## 1 Introduction
The ongoing more relevant global commitment is the 2030 Agenda for Sustainable Development [59], which includes two targets, 11.2 and 11.6, underlining the effects of urban transport on sustainability. The target 11.2 is related to \"affordable and sustainable transport systems\", states that \"by 2030, provide access to safe, affordable, accessible and sustainable transport systems for all, improving road safety, notably by expanding public transport, with special attention to the needs of those in vulnerable situations, women, children, persons with disabilities and older persons, meanwhile the target 11.6, that refers to \"reduce the environmental impact of cities\", states that \"by 2030, reduce the adverse per capita environmental impact of cities, including by paying special attention to air quality and municipal and other waste management\". Achieving the targets set in international agreements, such as the Paris Agreement [60] and the Kyoto Protocol [61], as well as respecting the air quality recommendations of international bodies, is not only a moral commitment of European cities, but also from a practical point of view it is necessary in order to avoid sanctions imposed by the European Commission for non-compliance with World Health Organization and European Commission recommendations. European Commission request the member states to comply with World Health Organization air quality requirements [62], taking an active role on monitoring and forcing Member states to reduce local levels of air pollution to keep EU legislation on ambient air quality [63]. In case pollutant particles exceed the legal threshold, and no effective measures to reduce air pollution are implemented, then European Commission refer countries to the Court of Justice of the EU, as it happened to Bulgaria and Spain on 2019 [64]. Public transport is a very common means of transport in Spain, although its use varies considerably depending on the urban form. Specifically, Madrid residents use public transport 34% of journeys made in the city, with an average annual use per citizen of 244 times a year [65]. After the aforementioned sanctions, the city council of Madrid has made a firm commitment to alternative fuels in compliance with the Euro V European regulations on emission levels of air pollutants, resulting on a seventy-two percent green public transport buses fleet [66]. Green fleet is a broad concept, which includes vehicles propelled by different types of fuels and different engine technologies. In the case of Madrid its 72% green fleet is made up 42% of natural gas buses, 29% of Euro V diesel buses, and 1% of electric buses. Nevertheless, not all green fleet buses are equally effective in terms of air pollution. Hence, the aim of this research is to compare, under a ceteris paribus condition, the main fuel alternative technologies available in the urban public buses market, that is diesel, diesel hybrid, CNG, plug-in electric, and induction electric, aiming to identify which one is the most efficient in terms of air pollution and therefore environmentally suitable for use in public road transport systems. For that purpose this study proposes a novel and consistent data envelopment analysis, which objective is to identify which Madrid urban public transport vehicle is the most efficient in terms of air pollution. Therefore, this study raises the following research question:  
* Research Question: Which type of alternative fuel technology for urban public buses is the most efficient in terms of air pollution? The result of previous research question can be useful and unambiguous evidence to support public administrations decisions regarding sustainable urban transport policies. The rest of this paper is organized as follows. Section 2 presents the data source and methodological description. This section explains the data envelopment analysis model and formulas. Section 3 includes the model results and discussion.