PIARC's design-fire guidance and the SOLIT tunnel tests are two of the most cited pieces of third-party research behind tunnel fixed firefighting system (FFFS) decisions worldwide — and they answer two different questions. PIARC's report tells a design team how big a fire to design for. SOLIT's tests measured what one specific type of water-based system did to a fire of roughly that size, in a real tunnel, at full scale.
What PIARC's design fire report says
PIARC — the World Road Association — publishes report 2017R01EN, "Design Fire Characteristics for Road Tunnels" (2016). A design fire is defined by three numbers: peak heat release rate (HRR), how fast it grows, and how long it burns. Those numbers drive the sizing of ventilation, drainage, structural fire protection and any FFFS.
The report's central point is that a single prescriptive number no longer reflects practice: international design fires now range from 20 to 300 MW peak HRR, far above the 30 MW figure many older guides used as a default. Rather than picking one number for every tunnel, PIARC recommends a performance-based selection that weighs the traffic permitted (cars only, or HGVs and tankers), the ventilation strategy, the tunnel's geometry, and — explicitly — whether a fire mitigation system is present and what it is capable of. For the full methodology, vehicle-type HRR tables and PIARC's own words on how an FFFS changes the design fire, see PIARC's Design Fire Guideline for Road Tunnels.
That last point matters for FFFS suppliers and buyers alike: a tunnel with a documented, evidenced FFFS can sometimes justify a lower design fire in the ventilation and structural design than an identical tunnel with none. It only works if "documented, evidenced" is true — a supplier's marketing claim is not evidence a fire engineer can put in a design report.
What the SOLIT tests measured
SOLIT and SOLIT2 were EU-backed research programmes that ran large-scale fire tests at the Runehamar tunnel in Norway, in 2013 and again in 2016. The fire load was standardised: 420 wood pallets, sized to represent an HGV-class tunnel fire, with a further stack of 21 pallets placed 5 metres away to test fire spread. A deluge-type water-based system, discharging at roughly 10 mm/min, was activated by a ceiling thermocouple set to trigger at 141°C.
The measured results: heat release rate with the system active came down to roughly 20–45 MW, against about 100 MW for an equivalent free-burning fire (and 75 MW in one run where activation failed). Fire spread to the target 5 metres away was prevented. Researchers also found suppression performance was sensitive to working pressure and droplet size, and to how early the system activated relative to smoke-layer buildup — findings that line up with why droplet size and pressure class are treated as first-order design variables across the water mist industry, not just a Mistelix concern.
Why this isn't a validation of high-pressure fine mist specifically
It would be easy — and dishonest — to point at SOLIT's numbers and imply they describe any water-based tunnel system, including Mistelix's. They don't. SOLIT's system was a large-droplet, low-pressure, deluge-class design at a fixed 10 mm/min application rate. Mistelix's high-pressure systems atomise water to a Dv0.9 under 200 microns at 100–140 bar — a different droplet-size regime with a different balance of cooling, vapour expansion and radiant-heat attenuation, covered in detail in High-Pressure vs Low-Pressure Water Mist and Water Mist vs Deluge in Tunnels.
SOLIT is real, credible, peer-reviewed evidence that the category — water-based FFFS — measurably reduces HRR and can stop fire spread in a full-scale tunnel fire. That is genuinely useful context for any tunnel project team. It is not, and should not be read as, a test certificate for a specific manufacturer's specific system. Our own status against every standard we design to — stated using four words we never blur together — is published on the standards and approvals page.
How Mistelix's tunnel design responds to this research
Mistelix designs tunnel protection zones at 25–30 metres, with the fire zone plus one guard zone on each side activating together, a design pattern consistent with PIARC's framing that mitigation capability is one input into the tunnel's overall fire-safety design — not a substitute for getting the fire-size assumptions right in the first place.
One practical point for anyone specifying an FFFS for an Indian tunnel project: ask what design fire the ventilation and structural calculations already assume. If a fire consultant has reduced the design HRR because an FFFS is assumed present, that assumption should be written into the design report, not left implicit — PIARC's own guidance is explicit that this only holds when the mitigation system's performance is documented.