# Temporal Instability Analysis of Injury Severities for Angle and Non-Angle Crashes at Roundabouts
## 6 Results and Discussion
### Analysis of the Characteristics' Effects
#### 6.1.2 Environmental Characteristics  
Environmental characteristics that were statistically significant include daylight, dark-lighted, dark-not-lighted, clear, cloudy, rain, and fog. Particularly, some variables only impact a single collision mode (either angle or non-angle collisions) at specific periods. For instance, the dark-lighted indicator was associated with reducing the injury severity of angle collisions in 2012-2013 and 2018-2019, while the indicator was not statistically significant for non-angle collision models. As shown in Table A11, both the clear indicator in 2014-2015 and the fog indicator in 2012-2013 increased the probability of severe injury in non-angle collisions (0.0437 and 0.0005, respectively). Similar to findings from previous studies [55], increased risky driving behavior in clear weather may be responsible for the higher likelihood of severe injury. However, neither of the indicators (fog or clear) had a significant effect on angle collisions. In general, it appears that angle collisions tend to be affected by lighting characteristics, while non-angle collisions are more sensitive to weather characteristics. One of the reasons may be that lighting conditions directly affect the driver's field of view [56], which is particularly pronounced for angle collisions. The daylight indicator was positively correlated with minor injury (0.0104, see Table 11) while negatively correlated with no injury (\(-\)0.0062) and severe injury (\(-\)0.0042) for angle collisions in the 2016-2017 model. Turning to the impact of daylight on non-angle collisions, injury severity increased in 2014-2015 and reversed in 2016-2017, indicating significant temporal instability. The marginal effects in Table 11 revealed that crashes occurring in dark, not-lighted environments are less likely to result in severe injury. This finding contradicts previous conclusions in general road sections that dark, not-lighted areas can exacerbate the severity of crash injuries [57]. Given this difference, a reasonable understanding is that roundabouts involve more complex geometric dimensions and require greater driver concentration than straight lanes [58]. In this complex traffic environment, 'dark-not lighted' as an additional disadvantage might make the driver more focused and cautious. Therefore, 'dark-not-lighted' plays a role in reducing crash injury severity at roundabouts. As for 'cloudy', it had significant impacts on the probabilities of injury severity outcomes. For angle collisions, inconsistent effects of cloudy weather were observed in 2012-2013, 2014-2015, and 2016-2017, whereas the overall trend showed an increase in the chance of severe crashes over time. Perhaps it can be attributed to the fact that drivers gradually adjusted to the roundabout and improved their driving skills, mentally reducing vigilance against the poor external environment and causing more severe crashes. Furthermore, in the 2016-2017 model, cloudy weather affects the injury severity of both angle and non-angle collisions. However, angle collisions had a higher probability of severe injury and a lower probability of no injury or minor injury, while non-angle collisions were just the opposite. The finding suggests that collision modes may be related to driver heterogeneity, such as diverse responses to environmental factors. Under the same weather conditions, some drivers feel pleasant while others suffer discomfort, which leads to different driving behaviors and varying injury severity in collisions. Previous studies have found that cloudy weather is beneficial for reducing injury severity [11], which is consistent with the effect on non-angle collisions in this study. Rain was a non-negligible factor for both modes of collisions at roundabouts, although the effect was only statistically significant in some periods. As shown in Table 11, positive marginal effects for no injury and negative marginal effects for severe injury resulted in fewer serious crashes. It is inconsistent with previous research [12].