Risk-based tunnel design for consequences of road accidents. The role of tunnel lenght

Tunnel extension is an under-analysed variable in road tunnel accidents despite being a dimensioning parameter for the purposes of users’ safety according to Directive 2004/54/EC. Recent studies have shown a correlation between the tunnel length and consequences of accidents. The analysis of fire events which occurred in tunnels indicates that in many cases fires are triggered by road accidents. By analysing the road accidents in Italy, the study aims to assess the relative risk of accidents with serious consequences for different classes of road tunnels. The second objective was to assess, using a vehicle type (or size) approach, the corresponding probability of accidents involving vehicles or trucks and special vehicles resulting in serious consequences (domino effect). We analysed the Italian National Institute of Statistics (Istat) dataset on tunnel accidents which occurred between 2018 and 2020 on Italian public roads, involving at least one vehicle. Of these, we extracted tunnel accidents, classified by tunnel length and estimated the corresponding probability of serious consequences. The analysis identified 1,885 case studies of tunnel accidents that occurred in approximately 265 long tunnels and 450 short tunnels and underpasses. Compared with “controls”, “size” was found to be more than double in long tunnels where the related probability of serious accident consequences exceeded 50% more than those of short tunnels. We found that the related probability associated with serious accident consequences in tunnels over 500 m in length was higher than in short tunnels, except for trucks and special vehicles. Road accidents and research on risk evaluation of the effects associated with long and short tunnels are rare. The study aims to fill these gaps.