# Interaction between Rockfalls and Vehicles Studied for Speed Limit Zonation along Mountainous Roads
## 5 Discussion  
The risk zonation methodology presented herein, along with the results achieved, is a starting point for developing some aspects of a discussion. The first aspect concerns the rating system that was applied, which proved to be a suitable and relatively quick procedure for categorizing the risk along the studied road, even from the perspective of the practical use of the results. In fact, the availability of final scores that characterized specific road sub-segments allowed the drafting of thematic maps, which represent a key tool for territorial and risk management, providing a quick and intuitive overview on the variation of risk. The usefulness of such a rating system is in its adherence to reality in terms of the parameters considered for the risk rating. In fact, on the one hand, it takes into account the geological aspect and the engineering features of the infrastructure that is at risk, which are constant at each selected road segment in terms of the geomechanical quality of rock masses, expected mobilized rock volumes, and road geometry. On the other hand, the RHRS method considers the dynamic presence of other elements that are at risk, which are represented by passing vehicles; these are, however, characterized by a certain variability, especially in terms of speed. This aspect is crucial for mountainous roads, where the presence of narrow bends strongly conditions the road visibility. In fact, rockfall risk does not arise only from the spatial and temporal correspondence between a falling rock and a passing vehicle, but also from the possibility that a vehicle could crash into a previously fallen boulder lying on a road segment. In this specific case, the driver should have at their disposal enough time to make a decision to either stop the vehicle or avoid collision. The DSD parameter allows the consideration of this important aspect in the rating system; since it is specifically related to vehicle speed, the simulation of the five different scenarios presented herein sheds light on key risk variations. In fact, although there is usually a posted speed limit along roads, this does not necessarily coincide with the actual speed of traffic units, thus conditioning the computation of the DSD and AVR parameters. The first one was proportional to the vehicle speed, and in this case, it showed an increase of 470% from the 10 to 30 km/h scenarios and of 228% from the 30 to 50 km/h ones, thus suggesting a relevant incidence in the risk computation procedure. Consequently, the P-DSD scores, which were calculated according to the RHRS procedure, showed great variation as a function of the vehicle speed considered (Figure 7a). This proved, indeed, the variable that significantly affects the computation of the final RHRS score. For example, the bend sub-segments 6 and 12 fell within the intermediate risk class based on the 10 to 30 km/h scenarios and within the highest risk class according to the 40 and 50 km/h ones due to an increase in the P-DSD score (Figure 7a). Similarly, sub-segment 26 was in the lowest risk class according to the 30 km/h scenario, while it shifted upwards to the medium risk class when considering a vehicle speed of 50 km/h, with a relevant increase in the P-DSD score (Figure 7a). On the contrary, road segments that offered good visibility were subject to slight score variations, which may not have involved a class shift, e.g., sub-segments 40 and 16 (Figure 7a). With reference to the AVR, which relies on the spatial probability of occurrence of a vehicle in the rockfall hazard zone, it is inversely proportional to the speed, with a 66% reduction from the 10 to 30 km/h scenarios, a 25% reduction from the 30 to 40 km/h scenario, and a 20% reduction from the 30 to 50 km/h scenarios (Figure 7b). Nevertheless, these variations had small repercussions on the AVR-related score, whose average maximum final score decreased; from 10 to 50 km/h, it was 0.3. The AVR also depends on the daily traffic volume, which was considered herein as a constant value.