Dottorato in INFRASTRUTTURE E TRASPORTI

Titolo della tesi: The Improvement and Transferability of the European Network-wide Road Safety Assessment Methodology

Speed is a major risk factor in road safety that contributes to the increase in the probability and severity of a crash. However, the safety effects of vehicle operating speed related to the road infrastructure is often given less attention in road safety assessment procedures. In most cases, it is assumed that there is no risk of speeding if the vehicle travels within the speed limit. Nevertheless, several road traffic crashes still occur at below the speed limit. As such, the primary objective of this research is to estimate the safety effects of vehicle operating for two-lane undivided rural highways, and incorporate these effects into the current Network-wide Assessment (NWA) Proactive model. Since statistics have shown that speed contributes to over 30 percent of road fatalities, including speed in the safety assessment would lead to accurate identification of hazardous road locations. The safety effects of speed were obtained by developing a Crash Modification Factor (CMF) for speed using the simplified cross-sectional method. This factor is then converted into reduction factors and incorporated into the new NWA Proactive model. The performance of the proposed model with speed inclusive is then compared to the current model without speed on four different samples of two-lane undivided rural highways within the Lazio Region in Italy. Furthermore, the impact of the proposed model on the overall integration of reactive and proactive assessment results was examined. The second part of this thesis focused on investigating the transferability of the European NWA procedure to typical highways in Sub-Saharan Africa, particularly the Yaounde – Douala highway in Cameroon, which is a Trans-African Highway (TAH) serving as a major corridor for neighboring landlocked countries such as Chad and Central African Republic. The primary objective is to investigate if the European procedure is capable of identifying potential crash locations in this part of the world. The results in the first part of this thesis showed that by incorporating the safety effects of operating speeds into the model, the predictions were closer to the observed crashes than when speed was not considered. This means that the safety effects of speed improved the identification of crash locations. Also, the analysis revealed that this affects the integration results by ranking the road sections safer when speed is not considered. The results for the second part of this thesis revealed that there is a high possibility of implementing the European procedure in identifying the safety performance levels of rural highways in Sub-Saharan Africa. Overall, the research provides a significant contribution to the field of road safety assessment procedures by demonstrating the contribution of operating speeds in safety assessments, as well as the possibility of applying these procedures in other parts of the world. Keywords: Risk assessment, road safety assessment, road safety inspections, road safety evaluations, network screening, transferability.