fires start at an HGV. One freely burning HGV carrying ordinary combustibles easily reaches heat release rates (HRR) of the order of 100 - 200 MW, which then spreads the fire to catastrophic levels. Tunnel safety authorities often try to quantify performance requirements for fixed fire protection systems by asking whether the system can suppress a 100 MW fire, or a 200 MW fire. This, as such, is a meaningless question as there is no generic 100 MW or 200 MW fire but all fires are different and depend at least on the actual fuel, the way the fuel is loaded, the tunnel dimensions, and the ventilation conditions. Also, the intent of any water-based fire suppression system in any application is to start the discharge before the fire is very large and prevent it from ever reaching the potential value. Therefore, the question should rather be whether the system can suppress or control a potentially 100 MW fire, or a 200 MW fire. Four different full-scale fire test programs with HI-FOG water mist systems will be analyzed primarily with respect to the relevance of the fuel packages for the intended tunnel application. The results will be evaluated and recommendations for acceptable performance requirements will be provided.