AbstractIn terms of both construction and operation, tunnels are the most energy demanding of road infrastructure assets. The construction of a road tunnel is energy intensive due to the volume of excavation required and the energy embodied in the materials that form the structure of the tunnel. In operating a tunnel, energy is consumed to provide adequate lighting, signing and ventilation for drivers, to maintain drainage systems and to deal with emergency situations. The European Directive on the Safe Operation of Road Tunnels requires the installation of engineering and safety systems to maintain the safe operation of the tunnel. As the operational lifetime of a tunnel is typically in excess of 100 years, energy usage through operation can soon outweigh that due to construction and therefore, the former provides the focus for this project. The Realistic Energy Efficient Tunnel Solutions (REETS) project aims to enhance the energy efficiency of road tunnel operation through the promotion and implementation of appropriate technologies.REETS commenced with a wide-ranging review of energy-efficient technologies. This covered technologies developed for tunnels, which could bring benefit through wider adoption, and those used for other applications that have proved effective, and could be used in tunnels. Technologies were considered for each of the main services that together maintain the functionality of a tunnel including: lighting, ventilation, drainage, and temperature regulation, with emphasis placed on the more energy demanding services. The review also covered technologies that can reduce energy consumption, by addressing energy provision and storage, and vehicle-oriented technologies that can improve safety and traffic flow. The various technologies might be implemented during the construction of the tunnel or be retrofitted later in the life cycle. The minimum criteria for technology were to (i) maintain or enhance the current levels of safety and comfort experienced by drivers using tunnels on national road networks, and (ii) meet or challenge existing standards for constructing and operating road tunnels.In the next stage of the REETS project, the most promising technologies will be evaluated through a quantitative assessment of carbon emissions and cost of implementation. The final stage will focus on facilitating the adoption of the most promising technologies through ‘hypothetical deployment’ case studies to identify barriers to the adoption of technologies and appropriate reconciliatory measures.