The Issue of Bus Fleet Renewal in Terms of Increasing the Share of Clean Vehicles: A Case Study for Slovakia

Abstract

The renewal of the vehicle fleet with environmentally friendly buses that constitute urban public transport within an urban territory, or a proportion of the transport within the territory of cities and municipalities as part of suburban public passenger transport, can make a significant contribution to reducing greenhouse gas and environmental pollutant emissions from transport. As part of the research, we dealt with the research question as to whether the application of the Act on the Promotion of Clean Vehicles (EU (European Union) Directive 2019/1161) will significantly increase the share of environmentally friendly buses by 2032 in the Slovak Republic (SR). The paradox of the application of the new legislation in the Slovak Republic is that, in public transport, the renewal of the vehicle fleet has significantly reduced, and will further reduce, the negative impacts of vehicle operation, but nothing will change significantly in suburban bus transport while a substantial part of the lines start and end at bus stations in city centres and a number of lines are for short distances, which can be operated by electric buses. Thus, the percentage of environmentally friendly automobiles in the Slovak Republic in urban bus transport will increase significantly. In suburban bus transport, we propose to change the legislation of the Slovak Republic in order to partially start increasing the share of environmentally friendly vehicles. Another follow-up research question was whether gross domestic product (GDP) per capita influences the increased share of environmentally friendly buses in the European countries studied. Based on the correlation measure, there is a significant connection between GDP per capita and the proportion of eco-friendly buses in certain nations. In areas with higher GDP, or in more advanced regions, there is a larger percentage of environmentally sustainable buses. The largest share of environmentally friendly buses is in the Nordic countries of Europe, at 13.44%.

1. Introduction

In its communication of 20 July 2016 titled A European Strategy for Low-Emission Mobility, the European Commission proclaimed that, to fulfil the obligations endorsed by the European Union (EU) during the 21st Conference of the Parties to the United Nations Framework Convention on Climate Change convened in Paris in 2015, the reduction of carbon emissions in the field of transportation has to be expedited. Moreover, emissions of greenhouse gases and air pollutants from transportation will consequently require gradual diminishment, aiming for elimination by the middle of the century. Furthermore, the reduction of air pollutants emitted by transportation, detrimental to both health and the environment, necessitates immediate and substantial mitigation. This objective can be accomplished through diverse policy endeavours, encompassing initiatives fostering a shift towards public transportation and leveraging public procurement to advance the adoption of eco-friendly vehicles.

Establishing mandatory minimum benchmarks for the acquisition of environmentally friendly vehicles, to be attained at the national level within the specified periods concluding in 2025 and 2030, is anticipated to enhance policy predictability in sectors where investments in emission-free transportation are imperative. These benchmarks are poised to foster the development of markets for eco-friendly vehicles throughout the EU, providing a timeframe for adjusting public procurement protocols while imparting a distinct market indication. Furthermore, mandating that fifty percent of the set minimum benchmarks for procured buses during these periods be accomplished through the acquisition of zero-emission buses reaffirms the dedication to reducing carbon emissions in the transportation domain. Trolleybuses are acknowledged as emission-free buses if they solely operate on electricity or employ solely zero-emission power systems when disconnected from the grid; otherwise, they are categorized as environmentally friendly vehicles. EU Directive 2019/1161 stipulates varying benchmarks tailored to individual Member States based on their economic capabilities (GDP per capita), exposure to pollution and urban population concentration. Market predictions anticipate a continuous decline in the acquisition costs of eco-friendly vehicles. Reduced operational and maintenance expenses already translate to a more competitive overall ownership cost. The anticipated decrease in acquisition costs will further dismantle obstacles hindering the availability and adoption of eco-friendly vehicles in the ensuing decade. Aligned with the objectives outlined in the ratified EU directive, the subsequent research questions were formulated:

• Will the application of the Act on the Promotion of Clean Vehicles (EU Directive) significantly increase the share of environmentally friendly buses in Slovakia by 2032?
• Does GDP per capita influence the increased share of environmentally friendly buses in the European countries studied?

2. Literature Review

CO₂ emissions from transport continue to rise globally, despite advances in low-carbon technologies and target setting by many governments. In this perspective, Axsen et al. summarise available evidence on the effectiveness of climate change policies and policy mixes in road transport against the mitigation objectives of the Paris Agreement in 2030 and 2050. The current policy mixes in most countries are not strict enough. Most regions need a stronger and more integrated policy mix led by efforts to limit vehicle travel [1].

Air protection and the negative impacts of air pollution on human health clearly need to be paid attention to, and measures taken to improve this issue. The authors of the studies published in the scientific journal The Lancet Planetary Health examined the impact of air pollution from transport on the health of children and adults in more than 13,000 cities around the world [2,3]. According to studies, nearly two million new cases of asthma in children worldwide each year can be caused by air pollutants, and the biggest problem is in large cities, with nitrogen dioxide most dangerous to children in causing asthma. Transport is a significant source of NO₂ emissions, so cleaner transport can bring health benefits to populations and especially in cities. It is crucial to pay attention to particulate matter in the air, which can pose a risk to human health. Particulate air pollution, which is predominantly the result of burning non-renewable energy sources, is considered a form of air pollution that threatens human health. A study shows that, on average, among the worldwide population, particulate air pollution shortens average lifespan by almost 2.2 years compared to if particle concentrations were everywhere at the level of particulate matter which the World Health Organization considers safe [4]. The problem with air quality due to particulate matter pollution also occurs in Slovakia as well as in Europe generally and causes negative impacts on human health [5,6,7]. In this context, the authors of the paper focused on evaluating the volume of particulate matter discharge in urban zones [8]. Selected measurement points (bus station, primary schools, level crossings) were defined as places with increased traffic intensity and places where high pedestrian movement is permanently monitored. The task of the research was to monitor particulate matter in the air and at the same time the intensity of vehicles near measuring points was monitored. As a result of the research of the authors of the paper, it is possible to point out a direct dependence between vehicle intensity and particulate production [8]. With high traffic volumes near the measuring points, air quality has also deteriorated. Strong air pollution also promotes stable, cool, and windless weather. In this state of weather, pollutants begin to accumulate, especially particulate matter PM10 and PM2.5 [9]. A paper presents data on energy consumption and CO₂ production from transport and road transport in the Nordic European countries, a proposal for an energy transition path environment, and an analysis of how the Nordic countries can achieve a nearly carbon-neutral energy system by 2050 [10]. The researchers explored the degree to which updating the public transportation fleet in a designated urban area can contribute to diminishing atmospheric contamination [11]. Their investigation aimed to present a methodology for assessing both direct and indirect emissions. Indirect emission metrics (WtT—Well-to-Tank) for various fuel varieties were derived using regression models. These WtT emission coefficients, combined with existing tank-to-wheels (direct emission) coefficients, offer a means to appraise the ecological ramifications of specific vehicle types concerning their energy source, fuel or power system, and operational pattern. As per the findings of their analysis, modernizing the fleet by altering a vehicle’s propulsion mechanism can serve as a method for curbing energy usage and mitigating environmental impacts. The manuscript elucidates the outcomes derived from the Transport and Environment initiative concerning the trajectory of transport demand in Germany across three situations, projected until 2040 [12]. The findings underscore the feasibility of steering the transportation domain towards a more environmentally sound state provided that robust policy interventions are implemented to curtail road-based transit while enhancing the appeal of alternative transportation modes. Nevertheless, the study’s results also indicate that road transport will persist as a cornerstone of the forthcoming transportation framework across all scenarios, implying that mitigation of ecological repercussions can only be realized through a synergistic amalgamation of technological advancement and fleet modernization.

