Although in recent years the number of traffic accidents and collisions on Polish roads has been decreasing, it is important to identify their most significant causes. This study presents an analysis of 34 causes of road incidents reported by the police in the Wielkopolskie province in the years 2011-2013. A total of 34 causes of traffic incidents (accidents and collisions - X1, ..., X34, respectively) were grouped by means of hierarchic cluster analysis using the nearest neighbour method with Euclidean distances, in a four-dimensional space of parameters (the number of accidents - LW, the number of killed - LZ, the number of casualties, including slightly injured in collisions - LR, the number of collisions - LK). Calculations were conducted using the Statistica ver. 12 package. Analyses showed that the most common cause of traffic incidents included side collisions and rear collisions of vehicles, failure to yield to the right-of-way, failure to adjust the speed to the traffic conditions and failure to keep adequate distance between vehicles. Hitting a tree was only the 19th cause (out of 34 analysed ones); however, consequences of these incidents were tragic (8% of fatal accidents). ...
