the HSR helped in increasing passenger volume and mode share ratio, particularly with respect to air transportation. However, despite a growing concern about the impact of new HSR construction, there have been limited studies because of a lack of data; studies were only restricted to travel surveys and passenger statistics. Recently, large amounts of data have been collected via mobile communication; these data include the spatiotemporal locations of phone users, as well as their gender, age, and home location. This is a reliable source for investigating urban issues related to mobility patterns and spatiotemporal demographic dispersion of a city. Thus, it is expected that mobile phone data allow for the provision of disaggregated, microscopic information without incurring significant costs involved in data collection. The mobile phone data offered by SK Telecom (SKT), Korea's largest telecommunications company with more than 29 million mobile subscribers; this accounts for around 50% of the total market in Korea (IT STAT 2015). In order to represent the total mobile phone users, SKT expanded its mobile phone records by adjusting the number of SKT users by their market share in each region. Therefore, the mobile phone data represents the total population of mobile users (hereafter, we use'mobile population' for short). One of major benefits using SKT mobile phone data is that the dataset contains all mobile phone signals unless users turn off their phone. That is because a cellular base tower sends a push-alarm signal to the user's phone every 15 min, the mobile signal is recorded as long as the phone is turned on. According to SKT, the turn-off ratio is less than 7% of total users. Due to concerns of privacy, the provided data is not available at an individual person level and does not include location trajectories of each user. Instead, SKT aggregated the individual record to a grid cell (\(50\times 50\) m) and provided to the public. Therefore, these valuable data that can be analyzed represent a broad and significant sample; these data will be able to help in comprehensively understanding spatiotemporal patterns of mobile phone users. The main purpose of this study is to demonstrate the research potential of mobile phone data for analyzing the impact the HSR has had, as a case study.