The study delves into the determinants influencing the decision-making process for the acquisition and utilization of low-emission heavy-duty vehicles, with a focus on identifying the most salient factors [13]. As per expert evaluations, pivotal considerations encompass truck reliability, the accessibility of fuel/charging infrastructure, the capability to access low-emission zones, and prevailing as well as anticipated fuel expenses when procuring and operating heavy-duty vehicles powered by alternative fuels. The insights gleaned from this research can also be extrapolated to the realm of bus transportation, where an examination of the deployment of electrically powered or other alternative fuel buses becomes imperative. Such scrutiny extends to evaluating their efficacy in mitigating the adverse effects of bus operations on designated public passenger transit routes, particularly concerning aspects such as air quality [14,15].

The authors of another paper addressed a comparable challenge involving the substitution of diesel buses with new electric ones, aiming to enhance sustainable public transportation and mitigate CO₂ emissions [16]. Presently, the acquisition costs of alternatively fuelled vehicles surpass those of utilizing conventional propulsion methods. This cost disparity could pose a hindrance to fleet modernization [17]. Consequently, the authors of the study concentrated on evaluating investments in electromobility or, as an alternative approach, investments in electric vehicles in conjunction with automobiles employing compressed natural gas as an energy source [18].

The objective was to conduct an examination into the feasibility, ramifications, and expenses associated with a public transportation bus modernization initiative in Milan, with the goal of transitioning the fleet to comprise 1150 electrically powered buses by 2030 [19]. Results from a study on the full life-cycle assessment of electrifying public bus transit, as part of a sustainable city planning strategy, have shown that increasing the share of electric buses in urban areas can lead to notable improvements for public health and climate protection, as long as the electricity used comes from a broad mix of clean and low-emission energy sources [20]. The overhaul of urban public transportation fleets yields favourable outcomes for air quality in urban centres, as corroborated by the findings [11]. Hence, it is imperative to unequivocally endorse these endeavours, particularly from the urban perspective.

Findings from a Brazilian study validate the imperative nature of fleet rejuvenation in aligning with overarching environmental objectives, with the authors additionally proposing a methodology for gauging the green performance of public bus services [21]. The conversion of bus fleets to electric propulsion is poised for widespread expansion across numerous cities owing to its notable environmental merits. These benefits encompass diminished traffic noise levels, attainable through the heightened integration of electrically powered vehicles [22].

While there may exist certain constraints regarding the purchase and deployment of electric-powered buses, the authors of the paper directed their attention towards orchestrating the operational logistics of these buses, taking into account charging infrastructure requirements and the imperative for minimizing operational expenditures [23]. Moreover, the strategic planning of vehicle operation must also factor in adherence to social legislation pertinent to road transport [24].

Certain studies delve into the necessity of establishing an electric grid for battery charging of electrically powered buses in the context of public transportation [25]. The authors of another paper meticulously scrutinized compressed natural gas (CNG) buses, comparing them with traditional buses fuelled by diesel, which currently dominate the market in many countries [26]. Additionally, diesel blends incorporating bio components are presently in use, as explored in the study, offering a comprehensive evaluation of the impact of various biodiesel blends on the emission profiles of city buses under real-world operational conditions [27]. The transition to eco-friendly fleets by urban or suburban bus operators necessitates coverage under public service agreements, with public procurement processes mandated to consider the cost implications associated with operating vehicles utilizing alternative propulsion technologies [28]. Similar inquiries are addressed in other studies, wherein authors assess whether passengers exhibit a willingness to pay premium fares for the utilization of environmentally sustainable buses in public transit, alongside an examination of the competitive landscape of public passenger transportation pre-COVID-19 crisis [29,30].

3. Materials and Methods

3.1. Objectives and Focus of EU Directive 2019/1161 on the Promotion of Clean and Energy-Efficient Land Motor Vehicles

Initially, we scrutinized the aims and emphasis of the directive, with a specific emphasis on bus transportation. In an endeavour to mitigate the emissions of greenhouse gases and environmental contaminants, the EU has ratified EU Directive 2019/1161 concerning the advancement of clean and energy-efficient Land Motor Vehicles, envisaging it as a pivotal instrument in fulfilling its pledges. Through this directive, the EU endeavours to compel its Member States to enforce a green public acquisition strategy, entailing the acquisition of zero-emission and ultra-low-emission vehicles, with a target of phasing out new vehicle CO₂ emissions by 2035.

EU Directive 2019/1161 pertains to procurement procedures within the EU:

• EU Directive 2019/1161 is applicable to arrangements involving the acquisition, leasing, renting, or hire-purchase of automobiles, which are granted by purchasing organizations or offices, provided they must adhere to the procurement protocols outlined in EU Directives 2014/24 and 2014/25. In the context of Slovakia, this encompasses contracts above the threshold that are awarded by the contracting authority.
• Public service contracts as defined in Regulation (EC) No 1370/2007 of the European Parliament and of the Council, which pertains to the provision of road passenger transport services utilizing a vehicle subject to the provisions of the directive and the mean annual price of the, or its variability in annual mileage exceeds 1 million € or 300 thousand km, respectively [31].
• Service agreements delineated in

  Table 1. Distribution of clean vehicles in passenger transport under EU Directive 2019/1161 on the support for green vehicles.

