Alternative propulsion technologies—including battery-electric vehicles (BEVs), hydrogen fuel cell vehicles, and other gaseous energy carriers—are becoming increasingly prevalent in global vehicle fleets. While these vehicles currently represent a small proportion of total road traffic, the combined effects of government policy and rapid technological advances are expected to accelerate their adoption in the coming years. In certain regions, such as airport zones or urban centres, higher concentrations of alternatively fuelled vehicles may be observed sooner than on the open road. Thanks to the penetration in the road context, and taking into account the nature of tunnel safety, particularly those associated with fire and explosions, it is expected to evolve. This necessitates detailed consideration of the risks posed by significant incidents involving such vehicles, including the evaluation of incident consequences with particular attention to fire characteristics, toxic emissions, explosions, and their impact on tunnel users and emergency intervention strategies.

PIARC Technical Committee 4.4 has prepared this literature review to present the current state of knowledge regarding the impact of new energy carriers on tunnel safety. Recognizing that the number of real-world incidents involving vehicles propelled by new energy carriers remains limited, and that statistically significant conclusions are not yet possible, this review collates available information from ongoing and completed research programs and scientific publications, offering insights into the most relevant findings for tunnel safety stakeholders. The report provides a comprehensive overview of recent research activities, highlights key findings, and identifies emerging trends and knowledge gaps.

The scope of the review is limited to those alternative propulsion systems and energy carriers considered most relevant to current and near-future road tunnel operations, with a particular focus on battery-electric vehicles (PHEV, HEV, …) and hydrogen or other gas-driven vehicles. For each propulsion type, the technological landscape, system architectures, and operational characteristics are presented, followed by a systematic overview of the surveyed literature. Extended summaries of selected studies offer deeper insight into methodologies, findings, and implications for tunnel operations and emergency response.

In this sense, the report continues the review work performed in the previous PIARC cycle, updating stakeholders on the evolving landscape of tunnel safety in the context of alternative propulsion technologies.