**4. Discussion**

A discussion should first refer to the existing research on similar issues, using the same methodology. Although an in-depth review of the literature in the Scopus, Web of Science, and EBSCO databases has not brought the expected results, a number of similar topics were raised within scientific publications. However, it is worth emphasizing that no description of studies alike using the Bass model to describe the diffusion of innovation understood as an increase in the number of ZEBs in the total bus fleet of a given city has been identified. Therefore, the research was conducted in a similar thematic scope, although with the use of different tool or with the same tool, but referring to other issues.

In the study from 2018 conducted by Ma and Zhang [40], the Bass model was used to optimize and predict the number of charging stations for electric vehicles. In order to solve this issue, the researchers used the exhaustion method, regarding minimum cost as the objective function. To finish up their work, they tested the given model using data from a particular Chinese city.

Akbari, Brenna, and Longo [41] adopted similar assumptions when using the Bass model. The authors focused on Milan, Italy. The main purpose of using the model was to calculate how many electric vehicles (EV) will be in 2024, and thus will need charging stations. In further analyses, the aim was to indicate the optimal location of the stations so as to meet the demand generated by customers on the one hand, and on the other hand, to minimize the costs of vehicle charging and management.

What appears to be complementary in the presented approach is the reference of the optimal number of charging stations for electric vehicles to the public transport infrastructure. Thus, the subject of city managemen<sup>t</sup> in the context of creating optimal urban spaces and the optimal use of ZEBs would find a wider application. A similar objection regarding the narrow approach to the subject of electric vehicles can be formulated against the authors of this article. Taking into account the holistic approach to electric vehicles in cities, it seems to be an interesting research direction.

Rogge, van der Hurk, Larsen, and Sauer [42] also looked at the problem of electric vehicles in an interesting way. Similarly to the authors of this article, they analysed public transport in the context of developing the most optimal saturation of the city transport fleet with electric buses. What was different about their approach was that they did not assume that the fleet should be fully electrified, but rather that the fleet should have different proportions of both electric and conventionally powered vehicles. Undoubtedly, this is a beneficial direction for further research with high potential.

The results of the 2018 research by Mohamed, Ferguson, and Kanaroglou [43] may provide some kind of valuable inspiration for the authors to carry out in-depth EU country-by-country analyses. This study identified factors that hinder the implementation of the electric bus in the public transit context as seen from the perspective of Canadian-based service providers.

With regard to modelling the market share of a specific group of electric vehicles, such as the zero-emission buses described in the article, it should be emphasized that there is a clear inconsistency between the actual market share of ZEBs and the feasibility models. This inconsistency shows a clear gap between the theoretical evidence for the positive environmental impact of ZEBs as well as the benefits from electric buses in the public transport fleet, and its practical application. While some

argue that a lack of political support, technological immaturity, and inertia to change are key factors contributing to low participation, others attribute this situation to the sensitivity of technical-economic models related to the operational context. This sensitivity is common ground for all evaluation models of electric buses [44–46]. As a result, this sensitivity increases the uncertainty about the operational benefits of the electric bus in the context of the network, thus limiting market share.

The adoption of the electric bus globally is geographically uneven and limited in scale [47]. Predictions for the development of the ZEB fleet are quite difficult, as replacing the fleet with an electric one faces many obstacles. These obstacles can primarily be divided into two groups dependent on each other. It is a matter of available technologies that affect bus electricity demand and both initial and operational costs that affect the economic efficiency of investment in these solutions. Polish geographers identified the main factors and mechanisms behind the development of low-emission public transport vehicles in Polish cities. They included energy challenges, environmental requirements, governance strategies, and manufacturing capacities [48].

One issue is the high upfront cost of zero-emission buses. The huge costs associated with the investment, both the purchase of a new fleet and the appropriate infrastructure, cause a number of considerations about this technology, as well as its ecological, economic, and organizational effectiveness [49]. A few studies have analyzed the contracting and financing mechanisms that can help accelerate electric bus adoption [47]. The justification for using public funds when purchasing battery electric vehicles is the anticipated reduction of CO2 emissions [50,51]. Using the Bass model, Brito and others investigated how governmental incentives can influence the diffusion of low emission technology in individual transport decisions. They were able to demonstrate how, for example, tax regulations can affect the increase in the adoption of zero-emission (to be precise, electric) technologies by the individual customer market [21]. There are also several other studies of the impact of the economic and social policy on the development of electric technologies in transport based on technology diffusion models [52].