  		Category	Until 31 December 2025	From 1 January 2026
  Clean vehicle	clean light commercial vehicle	M1	CO2 g/km: max. 50 Particulate matter and nitrogen oxides from Euro 5 and Euro 6 emission standards: 80%	CO2 g/km: 0 PM and NOx from Euro 5 and Euro 6 emission limits: not applicable
  		M2
  	emission-free large-scale vehicles	M3	using alternative fuels: electricity, hydrogen, biofuels (from biomass), synthetic and paraffinic fuels, natural gas (CNG and LNG), liquefied petroleum gas (LPG)
  Zero-emission heavy-duty vehicle		M3	using alternative fuels, and releases less than 1 g CO2/kWh

  , to the extent that contracting authorities or entities are mandated to adhere to the procurement protocols outlined in EU Directives 2014/24 and 2014/25. In the context of the Slovak Republic, these encompass contracts exceeding thresholds awarded by either a contracting authority or entity. The services in public road transport services, non-scheduled passenger services and special purpose road passenger transport services are covered by EU Directive 2019/1161 if they are rendered utilizing a vehicle specified therein [32].

The legislation Act No. 214/2021 Coll., which focuses on the Advancement of Eco-Friendly Road Transportation Vehicles, sets out regulations concerning the minimum quota of environmentally conscious vehicles in awarding public procurement contracts surpassing a designated threshold. This also ensures the provision of public road transport services. Consequently, it becomes imperative to delineate the specifications of contracts surpassing a specified threshold under the Public Procurement Act, along with the financial constraints within the Slovak Republic. Effective from 1 January 2022, as outlined in Decree 493/2021 Coll. by the Public Procurement Office, the financial limits for contracts beyond the threshold, concession over-limits, and design contests are stipulated as follows:

• The financial ceiling for contracts exceeding the threshold stands at €140,000 for the contracting authority, representing the Slovak Republic through its agencies.
• For other contracting authorities, the threshold for such contracts amounts to €215,000.
• The contracting authority’s threshold for contracts surpassing the above threshold is €431,000 [33].

EU Directive 2019/1161 specifies the minimum proportions of eco-friendly vehicles within the overall fleet of vehicles encompassed by all agreements subject to the directive in each EU member state. When acquiring vehicles falling within the directive’s scope and aiming to fulfil the prescribed quota of eco-friendly vehicles, it is crucial to adhere to the criteria defining clean vehicles as per the directive’s stipulations.

Through these legislative measures, endeavours are made to augment the market share of eco-friendly vehicles, potentially encompassing those falling under the concept of emission-free large-scale vehicles. The delineation of vehicles deemed clean or, where applicable, emission-free large-scale vehicles within the relevant categories of M1, M2, and M3 buses, is detailed in Table 1. The M category encompasses motor vehicles principally designed for passenger transportation and luggage carriage, subdivided as follows:

• M1—accommodating up to eight passenger seats alongside the driver’s seat,
• M2—featuring more than eight passenger seats and a driver’s seat, with a total permissible weight not exceeding 5 tonnes,
• M3—incorporating over eight passenger seats and a driver’s seat, with a total permissible weight surpassing 5 tonnes.

Alternative fuels encompass energy sources or fuels that, to some extent, act as replacements for traditional fossil oil sources in fuelling transportation. These fuels possess the capacity to aid in the decarbonization of transportation and enhance the environmental efficiency of the transport industry. M3 category vehicles utilizing alternative fuels may qualify as environmentally friendly vehicles. However, M1 and M2 category vehicles must adhere to the threshold delineated in Table 1; failing to meet this criterion renders the vehicle ineligible for classification as clean [32].

Concentrating on M3 vehicle classes, it can be observed that available buses running on alternative fuels, such as electricity, CNG, LNG or LPG, are utilized as clean vehicles in public transport.

Required Proportion of Clean Buses in Passenger Transport

EU Member States are tasked with ensuring that the procurement processes for vehicles and services falling under the purview of EU Directive 2019/1161 fulfil the mandated minimum targets for acquiring clean vehicles. These targets vary for each Member State and are contingent upon the specified reference periods, namely the first period spanning from 2 August 2021 to 31 December 2025, and the subsequent period from 1 January 2026 to 31 December 2030. If new targets are not set beyond 1 January 2030, the existing goals for the second reference period will continue for additional 5-year spans.

The required minimum purchasing goals for the share of clean light commercial vehicles stay the same in both reference periods. However, for category M3 buses, disparities exist in the minimum requirements for clean vehicles between the two reference periods, with a heightened demand for eco-friendly buses in the latter period compared to the former. It is noteworthy that half of the minimum target for clean buses must be achieved through the acquisition of zero-emission buses, classified as ‘carbon-neutral heavy-duty vehicles’. The date of contract conclusion serves as the determining factor for calculating the minimum proportion applicable for the reference period [34]. The mandated minimum proportions of clean automobiles in the Slovak Republic, falling within the ambit of EU Directive 2019/1161, are enumerated in

Table 2. Lowest acquisition objectives for the percentage of environmentally friendly automobiles in passenger transport in Slovakia (according to EU Directive 2019/1161 on the support for green vehicles).

	From 2 August 2021 to 31 December 2025	From 1 January 2026 to 31 December 2030
M1, M2	22%	22%
M3	34%	48%

.

The mandated procurement objectives for the portion of eco-friendly vehicles in Slovakia, as delineated in Act 214/2021 focusing on the advancement of eco-friendly vehicles, mirror precisely the figures stipulated for Slovakia in Directive 2019/1161. Nonetheless, there exists the potential for Slovak legislation to stipulate higher percentages, thus necessitating a greater portion of clean vehicles. Given the substantially larger quantity of Category M3 buses, totalling 7767 units, primarily employed in public transportation services within Slovakia compared to Category M2 buses numbering 844 units, our study focuses on this bus category. These bus figures are as of 31 December 2023.

In the M3 category, these percentages are markedly higher, ranging mostly between 41% and 45% in the initial reference period and surpassing 59% in the subsequent reference period, with figures reaching up to 65% in many countries. This approach predominantly aimed to operate public bus transportation with eco-friendly vehicles. Conversely, certain Member States have been assigned reduced baseline acquisition goals for the portion of carbon neutral vehicles. These predominantly include the Baltic States, the V4 states (Czech Republic, Slovakia, Poland, and Hungary), as well as the Balkan and South-Eastern European countries. The specified values represent the minimum under EU Directive 2019/1161, while each state can set higher benchmarks through its own legislation.

