# Temporal Instability Analysis of Injury Severities for Angle and Non-Angle Crashes at Roundabouts
## 6 Results and Discussion
### Analysis of the Characteristics' Effects
#### 6.1.3 Roadway Characteristics  
Road characteristics showed significant differences across periods and collision modes. Considering the shoulder type, the paved shoulder was found to be statistically significant for non-angle collision models in 2014-2015 and 2016-2017, which steadily reduced the likelihood of minor and severe injuries while increasing the possibility of no injury. Concerning the angle collisions, the same indicator had similar effects on all models except 2016-2017. During the analysis periods, the paved shoulder contributed to a lower probability of severe injuries and a higher probability of no injuries (see Table 11). However, unstable effects were observed for minor injuries. In addition, the unpaved shoulder significantly affected the injury severity of angle and non-angle collisions but did not produce stable results. It can be seen that a paved shoulder is beneficial in reducing the injury severity of crashes compared to an unpaved shoulder. The possible reason is that the paved shoulder provides a broader and more reliable safety redundancy zone for vehicles entering and exiting the roundabout. In contrast, [PERSON] and [PERSON] [43] reported that the unpaved shoulder had varied influences on the injury severity for drivers of different ages involved in a run-off-road collision. This suggests a strong distinction in the role of shoulder type in different sorts of crashes, which has not been discussed in detail in previous studies. Regarding surface indicators, Table 11 showed that a dry surface was associated with lowering the probability of no injury and increasing the probability of minor injury for two collision modes. In contrast, the marginal effects of severe injury were inconsistent, with a lower probability for angle collisions (\(-0.0200\)) and a higher probability for non-angle collisions (\(0.0263\)). Interestingly, research findings on the effect of the dry surface on injury severity are inconsistent [44; 59]. Driving around roundabouts requires more steering due to circular lanes and counter-clockwise driving rules, which can potentially lead to accidents on slippery surfaces, such as sideslips. In contrast, the dry surface has higher ground friction, improving the effectiveness of emergency braking and reducing the risk of collision during steering. Consequently, the dry surface indicator could reduce the frequency and severity of angle collisions, which are prone to occur during steering maneuvers. Straight road alignment only affected angle collisions, which reduced the probability of severe injury in both \(2016\)-\(2017\) and \(2018\)-\(2019\) (\(-0.0099\) and \(-0.0134\), respectively). For no injury and minor injury, the straight indicator was found to be temporally unstable during the two periods (see Table 11). The left-curved road alignment influenced the injury severity outcomes for both angle and non-angle collisions. In the \(2014\)-\(2015\) angle collision model, 'curved left' resulted in more no injury, less minor injury, and more severe injury (\(0.0281\), \(-0.0187\), and \(-0.0094\), respectively). However, non-angle collisions occurring on the left curved road had a lower probability of no injuries and a higher probability of minor injuries, while the marginal effect of severe injuries was unstable across \(2016\)-\(2017\) and \(2018\)-\(2019\). Roads within the roundabout have different alignments, resulting in differentiated driving behaviors. There are straight roadways before entering or just after exiting the roundabout. Roads in the intersection area curved right, while loops within the roundabout curved left. Curved right is consistent with regular driving steering habits, which may have less impact on injury severity. In contrast, straight roads can reduce injury severity due to the simple traffic environment and operational requirements. Similar results can be found in [PERSON] and [PERSON] [42] and [PERSON] et al. [39].