This publication includes the Proceedings of the 11th International Symposium on Tunnel Safety and Security (ISTSS) held in Reykjavik, Iceland, April 9-11, 2025. The Proceedings include 43 papers and 17 posters. They were presented in 16 different sessions, i.e., 3 Keynote sessions, Fire Dynamics 1&2, Fixed Fire Fighting Systems, Alternative Fuel Vehicles, Poster session, Evacuation 1&2, Ventilation, Engineering Case Studies 1&2, Risk 1&2, and Emergency Management.

During fire tests in a Dutch tunnel thirteen tests were done with linear fire detection systems. One fire detection system was a glass fibersystem and the two other systems were line detector systems . The fire size, the place of the fire according to the position of the detectors and the longitudinal ventilation speed were varied. The aim was to determine the time between ignition of the fire and fire detection and to determine the place accuracy. Another four tests were done with an open deluge sprinkler system on a van and as build trucks . The sprinklers provided large water drops . The aim was to get information if a fire could be extinguis hed or not. The sprinkler tests were done in combination with or without longitudinal ventilation to know the effect of the longitudinal ventilation on sprinkler .