3.2. Correlation Analysis

The assessment of the relationship between two random variables is tackled through a straightforward correlation analysis, which prioritizes gauging the strength of the association over delving into causality (regression). It primarily scrutinizes linear dependencies, where correlation, derived from the Latin correlátio (cor—together + relatio—relationship), serves as a gauge of the linear interconnection. It is imperative to underscore that correlation does not imply causation. Although correlated quantities may exhibit interdependence, it does not imply causality. The aim of correlation analysis is to identify, quantify, and statistically examine correlations. An essential aspect involves a logical scrutiny of the issue to determine the significance of the correlation itself, which may be distorted or non-existent. When employing two-dimensional methodologies, spurious correlations frequently arise, falsely suggesting a strong correlation between variables X and Y due to other unaccounted-for variables. The correlation value is also influenced by sample heterogeneity and the formal relationship between quantities (e.g., correlation of percentage characteristics summing up to 100%). The primary tool in correlation analysis is the correlation coefficient (notated as r_xy), which gauges the degree of closeness (tightness) of the relationship. This coefficient serves as a metric for the linear relationship between two variables and can be expressed using the following Equation (1):

$$r_{xy}=\frac{\sum _{i=1}^n\left(x_i-\overline{x}\right)·\left(y_i-\overline{y}\right)}{\sqrt{\sum _{i=1}^n{\left(x_i-\overline{x}\right)}^2·{\left(y_i-\overline{y}\right)}^2}}$$

(1)

A frequent endeavour in mathematical statistics involves determining the correlation between the random variables X and Y. The correlation coefficient’s magnitude hinges on the randomly chosen elements. A correlation coefficient nearing zero indicates whether the observed value is merely coincidental (due to random selection) or genuinely reflects linear independence. A test for linear independence is employed for validation. To ascertain the correlation between the random variables X and Y, the null hypothesis H0: ρ = 0 is posited against the alternative hypothesis H1: ρ ≠ 0.

The correlation coefficient spans the interval from <−1, +1>. A correlation coefficient of 1 indicates a positive linear relationship between the variables X and Y, implying that high values of X correspond to high values of Y, and vice versa. Conversely, a correlation coefficient of −1 signifies a negative correlation between X and Y, suggesting a distinctly opposite association, where high values of X correlate with low values of Y, and vice versa. When X and Y values are independent of each other, the correlation coefficient assumes a value of 0. In such instances, the values of X and Y are distributed independently. It is notable that a correlation coefficient of 0 can still occur even if there exists a statistical relationship between X and Y that is non-linear. Positive correlation coefficients indicate direct dependence, while negative coefficients suggest indirect dependence. Other correlation coefficient values can be interpreted as follows (though it is important to remember that these thresholds serve as guidelines, and the interpretation of dependence also hinges on the nature of the compared data—these thresholds are based on the authors’ expertise):

• if 0 ≤ | RXY | < 0.4, there is a weak dependence between the characters X and Y,
• if 0.4 ≤ | RXY | < 0.8, there is a moderate dependence between the characters X and Y,
• if | RXY | ≥ 0.8, there is a strong dependence between the characters X and Y.

3.3. Data in the Analysis

For the research, we used specially requested data from the central vehicle register of the Ministry of Interior of the Slovak Republic, published data from ACEA (The European Automobile Manufacturers’ Association), data from the Statistical Office of the Slovak Republic, EUROSTAT data, data from the Vehicle Register of the Czech Republic, processed data from public procurement of transport services in urban and suburban bus transport in the Slovak Republic and data from the Association of Urban Public Transport Operators in the Slovak Republic.

4. Results

4.1. Share of Buses by Type of Drive Used in the EU and Slovak Republic

According to a January 2023 report by ACEA (The European Automobile Manufacturers’ Association), which provides an overview of the European vehicle fleet, it is possible to determine the proportion of vehicles for each country by the type of fuel used, including buses.

Currently, diesel fuels approximately 92.5% of all buses in the EU. The utilization of alternative fuels remains minimal in comparison, with diesel maintaining its stronghold, particularly when compared with trucks (refer to Figure 1). Specifically, natural gas powers approximately 3.7% of buses across the EU, while liquefied petroleum gas fuels 0.1%. Electrically powered buses constitute a mere 1.3% of the total bus fleet [35].

In terms of propulsion, there is still a relatively small percentage of buses of category M3 with alternative fuels in the Slovak Republic and therefore system support for fleet renewal by environmentally friendly buses is important. The distribution of M3 buses by drive types in Slovakia is in

Table 3. Buses of category M3 by type of drive as of February 2022 and December 2023 in Slovakia (according to Ministry of Interior of the Slovak Republic—vehicle registration).

Drive Type	Number (December 2023)	% Share (December 2023)	Number (February 2022)	Share in% (February 2022)
Petrol	5	0.06%	6	0.08%
Petrol + CNG	1	0.01%	5	0.07%
CNG	252	3.24%	253	3.37%
Electricity	55	0.71%	47	0.63%
LPG	3	0.04%	1	0.01%
Diesel	7407	95.33%	7070	94.29%
Hydrogen	4	0.05%	0	0.00%
Unknown	43	0.55%	116	1.55%
Together	7770	100.00%	7498	100.00%

.

By 31 December 2023, there was an augmentation in the count of electric buses in the Slovakia compared to February 2022. Additionally, four hydrogen-powered buses were officially registered. The most substantial surge occurred in the diesel category, with a notable increment of up to 337 buses. Primarily, these comprise new buses falling under category M3, procured for suburban bus services.

4.2. Renewing the Bus Fleet in Europe

Based on the findings outlined in paper authored by the Department of Road and Urban Transport at the University of Žilina, it is advised that the EU and other nations continue their backing of public transport fleet modernization initiatives in urban areas [11]. This support stands to significantly mitigate the adverse environmental repercussions associated with urban public transport. Moreover, it leads to enhancements in the quality of public passenger transport services and fosters increased utilization. Consequently, this positively influences the distribution of transportation labour between public passenger transport and individual car usage, highlighting the importance of suburban bus fleet modernization efforts.

The aforementioned paper, which delves into the emissions production impacts following the renewal of urban public transport fleets, calculates CO₂ savings from fleet renewal for the period spanning 2019 to 2023. These savings total 1.3 million metric tons when compared to running the original fleet, while still upholding the same driving capabilities. Given the fleet’s size and mileage, transitioning to a greener fleet can also aid in achieving global climate change objectives.

