PIARC's report 2017R01EN, "Design Fire Characteristics for Road Tunnels," is the reference document most tunnel fire consultants reach for when deciding how big a fire to design a tunnel's ventilation, drainage and structural protection against. It does not hand over one number. Its crux is a methodology — informed by decades of full-scale tests and real tunnel fires — for choosing a defensible design fire, and a direct statement that a fixed firefighting system changes that number.
Why one fixed number stopped working
Tunnel fire-life safety design historically used a single prescriptive design fire, often capped at 30 MW regardless of vehicle type. Serious European tunnel fires since the late 1990s, followed by full-scale test programmes, showed this was frequently too low. PIARC's report documents that current national practice now ranges from 20 to 300 MW peak HRR, and that countries which rely only on longitudinal ventilation tend to accept higher design fires, because adding jet fans to handle a bigger fire is comparatively cheap — a transversely-ventilated tunnel scaling up its extraction rate for the same fire is a far more expensive proposition.
What actual fire sizes look like, by vehicle
| Vehicle type | Peak HRR (typical range) |
|---|---|
| Passenger car | 5–10 MW |
| Light-duty vehicle | 15 MW |
| Coach / bus | 20 MW |
| HGV up to 25 tonnes | 30–50 MW |
| HGV, 25–50 tonnes | 70–150 MW |
| Petrol tanker | 200–300 MW |
These ranges come from full-scale tests (EUREKA 499, Runehamar, the Memorial Tunnel programme among them) and are broadly consistent with the design-fire values different countries have adopted. The report is explicit that these are ranges, not guarantees: real tunnel fires that spread from the initial vehicle to adjacent ones — which fire services in several documented incidents were unable to control — produced HRRs and durations well outside any single-vehicle assumption.
The shape of a design fire
Full-scale tests show a consistent pattern PIARC formalises as an idealised curve with four phases: an incubation period where the fire smoulders with little heat output, a growth phase where HRR rises rapidly, a steady-state period at peak HRR, and a decay phase as the fire burns out. Across the tests reviewed, incubation typically lasts 3–8 minutes and growth a further 4–15 minutes — meaning a fire is often already near its peak within 20 minutes of ignition, and total fire duration in these tests usually runs 30–40 minutes, occasionally extending toward 2 hours.
That timing matters operationally: the report notes that fire service arrival at real tunnel fires has typically taken 15 minutes or more — which coincides closely with the time a fire takes to reach peak HRR in the tests. In other words, by the time firefighters typically arrive, the fire has often already grown to close to its worst size. The report's conclusion is blunt about what that means for design: the design fire should not be reduced on the assumption that the fire service will arrive before it matters — ventilation and any fixed suppression have to carry that early period on their own.
Choosing a design fire: prescriptive or performance-based
PIARC lays out two routes. A prescriptive approach applies a code-given design fire directly — simple, but historically conservative in ways that don't always match a specific tunnel's actual risk. A performance-based approach starts from the tunnel's own traffic type, geometry and location, sets explicit safety objectives (user evacuation, safe fire-fighting access, asset protection), and works through a seven-step process — hazard estimation, objective-setting, design, risk analysis, acceptability check, and optional optimisation — to justify a design fire specific to that tunnel, rather than borrowing a nationwide default.
Either way, the report is candid that a single number can't capture the full picture: it recommends structural protection sometimes use a different design fire than the ventilation system, since the two serve different purposes (asset protection versus a tenable evacuation environment).
The line that matters most for FFFS specification
Here is PIARC's own statement, from the report's conclusions, on what a fixed firefighting system changes: "if a fixed fire suppression system is installed then it might be argued that the design HRR based on a prescriptive code may not be reached, and so a lower value taken for the design." And further: "the role of a fixed fire fighting system in reducing the design fire size is clear."
That is PIARC's own guidance document saying an FFFS is one of the legitimate inputs into choosing a smaller design fire — not a marketing claim from any FFFS manufacturer, Mistelix included. It also does not hand out a specific reduction factor: the report leaves that to the tunnel's own performance-based assessment, built on the FFFS's documented, tested performance. Australia's own national practice, cited in the report's country table, makes this link explicit in its standard — 50 MW, "with FFFS (deluge system), for ventilation only."
What the real fires add to the picture
The report reviews documented tunnel fires as a reality check on the numbers: Mont Blanc and Gotthard reached roughly 100 MW; the Viamala tunnel fire stayed near a modest 30 MW but still took ten hours to bring under control; the Nihonzaka fire spread to multiple vehicles and became very difficult to control, as did Mont Blanc. A tanker fire at the Caldecott tunnel, against a common 300 MW tanker design assumption, was dealt with in under three hours — prompting the report to note that solid-fuel fires which spread between vehicles may in practice be more severe than a single tanker incident, because drainage limits how a liquid pool fire develops. The report also flags a practical firefighting limit: heat radiation from fires above roughly 50–75 MW can prevent a fire brigade from approaching closely enough to fight it directly, sometimes even lower, around 20–30 MW.
What this means for a tunnel project in India
India does not yet have a single dedicated design-fire mandate for road tunnels; most projects work through the concessionaire's own fire safety design, referencing PIARC's methodology and standards such as NFPA 502 — the same framework covered on our tunnel fire protection page. One thing worth taking from this report into any Indian tunnel project's fire safety brief: get the design fire the ventilation and structural calculations actually assume written down explicitly, rather than left as an unstated assumption. Our own status against every standard we design to is published, honestly, on the standards and approvals page. For how this report's guidance interacts with the SOLIT large-scale water mist tests, see PIARC Design Fires and the SOLIT Water Mist Tests.
Source: World Road Association (PIARC), Technical Committee 3.3 Road Tunnels Operations, 2017R01EN, "Design Fire Characteristics for Road Tunnels" (2016).