## 1 Introduction  
The number of fatal road accidents increases sharply during adverse weather conditions such as fog, snow, or rain; this trend causes problems including socio-economic loss and increased social anxiety [1; 2; 3; 4]. The causes of traffic accidents can be divided into three categories: first, driver-related factors that include recognition error, decision error, and performance error; second, vehicle-related factors that include tire/wheel-related, brake-related, and steering-related; and third, environment-related reasons that include slick roads, fog/rain/snow, and road design [5]. This study focuses on rain-related factors. Precipitation has a particularly significant impact on the risk and frequency of traffic accidents that occur in relation to weather conditions [6; 7]. For example, accidents and injuries are 10% and 8% more likely, respectively, on rainy days, and higher rainfall intensity is associated with a 50% increase in the possibility of accidents [8]. Moderate rainfall (>10 mm) has also been associated with increased accident risk [9]. Rain causes accidents through a combination of physical effects that affect the driving environment [10], such as tire friction loss, and drivers' lowered abilities [11], such as poor visibility due to rainy conditions or secondary spray from other vehicles. Therefore, it is necessary to clarify the relationship between adverse weather and road safety to develop smarter technology that enables safe driving of both normal and autonomous vehicles [9]. Accordingto the Korea Road Traffic Authority, the total number of fatalities in road traffic accidents has decreased; however, the number of crashes during rainy conditions has increased [12]. In particular, in Seoul, traffic fatality rates on rainy days were the highest of all cities [13]. In the Seoul area, the observation network measuring rainfall information is sparse; hence, a limited number of studies have been conducted on the relationship between rain and traffic accidents and its effects [14].  
Previous studies investigating the effects of rainfall on accidents have focused on urban areas [9; 15] because global urbanization is increasing the relative importance of cities. Improved vehicle design and driver training have reduced weather-related accident rates in developed countries in recent decades, but urbanization and rising occurrences of extreme rainfall events have led to the need for more effective approaches to analyzing the links between rainfall and accidents [7; 16; 17]. For example, [PERSON] and [PERSON] [18] compared the relative accident rate (RAR) for precipitation events using both radar-derived and gauge-based rainfall data for two urban areas in the United Kingdom, finding that the former approach was far superior. RAR values vary based on rain data sources. Although many studies have assessed the cause-and-effect relationship between traffic accidents and rainfall, few have used such time-based high-resolution radar data, especially to analyze changes in the frequency of traffic accidents due to differences in spatial rainfall distribution within urban areas. In this study, we analyzed the relationship between rainfall and traffic accident rates in Seoul, South Korea, using the gauge- and radar-derived rainfall data to assess the influence of the type of rainfall data used on the spatial distribution of traffic accident patterns.