The whole point of a dedicated network comes down to one word: momentum. A bike trip where you almost never stop. On the surface, every intersection imposes a light or a stop — you lose speed, time, and the sheer pleasure of riding. Underground, in three dimensions, we can do what highways do: separate the flows onto different levels. Two tunnels that cross never meet. And when you really do need to change axis, a give-way roundabout lets you do it without ever putting a foot down.
An interchange therefore combines two simple ideas, borrowed from the best of road design and adapted to the bike: the two-level crossing to go straight through, and the roundabout to turn.
1. Straight through: the two-level crossing
When two tunnels have to cross, one dips down a few metres to pass beneath the other — exactly like a highway overpass, but buried. There is no junction, no weaving, no conflict point. The cyclist going straight keeps all their speed; often they don't even notice they just crossed another axis.
On the surface, such a level change demands a bridge: a heavy concrete structure, costly and bulky. Underground it's trivial — digging a tunnel a little deeper costs next to nothing, and the space below is unlimited.
Underground, the third dimension is almost free. Where the surface must choose between a red light, a bulky roundabout or a costly overpass, the tunnel simply dips a few metres. The crossing disappears.
2. Changing direction: the roundabout
That leaves the case where you want to leave your tunnel for another one. Here again, no stopping: you take the green ring. Short ramps connect each tunnel to a one-way circular tunnel. You enter it with a simple give-way, ride the ring to your exit, and join your new axis. No light, no stop, no four-way face-off.
Priority always goes to those already in the ring. The arriving cyclist barely slows, slots into a gap and exits at the right branch. It's the cycling version of the road roundabout — a geometry proven the world over to keep traffic flowing. And because a bike is small and slow compared to a car, gaps are continuous and easy to catch: the ring can stay compact.
A crossing, step by step
- I approach the interchange on my tunnel.
- Going straight? I do nothing — I pass over or under the cross axis, without slowing down.
- Want to turn? I take the ramp onto the green ring and give way to cyclists already engaged.
- I slot into a gap, follow the ring, exit at my branch. I never put a foot down.
3. Why it's almost all upside
⚡ Speed preserved
No stops at crossings: average speed stays high across the whole network. That's exactly where surface paths lose the most time.
🛡️ Safer
No head-on collision, no right-angle crossing: every movement is a low-angle merge, in the same direction. It's the geometry that makes roundabouts safer than intersections.
🔀 No bottleneck
Capacity holds where the axes meet — exactly the weak point of conventional cycling networks, which saturate at intersections.
🔧 Simple and robust
No lights to power, synchronise and maintain, no sensors. Just geometry and a "give way" sign. Nothing that can break down.
🚴 The joy of momentum
The cyclist keeps their pace. That's exactly what makes cycling enjoyable — and what a dedicated underground network can deliver continuously.
📐 Compact
At bike speed, the ring stays small. An underground interchange fits in a far smaller footprint than a highway interchange.
Where two axes meet
| Situation | Surface intersection | Underground interchange |
|---|---|---|
| Going straight | Red light or stop — frequent stopping | Passing to another level — no stopping |
| Turning | Waiting, turning across the opposing flow | One-way ring — give-way, no stopping |
| Conflict points | Right-angle crossings, possible head-on collisions | Only low-angle merges |
| Equipment | Lights, sensors, maintenance, electricity | Geometry + a "give way" sign |
| Average speed | Broken at every intersection | Continuous, preserved |
A network where you (almost) never stop.
The two-level crossing settles "straight through"; the roundabout settles "turning." Together they remove what slows the bike down in the city — the stops — while making crossings safer. That is the whole point of a dedicated underground network: giving the cyclist back their momentum.
Schematic diagrams meant to illustrate the principle; the exact geometries (ring radius, ramp gradient, level changes) are a matter for detailed engineering.