Cyclist casualty severity at roundabouts - To what extent do the geometric characteristics of roundabouts play a part?

Introduction: In general, priority junctions are converted into roundabouts to increase capacity and reduce vehicle accidents. However, previous research has indicated that roundabouts are dangerous for vulnerable users, especially cyclists.

Method: This paper investigates which design factors influence cyclist casualty severity at give way (non-signalized) roundabouts with mixed traffic, using the UK STATS19 National dataset of cyclist casualties. First, the correlation matrix was generated to observe the relationship between variables. Second, dimension reduction was applied to geometric design variables in order to reduce the number of variables and generate the factors. Finally, the binary logistic regression method, with serious and slight casualties as dependent variables, was applied in three steps. The first Binary Logistic Regression Model (BLRM) included speed limit, sociodemographic, and meteorological conditions. The variables in the second BLRM consisted of geometric design variables. The third BLRM included the factors that were generated by dimension reduction.

Results: The correlation matrix revealed that the number of lanes on approach and half width on approach were statistically significantly correlated, while the variables, such as geometric design (entry path radius, number of arms, number of flare lanes on approach, type of roundabout and number of circulating lanes), sociodemographic (casualty gender and age), speed limit and meteorologically related factors (daylight, weather and road surface condition), did not show any statistical significance. From the dimension reduction process, two main factors were identified, including Approach Capacity (Factor 1) and Size of Roundabout (Factor 2), and they were subsequently used as independent variables in the logistic regression analysis. The subsequent BLRMs showed that a higher speed limit reduces the safety for cyclists at roundabouts. The probability of a serious casualty increases by approximately five times (odds ratio 4.97) for each additional number of lanes on approach and by 4% (odds ratio 1.04) with a higher entry path radius. It was also found that Factor 2 (Approach Capacity) increases the casualty severity (odds ratio 1.86) for cyclists at roundabouts. Practical applications: While this research studied roundabouts in the UK, the methodological approach and statistical analysis techniques are applicable to other countries and the findings are likely to be of value to decision makers worldwide.