When you think « tunnel », you picture highway tunnels and their huge fans forever fighting vehicle exhaust and smoke. A bike tunnel is the opposite: no combustion engines, so no exhaust. The only things to manage are the carbon dioxide, heat and humidity given off by the cyclists themselves — the equivalent of a busy gym, not a highway.
How it works
The principle is the proven one of the big road tunnels: jet fans fixed to the crown push the air along the tunnel, like a chain of small gusts of wind. A breeze of barely 1.5 m/s — imperceptible to a cyclist already riding at 22 km/h — is enough to renew the air continuously.
Fresh air enters and stale air leaves through shafts connected to the surface, spaced about two kilometres apart and often built into the stations. Air-quality sensors every 200 metres drive the whole system: at night, when the tunnels are nearly empty, the fans idle; at peak hours, they ramp up. It is this modulation that keeps the electricity bill low.
Air quality, in numbers
For 100,000 users a day, the network holds only about 7,000 cyclists at a time at peak hour — the others are just passing through (a trip lasts ~22 minutes). Spread over 150 km, that is roughly one cyclist every 20 metres. It is this real presence, not the total crowd, that the ventilation has to serve — hence such comfortable numbers.
well below the 1,000 ppm comfort threshold
ambient air — the ground at 8–10 °C does the work
comfortable — drainage against condensation
a soft background, like a conversation
Carbon dioxide is the only real parameter to watch, and the margin is huge: even packed, the tunnel stays far from any risk threshold.
| CO₂ benchmark | Concentration |
|---|---|
| Outdoor air | ~420 ppm |
| Our tunnel at peak hour | ~645 ppm |
| Indoor comfort threshold | 1,000 ppm |
| Health-and-safety limit (8 h exposure) | 5,000 ppm |
Naturally cool, thanks to the ground
Ten metres underground, the rock stays at 8 to 10 °C year round in the Québec region. The tunnel walls act like an immense reservoir of coolness: in summer, the tunnel is naturally air-conditioned while the city swelters; in winter, only the entrances need heating. On the humidity side, the cyclists’ contribution stays modest — the real work is avoiding summer condensation on the cold walls, handled by drainage and treatment of the incoming air.
The real challenge: smoke, not CO₂
Day to day, keeping the air breathable is easy. The system is in fact sized for a rare but serious event: a lithium-battery fire (electric bike or scooter). The power of such a fire stays well below that of a car, but its smoke is dense and toxic.
The countermeasure is proven: the jet fans ramp up to push the smoke to one side only and keep the other breathable, while users evacuate through the emergency exits (every 200–300 m), refuge niches (every 100 m) and smoke-proof lobbies. It is this requirement, not everyday air quality, that sets the power of the fans.
What it costs
≈ $600M to build
Already included in the project’s $11.2B budget. In use, ventilation costs only about $3 to $4.5M per year in electricity — a minor item, thanks to Québec hydroelectricity and fans that idle down at night.
Most of this cost goes to the surface ventilation shafts, the most demanding part of the system; the fans themselves are relatively inexpensive. Building them into the stations shares the excavation and keeps the bill in check.
Download the ventilation analysis (PDF)
Main sources. This page draws on our detailed analysis and the standards it cites: download the ventilation analysis (PDF).