Heavy goods vehicle (HGV) platooning — trucks driving in coordinated, closely-spaced groups using vehicle-to-vehicle communication — is moving from trial to deployment in Europe, and tunnel operators are only beginning to work out what it means for fire safety. A 2023 PIARC World Road Congress paper by Mahdmina, Mongeot, Mos and Schmidt puts a number on the question that matters most for fixed firefighting system (FFFS) design: does a platoon's tight vehicle spacing make an FFFS less effective, and does droplet size change the answer?
What HGV platooning is, and why tunnels care
Platooning uses vehicle-to-vehicle and vehicle-to-infrastructure (V2V/V2I) communication so following trucks can drive at short, fixed distances behind a lead vehicle — closer than a human driver would maintain, coordinated electronically rather than by eye. It has been developed and trialled under the EU Horizon 2020 ENSEMBLE project, which demonstrated multi-brand truck platoons of up to seven vehicles under real traffic conditions across national borders, aiming at fuel savings, better road-space use and improved traffic flow.
Tunnels are a specific concern because platoons concentrate more HGVs, and more total fire load, into a confined space over a shorter stretch of road than the same trucks travelling individually would. The paper proposes that tunnels use I2V communication to learn a platoon's configuration (vehicle count, speed, spacing) before it arrives, and negotiate permitted speed and inter-vehicle distance based on the tunnel's own risk assessment — closer to how air traffic control negotiates with aircraft than to how a road normally interacts with traffic.
The fire risks platooning adds
The paper's risk assessment identifies two platooning-specific fire risks for tunnels, on top of the fire risks that already exist for any HGV:
- A higher total calorific load in the tunnel at a given time, simply because more HGVs are travelling closer together through the same stretch of tunnel.
- A higher risk of fire jumping from one HGV to the next, if the platoon is forced to stop in the tunnel with vehicles still close together — the paper notes this risk is rated "medium" probability, with mitigation resting on ventilation, detection, FFFS, and increasing inter-vehicle distance the moment a platoon is stationary.
The paper also notes that the tunnel's ventilation strategy changes the outcome: in longitudinally-ventilated tunnels, smoke and fire are pushed in the direction of travel, and a platoon fire may not grow much larger than a single-vehicle fire, just last longer without intervention. In transversely-ventilated tunnels — more common where traffic runs both directions — fire can spread faster between platoon vehicles and produce a larger heat release rate, because the longitudinal airflow that would otherwise carry it away is deliberately kept weak.
The finding that matters for FFFS design: droplet size
Here is the paper's central point for anyone specifying a tunnel FFFS: a fixed firefighting system would not be made less efficient by platoon spacing of roughly 5 metres or more — droplets still reach the fire and act as radiation barriers or absorb heat regardless of how close the next vehicle is. The paper then adds a specific caveat: "Smaller intervals could be problematic, especially for large-droplet systems (as opposed to 'water mist' systems)."
That is a direct, third-party statement that droplet size is a real variable in platooning-scenario tunnel fire safety — not just an efficiency question, but a question of whether the system keeps working as vehicle spacing tightens. It lines up with the physics we cover in High-Pressure vs Low-Pressure Water Mist and Water Mist vs Deluge in Tunnels: coarse, large-droplet discharge is more easily obstructed by whatever is directly in its path, while a finer atomised mist diffuses around obstructions more readily. We did not run this comparison — the PIARC congress paper did, in the specific context of HGV platooning — and we think it is worth reading in full rather than taking our summary as the last word.
The paper's other proposed mitigations for platoon-specific tunnel fire risk include increasing inter-vehicle distance further whenever a platoon is stopped, ordering trucks within a platoon by cargo risk (placing higher-risk cargo at the ends or separating it with lower-risk "buffer" vehicles), and using V2I communication to stop a platoon before it enters a tunnel that already has an incident.
What this means for Indian tunnel projects today
Connected and autonomous HGV platooning is not yet operating on Indian highways at any scale, but NHAI-class tunnel projects are being designed with multi-decade horizons, and European platooning trials are a reasonable early signal of what tunnel FFFS specifications may eventually need to answer. The takeaway from this research: droplet size (Dv0.9) is a real design variable, not just a specification detail — it is exactly what decides whether an FFFS holds up as vehicles get closer together. Our own status against every standard we design to is published, honestly, on the standards and approvals page.
Source: Mahdmina, A. (4way consulting), Mongeot, H. & Mos, A. (Centre d'Études des Tunnels, France), Schmidt, F. (Université Gustave Eiffel) — "Operational Safety Considerations for HGV Platooning in Tunnels," presented at PIARC's Technical Session 4.4, Road Tunnel Operations and Safety, winner of the PIARC UK Prize Competition 2023 (Road Safety category). The paper itself cites PIARC (2016), Fixed Fire-Fighting Systems in Road Tunnels: Current Practices and Recommendations, Report 2016R03 as its basis for FFFS effectiveness.