# Risk Assessment of Fixed Obstacles along Roads
## 3 Research and General Definition of Input Parameters  
The risk rating of fixed obstacles is based on a general consideration of the probability of hitting the object and an estimate of how severe the consequences of the accident will be. The calculation of the resulting risk level is based on the following relation [19]:  
\[R=Z.P, \tag{1}\]  
**R**:... **the resulting riskiness of the fixed obstacle** \([-]\);
**Z**:... **the severity of the consequences of the traffic accident** \([-]\);
**P**:... **the probability of a traffic accident** \([-]\).  
Several different factors enter into the calculation of the resulting riskiness of a fixed obstacle along the road, which are assumed to influence the severity of the consequences of the traffic accident. For each parameter, it is important to define the specific values that the parameters can take on. These parameters and their corresponding sub-parameters, which are mentioned in this part of the text, were repeatedly consulted with road safety auditors at the beginning of the work. At the same time, the knowledge base has also been supplemented with knowledge from established and widely used road safety manuals [9; 10; 11; 12; 13; 14]. As a result of this communication, there was overwhelming agreement on the assumption that the elements mentioned below have the most significant influence on the potential worsening of the consequences of a traffic accident due to a vehicle running off the road.  
Figure 1: The working process of the research.  
The upper limit of the height difference was determined on the basis of the main Czech standard related to road design [6]. The standard shows that if the adjacent slope is correctly graded, objects higher than 1.5 m above the road level may not be considered as fixed obstacles. Parameter \"a\" represents the traffic lane width, \"c\" represents the hard shoulder width, \"e\" represents the shoulder width, and parameter \"h\" represents the height difference between the traffic lane and the lowest part of the fixed obstacle.  
The third criterion is the relative position of the fixed obstacle and the road ditch. The effect of the road ditch on the deceleration of the crashing vehicle that has left the roadway is analyzed in determining the risk. From the conclusions of one study [20], which dealt with the riskiness of exits or separate exits from the point of view of vehicle crashes, it can be concluded that not only the depth of the ditch but also the gradient of the external slope of the ditch has a major influence on the deceleration of the vehicle. These model examples are again based on the long-term professional experience of the author's team. The following cases (Figures 9-11) are distinguished for vehicle movement in a road ditch:  
Figure 11: Example of a road ditch that significantly slows down a vehicle running off the road.  
Figure 9: Example of a road ditch that does not slow down a vehicle running off the road.  
The fourth criterion describes the relative position of the object in relation to the arrival of the crashing vehicle. For this situation, two states are distinguished, namely whether the fixed obstacle is on the right or left side of the road. The probability of the vehicle leaving the roadway on the right and left of the direction of travel of the vehicle was found to be almost identical. Specifically, the probability distribution of collision occurrence is 55% in favor of objects located on the right, i.e., closer in position than objects located on the left from the direction of vehicle travel [20]. This is applicable when right-hand traffic flow is considered. In the case of left-hand traffic flow, the probability distribution of a collision would be reversed--55% in favor of objects located on the left, i.e., closer to the position than objects located on the right of the direction of vehicle movement.