###### Abstract  
The development of a road safety performance index has been widely accepted as a supportive instrument to evaluate and compare the safety performance in different countries. However, most of the current studies concentrate on the index development for only one year. In other words, there is still a lack of cross-year comparison based on the constructed safety performance index, so as to assess the progress of road safety performance in different countries over time. In this study, by collecting data on four background indicators and seven safety performance indicators of 21 European countries for both 2008 and 2015, the hierarchical clustering analysis is first utilized to identify country groups based on the background indicators. Then, the principal component analysis (PCA) is applied for each group to construct a safety performance index, and a cross-year comparative analysis on country grouping, index ranking, and weight allocation is conducted. The results show that the members in the two country groups remain the same, implying that there was no dramatic change with respect to these countries' road safety policy context in these two years. However, the gaps between these two country groups with respect to their overall socioeconomic development, as well as their road safety performance, enlarged over this period. Moreover, by comparing the indicator weights assigned for each country group in different years and examining the changes in indicator values of each country, a useful insight into the areas of underperformance is gained, which cannot be revealed in single year index evaluation. All these findings provide policy makers with valuable guidance to prioritize their actions to improve the level of road safety. a 2022 25 September 2022 29 September  
measurement that is causally related to the number of crashes or to the injury consequences of a crash (e.g., the seat belt wearing rate of car occupants), and are used in addition to the figures of crashes or injuries to understand the process that leads to crashes [9]. Today, having been aware of the complex character of the road safety issue, a great number of SPIs have been developed [10; 11; 12; 13; 14; 15; 16; 17]. Hence, combining various SPIs into an index is regarded as a scientifically sound way to reduce the dimension of selected SPIs. Such an index can then be used as a supportive instrument to compare and prioritize road safety actions among various countries. Some of the successful examples are presented below. [PERSON] et al. explored a methodological framework for creating a road safety performance index for cross-country comparison, in which seven SPIs with respect to alcohol, speed, protective systems, daytime running lights, vehicle, infrastructure, and trauma care were defined, and five weighting approaches were applied to create one overall index for 21 European countries. The study concluded that comparing the road safety performance of countries by means of an index enabled earlier and goal-oriented action [10]. This study was further extended by [PERSON] et al. [11], in which some additional SPIs were considered, constituting a layered hierarchy, and the technique of hierarchical fuzzy TOPSIS was developed for the construction of an overall index. In the SUNflowerNext study [12; 13], three types of performance indicators, i.e., road safety performance indicators, implementation performance indicators, and policy performance indicators, were distinguished, and an overall road safety index was developed by two weighting approaches (i.e., principal component analysis and factor analysis) based on the data collected for 27 European countries. The analysis revealed that the countries' ranking based on the combination of different performance indicators may be different from the traditional ranking of countries based on crash data. This study was revisited by [PERSON] [14], in which the different treatment between quantitative and qualitative indicator data is considered when developing such an index.