Furthermore, fleet modernization endeavours aim not only to curtail transport emissions but also to enhance the ease of access to public transit for those with mobility impairments, improve the standard of offered public transport services, and bolster safety measures. Today’s vehicles come with a range of active and passive safety features, which effectively reduce accidents and consequently contribute to safeguarding human health and life [11]. Data were utilized to examine the progression of the average bus age in selected European countries, as depicted in Figure 2 [35,36,37,38].

In general, the average age of buses increased on average in the EU between 2018 and 2020, rising from 11.4 years to 12.8 years, and in 2021 the average age of buses falling slightly to 12.7 years. In the Slovak Republic, a trend of decreasing the average age of the bus fleet can be seen in the studied period, when the average age decreased from 12.3 to 11.1 years. In this case, the increase in the average age of the bus fleet in the EU may have been affected by the COVID-19 pandemic and the consequent impact on the economy and mobility. The lowest average age of the bus fleet from European countries in 2021 was in Austria, Luxembourg, and Sweden, where values range from 5 to 6.8 years.

Another segment of the study primarily centres on buses falling under category M3. These vehicles include automobiles with more than eight seats excluding the operator’s seat and a maximum weight over five tonnes, regardless of whether they have a standing passenger area [39,40]. Within the M3 bus category, there remains a notable presence of aging buses, surpassing the 20-year mark, registered in the Slovak Republic (refer to Figure 3). Nevertheless, this scenario is notably improved compared to the M2 bus category. As of 31 December 2023, the average age of category M3 buses in the Slovak Republic stood at 10.7 years.

In terms of registrations of new buses in the EU (Figure 4), the highest number of new buses registered with diesel in 2022 was 67.3%.

The proportion of new electrically powered buses is increasing. This share was specifically 12.7% in 2022 [41]. New buses with alternative fuel types, which include buses running on natural gas, account for 11.9%, and 8.1% are new electric–diesel hybrid buses.

A year-on-year comparison of [41] data for 2022 and 2021 (Figure 5) shows a decrease in diesel buses sold in the EU from 20,072 to 18,500 buses, in relative terms by 7.8%, representing a decrease of 1.5 percentage points from 68.8% to 67.3%.

Electric buses were sold in more numbers in 2022 than in 2021, as sales of new electric buses increased from 3083 to 3505 buses, or 13.7%, an increase of 2.1 percentage points. Sales of alternatively fuelled buses also increased from 3021 to 3262 buses, an increase of 8% and an increase of 1.5 percentage points. On the contrary, a decrease of 25.9% was recorded in sales of hybrid buses, from 3006 to 2227 buses in absolute terms, and a decrease of 2.1 percentage points in the share of all registered buses.

A closer examination of registrations of new buses in individual European countries by fuel type in 2022 from a data source [41] shows in Figure 6 that the highest proportion of electrically powered buses was recorded in the Nordic countries, namely Finland (66.9%), Denmark (60.3%) and non-EU Norway (50.9%). In Slovakia, of the new buses registered in 2022, 88.8% were diesel-driven, 0.3% electrically driven, and 10.9% with other alternative fuels.

Data pertaining to registered vehicles in the Slovak Republic as of 12 April 2022 furnish insights into buses categorized under M3, categorized by their registration year, which underwent further detailed examination. These data are categorized based on the number of buses registered in a particular year, delineating the count of new registrations alongside buses adopting alternative propulsion types, namely electric, CNG, or LNG-powered vehicles, recognized for their eco-friendly attributes. While the observed period witnessed the registration of several dozen environmentally friendly buses in the Slovak Republic, it is discernible that there isn’t a pronounced upward trajectory in the proportion of environmentally friendly buses among newly registered ones (see Figure 7). In select self-governing regions of the Slovak Republic, such as Košice, Banská Bystrica, Nitra and Bratislava, public procurement initiatives for providers delivering public services were executed during 2020 and 2021. Notably, the specifications and procedural guidelines stipulated in these agreements did not mandate the utilization of vehicles employing alternative propulsion types. Typically, these contracts extend for a duration of 10 years in practice.

Figure 7 additionally illustrates the total number of environmentally friendly buses registered from 2009 to 2021. This data provides the groundwork for computing the average expansion rate of these eco-friendly buses using the ensuing formula:

$$\overline{k_t}=\sqrt[n-1]{\frac{y_n}{y_1}}=\sqrt[13-1]{\frac{288}{51}}1.1551=>15.51\mathrm{\%}$$

(2)

where:

• $\overline{k_t}$ Average growth rate [-]
• y_n Value of the last element [-]
• y₁ Value of the first element [-]
• n Number of elements [-]

The computation reveals that the mean annual growth rate of environmentally friendly buses between 2009 and 2021 stands at 1.1551. This signifies that, on average, there has been a yearly increase of 15.51% in the count of eco-friendly buses during this timeframe. For the Czech Republic, the average growth rate of the number of ecological buses was also calculated, which is 43.99% [42]. Data for the research were taken from [43].

4.3. Transposition of EU Directive 2019/1161 on the Promotion of Clean and Energy-Efficient Land Motor Vehicles in the Slovak Republic

EU countries are mandated to enact the requisite laws, regulations, and administrative measures to adhere to Directive 2019/1161 concerning the advancement of clean and fuel-efficient automobiles within the European Union by 2 August 2021, and promptly inform the European Commission thereof. An examination was conducted to determine the timeline for EU Member States for incorporation of Directive 2019/1161 into their domestic legal frameworks. A comprehensive inquiry using EUR-Lex’s advanced search tool was conducted to discover the measures EU Member States have taken to incorporate the Directive into their national laws [44].

As per published records dated 26 October 2021, only 15 nations have integrated this mandate into their domestic legislation, albeit after the stipulated deadline. These countries include Belgium, Croatia, Cyprus, Czechia, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Ireland, Latvia, Lithuania, Luxembourg, Malta, the Netherlands, Poland, Austria, Portugal, Romania, Slovenia, Slovakia, Spain, Sweden, and Italy [32]. Conversely, Bulgaria remains the sole state yet to fulfil this obligation, having only done so on 2 October 2023.

Slovakia is one of the countries that complied with the directive by the required date, as confirmed by Act No. 214/2021 Coll. on promoting eco-friendly Land Motor Vehicles, dated 4 May 2021, and effective from 2 August 2021.

