The TransMilenio express bus network in Bogotá is considered a model for many megacities in developing countries: commuters fill up the free busses, and bus lanes and bicycle paths take the place of car-filled streets. They're almost as effective as an underground railway system, but cost only a fraction to operate. CO2 emissions from passenger and goods traffic are rising steeply in developing countries. Colombia is trying out various projects to curb that growth.