Accidents and Geometric Characteristics of Road Layout: Some Quantitative Evaluations

Driver behaviour and high speed are constantly advised as the greatest causes of road accidents, as annually referred from ACI-ISTAT statistics (Automobile Club of Italy and National Institute of Statistics). Therefore it is necessary to deepen our knowledge on these two accident factors in order to formulate objectively quantitative evaluations on road safety. It'll be possible to realize rigid measures to obtain a reduction in the accident rate without penalizing more transport acts. These quantitative evaluations will affect road design in the future. Accidents caused from skidding between wheels and pavement are very frequent with low traffic flow: possible consequences are bumps against other vehicles or obstacles to the margins of the road (e. g. crash barrier, retaining or gallery walls) and run off road. Accidents due to hazardous maneuvers (overtaking), and accidents on crossing start when traffic flow increases (medium flow). In high flow conditions accidents caused by sudden vehicles deceleration (stop and go) or by poor visibility conditions. Therefore it's reasonable to give some responsibility to road environment (e.g. road layout, road width, visibility distances, pavement state, road sign). Accident survey has been developed collecting ACI-ISTAT statistics integrated with ANAS (Italian Roads Organization) traffic data. It must be pointed out that certain roads sections have a higher accident rate. It is possible to define true speed for a single vehicle reasonably for an undivided and two-way country road with low traffic conditions and control of geometric road characteristics. By the knowledge of speed and a direct study on accident reports compiled by Traffic Police, it has been possible to explain high accident rate due to road geometry.