Add to ORKGA parametric study for evaluating the impact due to vehicles transporting dangerous goods on the risk level in road tunnels is presented. Unidirectional tunnels having characteristics in compliance with the European Directive 2004/54/EC were more especially investigated. Different combinations of tunnel length (L), annual average daily traffic (AADT), percentage both of heavy goods vehicles (HGVs) and dangerous goods vehicles (DGVs) were considered. The results, expressed in terms of social risk (as F/N curves) and expected value EV, show that the risk level is positively associated with: L, AADT per lane, percentages of HGVs and DGVs. Furthermore, certain F/N curves were found to be beyond the threshold of intolerable risk for higher perce...