Sustainable” is an oft-used expression, though people rarely explain what they mean when they use it. Sustainability can only be defined with respect to the needs of the future generation. Today, we have imperfect knowledge about future supply, demand and technology choices. Thus, in 1898, there was an international conference on horse dung, debating the problem of major cities (London, New York) being buried in copious quantities of horse-dung, produced from horse-drawn hansom cabs, buses and carts. The invention of the automobile was unanticipated then. That being said, sustainability can only be defined in terms of being environment-friendly. As a general proposition, public transport is more environment- friendly than private transport and should be more “efficient” too. After that, quite a bit depends on the context.
Do we mean intra-city?…