4.3.1. Application of Valid Legislation to Support Environmentally Friendly Buses in the Slovak Republic—Option 1

Based on the current legislation valid in the Slovak Republic, the examination of contracts for public transport services in eight regions and most cities of the Slovak Republic and processed data on the renewal of M3 category buses, the expected registrations of environmentally friendly buses in the period 2023–2032 in urban bus transport were calculated, which should approximately occur after the application of the Act on the Promotion of Environmentally Friendly Automobiles (Figure 8).

In 2024, 2025, 2027, and 2029, registrations of environmentally friendly buses would theoretically not have to be expected, since during that period no new contracts would have been concluded and there would be no obligation to provide bus transport with a certain proportion of environmentally friendly buses, or the share would be met through the use of trolleybuses, such as in the city of Žilina.

Conversely, in 2023 and 2026, annual registrations of clean buses would be expected in higher numbers of 166 and 136 vehicles per year, respectively. The situation would arise because new contracts would be concluded in the cities of Bratislava, Košice and Prešov during the given period. The rise in 2026 is also attributable to the fact that, starting from this year, the mandated minimum percentage of clean vehicles within category M3 will escalate from 34% to 48%. This means that carriers that will be contractually bound to operate 34% of environmentally friendly buses by 31 December 2025 should increase this share to 48% from 1 January 2026.

Thanks to the detection of expected registrations of environmentally friendly buses in individual years, it is also possible to indicate the number of expected registrations by the end of 2025 and from 2026. At the turn of these years, the statutory minimum required share of clean vehicles of category M3 will change from 34% to 48%. In the period 2023–2025, after the application of the Act on the Promotion of Clean Automobiles, the number of registrations of clean buses in urban bus transport would be assumed at the level of 166 vehicles and in the period 2026–2032 at the level of 297 vehicles.

From the vehicle registration database sourced from the Presidium of the Police Force in the Slovak Republic, the count of registered environmentally friendly buses within individual years spanning from 2010 to 2022 was ascertained. The trajectory of registrations of environmentally friendly buses during this period is depicted in Figure 9 through the blue curve. Given the current scenario, a projection (represented by the red line) of the anticipated registrations of environmentally friendly buses was formulated using MS Excel 2402 software in the absence of the Act on the Promotion of Clean Automobiles. This projection extrapolates forthcoming values utilizing existing temporal data and an exponential smoothing algorithm.

The forecast is accompanied by a 95% confidence interval, denoting the range encompassing each predicted value, where it is expected that 95% of future values will lie within this interval (assuming a normal distribution). Hence, it is probable that, following the prevailing trend of procuring and registering clean buses without the legislation on the promotion of clean Land Motor Vehicles, the future count would hover around the projected values, extending up to the upper confidence interval (depicted by the orange line). It is discernible that, subsequent to the enforcement of the legislation on the promotion of clean Land Motor Vehicles, particularly in 2023 and 2026, the number of registrations is anticipated to surpass what would be likely to occur under the current trend.

Using data on annual registrations of eco-friendly buses in Slovakia, it was possible to chart the growth of the cumulative total of such registrations. Figure 10 illustrates the cumulative anticipated registrations of eco-friendly buses after the enactment of the Act on the Promotion of Environmentally Friendly Land Motor Vehicles. This total is expected to closely match the ongoing trend of yearly registrations of eco-friendly buses in Slovakia.

These graphical representations indicate that the implemented legislation is not poised to exert a notable influence that would markedly spur an augmentation in the quantity and proportion of environmentally friendly buses within the Slovak Republic.

The previous figures contained information regarding new registrations of environmentally friendly buses in the Slovak Republic. The total number of buses, environmentally friendly buses, and their proportion, if the law on the promotion of clean Land Motor Vehicles applies only to urban transport, is given in Table S1. The number of buses in the period from 2023 to 2032 is determined as the value of the arithmetic average from 2019 to 2022. The number of environmentally friendly buses between 2019 and 2022 was determined, as well as the total number of buses from the available vehicle registration database in the Slovak Republic. In the following years, the expected number of registrations of environmentally friendly buses in the Slovak Republic was added to those in the current situation.

The forecast development of the total number of buses, environmentally friendly buses and their share is shown in Figure 11. When examining this issue, it was found that currently, as of 31 December 2022, 4.25% of buses in the Slovak Republic are environmentally friendly in accordance with the terminology of the Act on the Promotion of Clean Land Motor Vehicles, and in the future, due to the application of this Act, this share is expected to increase, which could reach almost 11% in 2032. However, it should be noted that this proportion would probably have been achieved even without the application of the law in question which, in its present form, will probably apply only to urban transport in the field of public service contracts.

4.3.2. Proposal to amend the Act on the Promotion of Clean Vehicles in the Slovak Republic—Variant 2

If the aim of legislation aimed at promoting clean Land Motor Vehicles is also to increase the share of environmentally friendly buses, then with variant 1, which will be applied, it can be seen that a significant increase in the share of environmentally friendly buses cannot be expected. Increasing the proportion of environmentally friendly buses in a more significant way would be possible if the legislation also applied to buses commonly used in suburban bus transport. The directive on promoting clean and energy-efficient Land Motor Vehicles does not explicitly state the specific reason for excluding buses typically used in suburban bus services from its scope. This may be related to concerns about the range of alternatively fuelled vehicles, refuelling and charging infrastructure.

There may also be opinions as to why a certain proportion of buses in urban bus transport must meet the condition of the definition of a clean vehicle, and similar buses in suburban bus transport do not have to be ‘environmentally friendly’; it is sufficient only to meet the technical and operational standards set by the contracting authority for the public service. Due to the fact that suburban bus stations are mostly located in city centres, these buses also perform some driving in cities and these buses could also contribute to improving air quality in cities. Therefore, a compromise proposal could be to modify the required proportions of environmentally friendly buses of category M3, except for class I and class A, used in urban bus transport.

In this way, it would be ensured that environmentally friendly buses would also operate in suburban bus services. This would be the required minimum share of 17% in the first reference period and at least 24% in the second reference period (

Table 4. Proposal to adjust the minimum percentage of environmentally friendly buses of category M3 in the Slovak Republic from 2 August 2021.

