# A Novel Methodology for Planning Urban Road Safety Interventions
## 3 Novel Methodology
### Cost-Benefit Analysis for Road Safety Interventions
#### 3.4.1 Proactive Estimation of Fatalities and Serious Injuries  
The proactive cost-benefit analysis estimates reductions in fatalities and serious injuries (FSIs) resulting from road safety interventions. Moreover, it quantifies the associated social cost savings, including human, healthcare, productivity, administrative, and material costs. It considers implementation costs to ensure financial sustainability.  
FSIs can be estimated through iRAP's SRS to predict the annual number of fatalities and serious injuries for four road user categories (drivers, motorcyclists, cyclists, and pedestrians). Calculations incorporate traffic flow (Annual Average Daily Traffic--AADT), crash type risks, and calibration factors tailored to local conditions.  
Calibration factors adapt iRAP's international model to local contexts by integrating historical crash data with proactive risk assessments. Furthermore, it adjusts predictions based on factors like vehicle safety standards and user behavior in specific regions.  
One of the novelties of the proposed model is the estimation of serious injury, which relies on a ratio of serious injuries to fatalities (5.5:1 in Italy, based on historical data) to estimate injury figures. In fact, it uses the Maximum Abbreviated Injury Scale (MAIS 3+) to classify serious injuries.  
The process described above can be applied to different design solutions, using their respective SRS to assess the associated road safety risks. This approach enables the estimation of the annual number of FSIs after implementing various design alternatives.  
For the calculation of the annual number of FSI under the current scenario (Option 0--Do Nothing), the proactive iRAP approach has been chosen. The available Istat dataset provides the number of fatalities for each road segment, but it does not distinguish between minor and serious injuries. Serious injuries are reported only in aggregate for the entire national road network without breakdowns for individual segments. To ensure consistency and comparability of results, the FSI for the \"Do Nothing\" option is calculated using the same methodology as the other design options, based on risk analysis from the SRS. This ensures that the reactive and proactive approaches remain separate in the cost-benefit analysis, providing a fully proactive calculation independent of historical incident data. The calculation of avoidable FSIs is performed by Equation (3).  
\[FSI_{sured}=FSI_{before}-FSI_{after} \tag{3}\]  
where \(FSI_{sured}\) is the estimate of avoidable FSIs per year; \(FSI_{before}\) represents the number of FSIs currently estimated annually; and \(FSI_{after}\) indicates the estimated number of FSIs per year after the implementation of a road improvement intervention.  
Estimating the number of avoidable FSI is a critical step in demonstrating the effectiveness of a proposed road safety solution. However, during the initial decision-making phase, particularly in the technical-economic feasibility study, the choice between different design options must also consider the financial resources available. Therefore, evaluating the benefit of an intervention should go beyond just the number of avoided FSIs and include an economic quantification of the results.