**5. Conclusions**

It should be stressed that it is likely that most European countries will not be able to replace the traditional fleet of diesel buses with ZEBs by 2050. The analysis shows that most countries will replace the fleet after 2050, around 2077.

As a result of the study based on available European data, the authors isolated two groups of EU countries. The first group consists of four countries for which the fit of the model is appropriate and these countries seem able to achieve the saturation of their fleet by 95% by 2040. For the remaining countries, owing to insu fficient data, the model fit is low and these countries do not seem to be able to replace their bus fleet before 2050.

The fact is that, today, the number of ZEBs in EU urban spaces is rising, despite many technological, organizational, and financial barriers. The future of the bus fleet will depend on which way the technology develops and how it can be financed. The economic situation of countries will also be of grea<sup>t</sup> importance, especially in the face of COVID-19, to which extent ecological priorities can be further financed in the face of the need to save national economies.

**Author Contributions:** Conceptualization, G.C., A.B. and J.J; data curation, G.C., A.B. and J.J.; formal analysis, G.C., A.B. and J.J.; funding acquisition, G.C., A.B. and J.J.; investigation, G.C., A.B. and J.J.; methodology, G.C., A.B. and J.J.; project administration, G.C., A.B. and J.J.; resources, G.C., A.B. and J.J.; software, G.C., A.B. and J.J.; supervision, G.C.; validation, G.C., A.B. and J.J.; visualization, G.C., A.B. and J.J.; writing—original draft, G.C., A.B. and J.J.; writing—review & editing, G.C., A.B. and J.J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was partially supported by the Ministry of Science and Higher Education (MNiSW, Poland) core funding for statutory R&D activities.

**Conflicts of Interest:** The authors declare no conflict of interest.