	From 2 August 2021 to 31 December 2025	From 1 January 2026
M3	17%	24%
M3 class I and A	34%	48%

). From the point of view of the bus fleet in the Slovak Republic, the share of environmentally friendly buses would increase more significantly than in the application of variant 1, where it is possible to expect the number of registrations of new environmentally friendly buses to copy the forecast state even without the application of the Act on the Promotion of Environmentally Friendly Buses.

Based on the proposal to modify the required share of clean buses for category M3, except for class I and class A, Figure 12 is expected to register clean buses in the period 2023–2032 in urban and suburban bus transport after applying minimum procurement targets for the share of clean buses in the Slovak Republic. In 2024 and 2027, registrations of environmentally friendly buses would theoretically not be expected, since during that period no new contracts would be concluded and there would be no obligation to provide bus transport with a certain proportion of environmentally friendly buses, or the share would be met through the use of trolleybuses, as in the city of Žilina. On the contrary, in 2023 and 2026, annual registrations of clean buses would be expected in the number of 475 and 296 vehicles per year, respectively. The situation would arise because new contracts would be concluded in the regions of Trnava, Trenčiansky, Banská Bystrica, Prešovský and in the cities of Bratislava, Košice and Prešov. The increase in 2026 is also due to the fact that from this year the minimum required share of clean vehicles of category M3 Class I and Class A will increase from 34% to 48%, respectively, and for other M3 buses the share would increase from 17% to 24%.

Thanks to the detection of expected registrations of environmentally friendly buses in individual years, it is also possible to indicate the number of expected registrations by the end of 2025 and from 2026. In the period 2023–2025, the application of variant 2 would envisage the number of registrations of clean buses in suburban and urban bus transport at the level of 554 vehicles and in the period 2026–2032 at the level of 894 vehicles.

From the database of vehicle registration in the Slovak Republic, which was obtained from the Presidium of the Police Force, the number of registered environmentally friendly buses in the Slovak Republic in individual years for the period 2010–2022 was determined. The development of the number of registrations of environmentally friendly buses in the given period is shown in Figure 13 by the blue curve. Knowing the current situation, a forecast (red line) of the predicted number of registrations of environmentally friendly buses was created in MS excel software if the Act on the Promotion of Clean Land Motor Vehicles were not applied. The forecast predicts future values using existing time data and an exponential levelling algorithm. The forecast has a 95% confidence interval, which is the range that encloses each predicted value where 95% of future values are expected to fall within this forecast range (at normal distribution). It is therefore likely that, with the current trend in procurement and registration of clean buses without the law on the promotion of clean Land Motor Vehicles, in the future this number would be around the predicted values, up to the upper confidence interval (orange line). It can be seen that, if the Act on the Promotion of Clean Land Motor Vehicles applied not only to vehicles used in urban bus transport but also to vehicles used in suburban bus transport with an adjusted proportion of required environmentally friendly buses, the number of expected registrations of environmentally friendly buses would exceed the predicted values in more years than the predicted registrations of environmentally friendly buses would occur with the current trend in the procurement of environmentally friendly buses.

By examining data on annual registrations of eco-friendly buses in Slovakia, we were able to chart the trend in the cumulative total of such bus registrations across the country. As can be seen in Figure 14, if the Act on the Promotion of Clean Land Motor Vehicles also applied to vehicles used in suburban bus transport with at least an adjusted required share of environmentally friendly buses, the sum of expected registrations of environmentally friendly buses would be higher in the period 2023–2032 than the sum of registrations of environmentally friendly buses, which would probably occur with the current trend of annual registrations of environmentally friendly buses in the Slovak Republic. These curves suggest that the implemented legislation would indeed influence an increase in the quantity and proportion of eco-friendly buses in Slovakia.

Table S2 presents the total number of buses, environmentally friendly buses, and their share if the Act on the Promotion of Clean Land Motor Vehicles would apply not only to vehicles used for urban transport but also to vehicles used in suburban transport, with at least an adapted required proportion of environmentally friendly buses. The number of buses in the period from 2023 to 2032 is determined as the value of the arithmetic average from 2019 to 2022. The number of environmentally friendly buses between 2019 and 2022 was determined, as well as the total number of buses from the available vehicle registration database in the Slovak Republic. In the following years, the expected number of registrations of environmentally friendly buses in the Slovak Republic for variant 2 was added to the current state. Figure 15 shows the development of the total number of buses, environmentally friendly buses, and their share. When examining this issue, it was found that currently, as of 31 December 2022, 4.25% of buses in the Slovak Republic are environmentally friendly in accordance with the terminology of the Act on the Promotion of Clean Land Motor Vehicles, and in the future, due to the application of this law for variant 2, an increase in this share would be expected, which could reach almost 24% in 2032. When comparing variant 1 and variant 2, it can be seen that, if the mentioned law also applied to vehicles used as standard in suburban bus transport with at least an adjusted required share of environmentally friendly buses, the overall share of environmentally friendly buses in the Slovak Republic would be almost 2.3 times higher than if it applies only to vehicles normally used in urban bus transport.

4.3.3. Verification of Research Question 1

Will the Application of the Act on the Promotion of Clean Vehicles (EU Directive) Significantly Increase the Share of Environmentally Friendly Buses in Slovakia by 2032?

Act No. 214/2021 Coll. on the promotion of clean vehicles applies to public service contracts. On the basis of these contracts, services for suburban bus transport and public transport are provided in the Slovak Republic. That Act applies to a vehicle of category M3, but excluding a vehicle of category M3, which is not a vehicle of Class I or Class A according to UNECE Regulation No 107. In practice, it is standard that the service of suburban bus transport exclusively uses vehicles of category M3 class II, which are not covered by the Act on the Promotion of Clean Land Motor Vehicles. It can therefore be expected that the proportions of clean vehicles stipulated by the Act on the Promotion of Clean Land Motor Vehicles will apply only to urban bus transport, as this is provided by automobiles of category M3, class I, which by their nature are intended for urban public transport.

Through the proposed method for evaluating the impact of legislation promoting eco-friendly vehicles on bus transportation and thorough analysis, it has been determined that, as of 31 December 2022, approximately 4.25% of buses in Slovakia adhere to environmentally friendly standards as defined by the Act on the Promotion of Environmentally Friendly Land Motor Vehicles. Furthermore, it is anticipated that this percentage will ascend, potentially reaching close to 11% by 2032 owing to the implementation of this legislation. However, it is likely that this ratio would have approached similar levels even without the enactment of the aforementioned law, which, as it currently stands, is projected to predominantly affect urban transportation within the realm of public service agreements.

