# Temporal Instability Analysis of Injury Severities for Angle and Non-Angle Crashes at Roundabouts
## 7 Summary and Conclusions  
To investigate the heterogeneity and temporal instability of factors affecting the injury severity for roundabout angle and non-angle collisions, this study applied the 2012-2019 Florida roundabout crash data to estimate the random parameter logit models with heterogeneity in means and variances. With possible injury severity outcomes of no injury, minor injury, and severe injury, several potential factors are considered in the analysis, including time, environment, roadways, spaces, vehicles, crashes, and humans. Overall temporal instability was captured by a series of likelihood ratio tests, whereas some variables still exhibited relative temporal stability. For instance, weekend and dark-lighted were found to be consistently associated with lower severe injury probability in angle collisions, while the paved shoulder and cyclist indicators had a stable effect on non-angle collisions. The findings can provide valuable insights for decision-makers to specify effective strategies from a long-term perspective. This paper also reveals the non-transferability between angle and non-angle crashes. Some factors were only statistically significant in the non-angle crash model, including clear, fog, commercial vehicles, and aggressive driving. In contrast, there are several contributing factors specific to the injury severity of angle crashes, such as dark-lighted, straight alignment, merge, lane departure, and disabled drivers. It is worth noting that certain indicators, cloudy and town, had opposite effects on the injury severity of angle and non-angle collisions even within the same period. According to the divergence, some differentiated countermeasures should be developed targeting different stakeholders. First, a straight roadway is less likely to cause serious injuries in angle crashes. In contrast, it is necessary for traffic managers to place signs on the curved roadway after entering the roundabout to raise the driver's awareness of driving carefully in this relatively unsafe situation. Furthermore, it is crucial to enhance the supervision of roundabouts in rural areas, while effective administration in town requires specialized strategies considering the specific traffic volume and local characteristics. Road engineering professionals are advised to prioritize the installation of shoulders over curbs, particularly emphasizing the implementation of paved shoulders to improve safety at roundabouts. In addition, dark-lighted crashes were found to be positively associated with no-injury-angle crashes, indicating a favorable effect of reducing injury severity. Therefore, lighting infrastructure should be regularly maintained or upgraded in roundabouts where natural light is poor. The findings of this study also provide valuable perspectives for distinct groups of drivers. The importance of enhancing skills training and traffic safety education for commercial vehicle drivers was highlighted due to the statistical significance of commercial vehicle indicators in all the non-angle crash models. There is another need for educational outreach and safety training targeting bicycles, as indicated by the unstable effects on roundabout safety. Further, additional enforcement countermeasures should be implemented to prevent aggressive driving, thereby decreasing the chance of serious injury in non-angle crashes. It should be acknowledged that this study still has certain limitations. Research could consider more roundabout types and more relevant variables. In addition, the causes of temporal instability are not fully understood. The discrepancies and instabilities may be due to factors not considered in the analysis--these need to be further studied in future work. Y.L. : Conceptualization, Data--processing and analysis, Methodology, Writing--original draft. C.G. : Conceptualization, Data--processing and analysis, Writing--original draft. Q.C. : Conceptualization, Writing--original draft. C.Y. : Conceptualization, Methodology, Supervision, Writing--review and editing. L.X.