This also means the bus driver doesn't have to worry about checking tickets. There aren't guards to check if people are jumping over turnstiles, etc. Moving to 'totally free' eliminates all kinds of overhead.
There is no reason it couldn't scale.
And the question of scaling hides an interesting assumption: what is the purpose of charging fares?
If it is to reduce usage (to thus ensure that the system isn't used past capacity), then why do you want to limit the usage of public transport? If people don't take public, they will take private transport (cars, etc) which have much higher social cost (you can move many more people by bus than by car).
If it is to 'raise revenue to pay for the system', does the same argument apply to use of roads? What is a fair road tax, given that this method of transport has such high social costs?
This ignores two elements:
- congestion differs by time of day, and
- capacity is constrained differently by mode
In London:
- it's cheaper to take a bus than to take an underground train, and
- there are ticket types that are only valid after the morning rush hour
Both of these decrease peak congestion on the transport system.
The differential pricing would not push people to private transport, but might push them to buses (whose capacity can more easily be increased) or to postpone their journey to later in the day.
You know what might push people to private transport? Severe congestion on the underground. If you are not fit and aggressive, then trying to get on a Central Line train in the morning might mean waiting for 2-3 full trains to pass before you can get on one.
Increasing bus capacity might not be easy either. That requires more busses, more staff, more service, more whatever the source of power is for the busses. It all costs ££££.
In theory population size 12x of the population of Luxemburg would make it easier to distribute the cost of free transport. But this glosses over the fact that London is huge and only zone 1 and maybe 2 are a pain to drive. Not everyone in London works in zone 1 / 2.
I wonder if there’s anyone here able to give an answer if free transport in London would be financially realistic.