This idea is so simple that I'm surprised it hasn't been done on a large scale (I've researched it).
It is mainly intended for dense cities such as central London and Paris, where distances are fairly short and people don't normally travel by car.
Apply the same principle used for highways to cycle
* no junctions
* no traffic lights
* entry and exit via ramps
* two cycle lanes in each direction to allow overtaking
* Good signposting for easy navigation
* Only bikes are allowed
In a big city, this can be achieved in three ways:
* Elevated cycleways
* Underground cycleways under trunk roads, build in cut-and-cover method
* Street level cycleways with underpasses for pedestrians and cars (for short distances only)
* Unbeatable speed over shorter distances (below 3km), up to about 20 km speed is similar to underground.
* Much cheaper than building new underground lines, but similar capacity and jouney time (the *average* speed of the underground is only 15 km/h - similar to cycling!)
*Safe, traffic free routes protected from the elements would persuade the masses to cycle, justifying the investment
* Low land use
* High capacity per unit cost
Some of the investment costs could be offset by renting out retail space along the cycleways
The network of cycleways needs to be fairly dense, and it needs to cover the whole of the city, to take advantage of network effects