## 6 Discussion and Conclusions  
In the C-ITS environment, various in-vehicle warning information is implemented. Thus, it is essential to evaluate the warning services using appropriate quantitative indicators and to monitor whether the effect continues during the operation period. The evaluation process is also important because the results of the evaluation can be applied to calibrate the service parameters (e.g., activation conditions and exposure duration).  
This study aims to quantitatively assess the impact of in-vehicle warning information using PVD collected from freeway C-ITSs. Leveraging the background of traffic flow theory and individual vehicle data collected in every second, we evaluated whether the warning information induces drivers to drive in a conservative way. For the evaluation, this study employed PVD collected from 700 vehicles on freeways over two different time periods: one period when the in-vehicle warning information was provided to drivers and the other when it was not.  
After identifying the driving characteristics of conservative drivers in a speed-spacing plane, we compared various statistics related to speed and spacing between the two data collection periods. Our analysis revealed several significant findings regarding the effectiveness of in-vehicle warning information as follows.  
* During the \"with\" period, drivers tend to decelerate and increase spacing when they receive an in-vehicle warning.
* Compared with the \"without\" period, we found empirical evidence that the in-vehicle warning information was effective in encouraging drivers to drive more conservatively.
* Since the \"with\" period was followed by the \"without\" period, some drivers selectively react to the in-vehicle warning after going through an adaptation phase (i.e., \"with\" period).
* Drivers' reliability and compliance rate to the warning information could be time sensitive.  
These findings provide an insight that it is imperative for the C-ITS operators to devise an MOE to evaluate the reliability of warning information and continually monitor these measures.  
Despite the C-ITS operation period in this study extending over a year, it was difficult to obtain a sufficient number of valid samples. Especially, we collected the \"without\" data for 3 months and a relatively smaller number of samples were collected as compared with the \"with\" period. Expanding the data collection would enhance the comprehensiveness of this study. Of the many different types of warning information, this study only considers the FCWS warning. In future research, we intend to investigate whether other warning information has a different impact on the driver in terms of conservative driving behaviors. We emphasize that the speed-spacing relationship was applied to evaluate the effectiveness of the in-vehicle warning information. However, lateral movement, including lane-changing maneuvers, represents another potential driving behavior to be expected after receiving an in-vehicle warning. Analyzing lateral movement can be accomplished using the PVD by collecting additional in-vehicle records such as steering angle, angular velocity, and lane information.  
Study conception and design, H.K. and S.K.; data collection, J.K. and A.J.; analysis and interpretation of results, H.K., J.K. and S.K.; draft manuscript preparation, H.K. and S.K. All authors have read and agreed to the published version of the manuscript.  
This work is supported in part by the Korea Agency for Infrastructure Technology Advancement (KAIA) grant funded by the Ministry of Land, Infrastructure and Transport (Grant RS-2022-00143579).  
Not applicable.  
**Data Availability Statement:** The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding author.