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Current massive urbanization process concentrates high amount of population and impose an increased demand on transport systems. In this context, transit bus system plays an important role, as the most dynamic and less capital intensive transit option available. At the same time, it is strongly dependant on fossil fuels, predominantly diesel fuel, with its intrinsic polluting and greenhouse (climate change) effects. This has boosted research and investments for alternative and renewable fuels. One solution currently receiving widespread recognition is biogas use in transit bus fleets, as it allows the use of a renewable fuel, made from substrates derived basically from waste and sewage that otherwise would produce methane released to the atmosphere.

From the nineties there was a great interest in the use of compressed natural gas - CNG (predominantly composed of methane) on transit bus fleets around the globe. In a first moment, developed countries (US, EU and Japan) have focused their efforts to address serious urban air pollution problems caused by heavy duty diesel engines - since PM and NOx emissions were initially easier to control from natural gas engines than from conventional diesel engines - and also to offset growing oil imports. As such, for many years, dedicated methane fuelled city buses meeting emission requirements (Euro IV, V and EEV, US Federal and California, and Japan) either in a lean burn or stoichiometric technology, have been offered to the market.

The increasing demand for urban mobility, combined with the constriction of investment capacity of transit authorities and private companies make bus based systems a great option for public transport systems, since they allow the provision of high quality services at a fraction of the costs of rail based systems. In this scenario, Bus Transit System - BTS and Bus Rapid Transit - BRT allow the implementation of transport networks at considerably lower costs than their rail system counterparts. This is specially true to developing-nation cities, that have infrastructure costs as a pre-eminent decision-making factor in technology selection. From an environmental perspective, traction technology and fuel option are decisive to define systems' performance.