Therefore, one of the most important discussions in the literature on clean buses is the issue of their real impact on reducing greenhouse gases. The environmental benefits of ZEB in cities should be calculated and assessed from two points of view: emissivity and operational harmfulness as well as emissivity, harmful to the environment at the time of energy production. Ultimately, however, the environmental benefits of ZEB will really depend on what sources electricity is obtained from in the country or city. Some research papers propose the application of a life cycle assessment (LCA) [46,53,54], or through a combination of LCA with an economic analysis [55], or through a cost–benefit analysis [56,57]. The methods adopted to evaluate the transport impact of CO2 emissions are rather heterogenic, including different phases of the fuel production and the emission phase. Cavallaro et al. [50] propose a well-to-wheel analysis, including the well-to-tank and tank-to-wheel phases, while Topal and Nakir [58] propose a total cost of ownership calculation model. Total cost of ownership from well-to-wheel has been proposed for the three groups of transportation, namely diesel, CNG (compressed natural gas), and electric buses. The analysis showed that the total costs of ownership for electric buses is greater than for those with diesel and hybrid engines. Nonetheless, the authors conclude that high initial costs and amortization points in electric buses can be caught because of low operating costs. Total costs of ownership are directly influenced by the costs of purchasing technology (buses and necessary infrastructure, that is, charging stations) as well as operating costs.

The total operation costs also depend on several factors. The buses' electricity demand depends on the operating time on one battery charge. Electric buses have a limited driving range and need to be charged during the day [13,59–62], which turns them off for some time. That creates the need for more buses to fulfill the transportation demand than the diesel ones. This goes for battery electric buses and, to a lesser extent, for hydrogen fuel cell buses. Regular diesel buses can drive all day without refueling, while battery electric buses need to recharge after about 200–250 km, depending on circumstances such as the climate and road conditions, and hydrogen fuel cell buses need to refuel after about 200–400 km [63]. This also affects the problem of planning courses. The electric bus scheduling problem requires not only satisfying timetable constraints, but also considering battery range limitation and vehicles recharging plans, including available charging infrastructure [64]. Moreover, for example, May [65] proposed planning bus lines served by ZEB based on local environmental impact assessment. The interdisciplinary approach was applied to evaluate the local environmental relief potential of electric buses in comparison with diesel buses. The issue of timetable planning is connected with the problem of charging station location and charging method solutions [62,66–68].

Bus demand for electricity depends on battery type, driving cycle and style, number of stops, traffic level, elevation profile [69,70], and weather conditions including temperature and humidity [8]. In order to improve the driving style, and thus reduce the bus's energy demand, various technological solutions are proposed, for example, the robotized manual gearbox [56,71,72]. Cost effectiveness also varies depending on the energy storage systems in electric buses [71,73] and the choice of charging technology [74]. Operating costs also depend on the price of the battery and the battery life [75]. Ufert and Bäker [9] propose a model for predicting battery life. According to them, the ability to predict battery life can reduce total operation costs by up to 17%.

In turn, Bakker and Konings [13] argue that the technological barriers for replacing the diesel bus fleet with ZEB are not that grea<sup>t</sup> compared with institutional barriers in individual countries and cities. Veeneman [76] also draws attention to the tender processes that must take place when purchasing a bus fleet owing to the fact that the funds involved in the purchase are public. What is important here is the low quality of tenders, which are based on the lowest price, which ultimately leads to the purchase of low quality products and services. There are also some studies addressing the public's willingness to pay for environmental-friendly buses [12,77,78].

Knowing the multidimensionality of issues related to the ecological and economic efficiency of electric buses, it is difficult to predict to what extent the forecast presented in this article will be implemented. This will probably depend on both, the direction of technology development, which will allow reduction of electricity demand and greater operational efficiency and on the funds available for cities in the future. The latest reports from the technology market inform about a new type of battery. Catl, a Chinese car battery-maker, says it is ready to manufacture a product capable of powering a vehicle for 1.2 million miles (two million kilometers) across the course of a 16-year lifespan [79]. For now, this technology is to be used in cars, but it is probably only a matter of time before it will also be used in larger vehicles with a higher energy demand.

The current global coronavirus pandemic problem should also be considered. It seems that the impact of Coronavirus disease 2019 (COVID-19) on financing clean buses in the near future cannot be predicted. According to international experts, despite many barriers and the crisis in the automotive industry, electromobility, including the ZEB sector, is developing dynamically and will continue to develop. This is owing to the fact that its expansion is based on very solid foundations, such as EU, national, and regional legal standards or multi-billion investments by automotive concerns [80].

The obligations arising from the Act on electromobility and alternative fuels in the field of zero-emission collective transport remain unchanged, new tenders are announced, subsequent deliveries are carried out, and leading manufacturers such as Solaris record a dynamic increase in orders (506 contracted ZEBs in 2020 compared with 162 in 2019) [80].

However, the survival of many enterprises often operating in key sectors to promote zero-emission transport, which is directly connected with the ZEB sector's development, depends on the government's rapid and decisive action on both local and international levels.
