Following up on Simon Norton comments here of 2016/08/07
“Public Space” is generally mostly free. This includes footpaths, parks, and town squares. If one advocates charging for public transport, it would seem most of the same arguments would apply to public space. And yet few would actually support such a position, principally on grounds of equity.
There are also ways to make public transport funded on a sustainable basis while making it free to the user. There are cities which utilize a parking levy to completely cover all public transport costs.
Such modal funding transfers also carry a great deal of appropriateness when one considers the actual societal costs brought by private motor vehicle use and the actual societal benefits of collective transport.