## 1 Introduction  
The term connected vehicle (CV) denotes a technology and system that utilizes wireless communication technology to provide information about traffic conditions downstream. This communication occurs between vehicles as well as between vehicles and infrastructure. The driver receiving the information is expected to quickly and safely respond to unexpected situations to prevent accidents from risk factors [1; 2]. Since the early 2000s, various studies using real-time in-vehicle data have been conducted in the United States and Europe as communication technologies and services advance [3; 4; 5; 6]. In South Korea, the first pilot study of the cooperative intelligent transportation system (C-ITS) started in 2014 in Sejong and gradually expanded to other cities, including Seoul, Jeju, and Gwangju. In each study, the C-ITS infrastructures, including in-vehicle warning systems, roadside units (RSU), and other wireless communication units, were installed, tested, and operated. Apart from the study in urban areas, the Korea Expressway Corporation (KEC) installed C-ITSs in some sections of expressways in the metropolitan area in 2018. This installation encompassed the development and implementation of over 700 in-vehicle warning systems, a digital tachograph (DTG), and an advanced driving assistance system (ADAS). From the system, one can easily obtain various in-vehicle data, including speed, acceleration rate, latitude, longitude, time to collision (TTC), and spacing (often referred to as PVD).  
In the C-ITS environment, we anticipate that drivers who receive vehicle-to-everything (V2X)-based warning information will likely adjust their behavior by either reducing their speed, increasing spacing, or changing lanes to avoid potentially hazardous traffic conditions downstream. Thus, assessing the quantitative impact of the warning information provided on driver behavior would play a crucial role for traffic operators.  
This study suggests a method to quantitatively evaluate the effect of in-vehicle warning information using per-vehicle data (PVD) collected from freeway C-ITSs. Based on findings from a previous study [7], aggressive drivers (\"Aggressive driver\" means a driver who maintain shorter spacing at a given speed than ordinary drivers) typically maintained speeds ranging from 75 kph to 140 kph while keeping a spacing of 32.4 m (i.e., standard deviation: 27.4 m), whereas conservative drivers (\"Conservative driver\" means a driver who maintain longer spacing at a given speed than ordinary drivers) speeds were distributed between 74 kph and 105 kph with a spacing of 62.2 m (i.e., standard deviation: 32.9 m). Leveraging insights from this earlier research, this study aims to quantitatively evaluate the impact of warning information from the C-ITS. To achieve this, two experiments were designed within the same C-ITS environment: one experiment where the in-vehicle warning information is provided to drivers and the other where it is not. We examined the PVD collected at second intervals from the two experiments to investigate the relative changes in speed and spacing within the speed-spacing plane and see if driving behavior tends to become more conservative when the warning information is actually provided versus when it is not. Furthermore, this study quantitatively assessed the number of vehicles responding to the warning information and a comparative analysis was conducted between the two experiments.  
The remainder of this paper is organized as follows. Section 2 presents previous studies related to the evaluation of in-vehicle warning information. Section 3 presents the methodology to evaluate the in-vehicle warning information. Section 4 introduces the analysis area and empirical data used in this study. Section 5 shows the results of the evaluation and finally this paper closes in Section 6 with the conclusions of the study.