Analysis of the operative safety conditions in the road curvilinear elements

Road Safety Audits strongly depend on the operative conditions roads are exposed to, in terms of both influence of local environmental parameters (light, visibility, troubling elements, etc.) and vehicle dynamics related to driver behaviour. This is particularly important in the case of circular curves, where the influence of the “human” factor becomes highly determinant in the road safety assessment: both the choice of trajectory and transit speed along the geometric element unavoidably rebound on vehicle behaviour and, with this, on the tyre-road friction characteristics. These aspects are reflected in Road Safety Audit procedures, mainly through the integration of the “human” factor in road safety assessment. The border-line conditions to be used in safety audits must reflect the real road geometry covered by drivers (free geometry or free trajectories), the transit speed accepted by drivers (in terms of S85) and the real dynamic conditions undercurrent between tyres and road pavement. Road Safety Audits conducted considering the “human” factor will show a different safety degree between the operative conditions and the theoretical ones forecast for the geometric element, especially in terms of required skid resistance. The adoption of higher transit speeds than the theoretical ones, united with the choice of trajectories not coinciding with the road axis, unavoidably involves higher friction properties than those foreseen by designers. In particular, comparing required friction and offered friction in both cases (operative and theoretical), a different safety coefficient can be found for the considered geometric element. This paper discusses the importance of the interaction among the three factors “driver-vehicle-road” in Road Safety Audits in the case of circular curves.