Support public transport, biking and walking
Right now, the extension always assumes that Google Maps suggested routes are for cars. Therefore, it gives wrong estimations when the user has actually asked for biking, walking or public transport directions. This should be fixed. At least, no estimation should be shown for these other modes of transportation (or it should be made clear that the estimation refers to prevented emissions in the case of walking or biking). Ideally, we should also provide good estimations for public transportation. However, the CO2 emissions per passenger may vary greatly depending on the city, on the average number of passengers, on the time of the day, on the type of public transportation (bus, electric bus, tram, metro,...) and so on... Therefore, such parameters should be configurable by the user in the options page. Nevertheless, we should provide good default values for the parameters. For that, we need good sources of information about emissions from busses, trams, metro, train. Some cities (e.g. Helsinki) seem to have public data available about this. We could use the same data (possibly an average of the available data from many cities) for all cities, to simplify the estimation.