Georges, This is a key issue in the development of public transport in cities with high m/c ownership. I encountered this issue when attempting to forecast patronage and revenue for bidders of Indian Metro concessions: Delhi metro extension to Airport, Mumbai Line 2 and Hyderabad lines. The operating costs of motorcycles are close to those of a metro and they offer door-to-door service; somewhat unsafe and wet in the rain but cheap. I discovered that there were no mc ownership models in the region (but there were some car ownership models) and that there was not much thought on the link between car and mc ownership. We tackled the issue somehow with a joint mc and car ownership model and realised how critical the issue is. Why is it that mcs are abundant in some cities and quite scarce in others, for example in Latin America? This is not just a regulatory issue.