## 4 Conclusions and Recommendations
### Research Limitations and Future Lines of Research  
The main limitation of our model is to only incorporate the traditional variables to assess the air pollution caused by vehicles, such as the NOx (kg), PM (kg) and CO2 (kg) emissions for each engine type. Following mainstream theory, some other relevant variables have been ignored.  
The buses pollution is not only atmospheric, as they also produce acoustic pollution. Additionally, the production of electricity to charge electric vehicles does pollute, unless it is totally obtained from renewable energies, such as solar or wind.  
Aforementioned limitations are an inspiration of future lines of research, which could incorporate new pollutant variables in order to obtain a much more accurate analysis.  
In addition, from a methodological perspective, the following ideas can be taken into consideration in future research: (1) to use alternative methods to calculate external costs, such as the classical one; (2) to use non-DEA approaches to calculate efficiency, such as efficiency calculation based on the weights from the Delphi method, or efficiency calculation based on using the weights obtained from the PCA, which could be a good comparison points with DEA results; (3) to consider additional lines, ideally in diverse different areas of Madrid, to create a larger sample for a DEA windows analysis, which could be more representative of the city; (4) to consider three different approaches of DEA without, and with the two proposed weight restrictions.  
Another avenue for future research could be related to the feasibility of implementing new alternatives depending on the degree of urban development and the city form. There is agreement in the scientific community that the electric public road transport is the benchmark to reach for mobility and travel behavior in cities.  
In the case of buses, there are different possibilities when choosing an electric motor. According to the research results, the most environmentally efficient is the use of plug-in electric motors. Nevertheless, the urban form, and especially its level of urban development, also determines the choice of the environmentally efficient bus from the point of view of economic efficiency and urban viability, i.e., the urban form also determines the choice of the environmentally efficient bus.  
As a consequence, it might seem that the choice of type of electric motor (plug-in or induction) depends primarily on the urban form. Those cities that are already urbanized require modifications to existing infrastructure that would not make induction motors viable. Non-urbanized areas or new planned cities could be the ideal location for such induction motors.  
An interesting potential study is the analysis of the viability of these new type of technology in newly designed cities, such as the new capital of Indonesia or the imminent new capital of Equatorial Guinea. The contrast between theoretical studies on the efficiency and feasibility of electric buses and practical cases of their implementation in new cities is a line of future research with great potential.  
Conceptualization, A.R.-A., L.R.G. and M.A.D.V.O.; Data Curation, L.R.G.; Formal Analysis, M.A.D.V.O.; Investigation, A.R.-A., L.R.G. and M.A.D.V.O.; Methodology, M.A.D.V.O.; Project Administration, A.R.-A., Resources, L.R.G.; Software, M.A.D.V.O.; Supervision, A.R.-A.; Validation; M.A.D.V.O.; Visualization, A.R.-A.; Writing--Original Draft Preparation A.R.-A.; Writing--Review and Editing, A.R.-A. and M.A.D.V.O. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The data presented in this study are available in the tables and references included in this paper.  
The authors declare no conflict of interest.