4.4. Results of the Study of the Impact of the Dependence of Environmentally Friendly Buses and GDP/Capita in Selected European Countries

The aim of the EU and each country should involve lessening the harmful consequences of human activities on the natural world. Among these spheres lies road transportation and its environmental impact on urban and rural areas. Alongside promoting the use of alternative fuels in private vehicles, significant attention should be directed towards enhancing public transportation as a means to enhance environmental conditions. A viable solution entails phasing out conventionally powered vehicles within urban settings, fostering the attractiveness of public transit, and integrating buses equipped with alternative power sources. Analysis of the overall proportion of buses utilizing alternative fuels across European nations in 2021, sourced from [32,35,45], reveals notably high shares in Sweden and the Netherlands (27.1% and 22.9%, respectively). The objective entails augmenting the collective count of buses employing alternative propulsion systems, a goal that can be realized through legislative interventions. Integral to this endeavor is state support for the acquisition of such vehicles and associated incentives; however, societal affluence and attitudes towards environmental conservation also wield significant influence. Figure 16 illustrates the distribution of alternatively powered buses across Europe in 2021, juxtaposed with GDP per capita values adjusted for each country. By scrutinizing the shares of alternatively fueled buses in European nations alongside calculated GDP per capita figures, the interplay between these key metrics was examined through correlation analysis and correlation coefficient calculations:

$$r=0.3660=>\mathrm{Weak} \mathrm{dependence}$$

(3)

The computation of the correlation coefficient indicates a certain level of association between the proportion of buses using unconventional fuels and gross domestic product, albeit exhibiting a weaker correlation. Consequently, it can be inferred that, aside from the economic development status of nations, the choice to acquire an eco-friendly bus is undoubtedly affected by various other factors. These may encompass the availability of refuelling stations, regulatory constraints, financial incentives, perks for environmentally sustainable vehicles, and, notably, societal attitudes towards environmental conservation.

Similarly, an examination of the interdependence of GDP per inhabitant and the percentage of eco-friendly buses in individual regions of Europe was carried out. The regions were divided into the Visegrad countries, the so-called EU-15 and the Nordic countries Norway, Sweden, and Finland. Also in this case, correlation analysis and calculation of the correlation coefficient were used to express interdependence:

$$r=0.9874=>\mathrm{Strong} \mathrm{dependence}$$

(4)

According to the correlation coefficient, there is a strong link between gross domestic product per capita and the percentage of eco-friendly buses in selected regions. This dependence is clearly visible in Figure 17. In regions with higher GDP, i.e., more developed regions, there is also a greater share of environmentally friendly buses. The largest share of environmentally friendly buses is found in the Nordic countries, at 13.44%.

4.4.1. Verification of Research Question 2

Does GDP per Capita Influence the Increased Share of Environmentally Friendly Buses in the European Countries Studied?

Likewise, an analysis of the correlation between gross domestic product per capita and the percentage of eco-friendly buses across distinct European regions was conducted. These regions were categorized into the Visegrad nations, EU-15 members, and the Nordic trio, comprising Norway, Sweden, and Finland. The correlation coefficient indicates a robust correlation between gross domestic product per capita and the portion of eco-friendly buses in these regions. In regions boasting higher GDP, denoting greater economic advancement, there is also a larger share of environmentally friendly buses. Notably, the Nordic countries lead with the highest proportion of eco-friendly buses, standing at 13.44%.

The computation of the correlation coefficient reveals a certain association between the proportion of alternatively fuelled buses and GDP, albeit displaying a weaker correlation. Thus, besides the developmental stage of nations, the decision to procure eco-friendly buses is undoubtedly influenced by various other factors. These may encompass the availability of fuel stations, legal constraints, incentives, discounts, perks for eco-friendly vehicles, and, notably, societal attitudes towards environmental conservation.

The study shows that, if a country is richer, it also has more buses with an alternative propulsion system.

5. Discussion and Conclusions

EU Directive 2019/1161 stipulates varying benchmarks tailored to individual Member States based on their economic capabilities GDP per capita) and exposure to pollution and urban population concentration. One of the results of the research published in this study is that the EU correctly based the proposal of the minimum procurement target values for the share of ecological buses of category M1 and M3 from economic capabilities GDP per capita. For example, the Scandinavian countries have set minimum target values for the procurement of M3 category ecological buses for the years 2021 to 2025 as follows: Sweden 45%, Finland 41%, and the Visegrad nations, e.g., Hungary 37%, Slovakia 34%, and Poland 32%; for the years 2026–2030, Sweden 65%, Finland 59%, Hungary 53%, Slovakia 48%, and Poland 46%.

Another message from the research results published in this study is that the Slovak Republic would achieve the minimum procurement target values for the share of M3 category ecological buses even without the adoption of EU Directive 2019/1161 in urban bus transport. Another finding is that directive 2019/1161 did not set more precise technical requirements for the type of buses and, as was demonstrated in this study in the Slovak Republic, the minimum threshold values do not apply to buses of suburban bus transport based on the adopted Slovak law on the support of ecological vehicles. They also significantly pollute the air in cities, which will not contribute to the improvement of air quality in cities.

The paradox of implementing the new legislation in Slovakia is that, for instance, in Žilina’s public transport system, updating the vehicle fleet has considerably decreased and will continue to decrease the negative environmental impact of public transport vehicles [11], but in suburban bus transport nothing will change significantly even on suburban bus lines, which operate over short distances to nearby villages with a relatively high number of connections per day, while a substantial part of the lines runs through the territory of the city of Žilina. Therefore, the authors proposed option 2 of the application of the EU Directive to promote environmentally friendly buses in urban and suburban transport. If the aim of legislation aimed at promoting environmentally friendly Land Motor Vehicles is also to increase the share of environmentally friendly buses, then with variant 1, which will be applied in the Slovak Republic, it is clear from the examination that a significant increase in the share of environmentally friendly buses cannot be expected. A rise in the proportion of eco-friendly buses would be possible in a more significant way if the legislation also covered a certain proportion of newly procured buses commonly used in suburban bus transport.

Due to the fact that suburban bus stations are mostly located in city centres, these buses also carry out some driving in cities and these buses could also contribute to improving air quality in cities. Therefore, a compromise proposal could be to modify the required proportions of environmentally friendly buses of category M3, except for class I and class A, used in urban bus transport. This would be the required minimum share of 17% in the first reference period and at least 24% in the second reference period. From the point of view of the bus fleet in the Slovak Republic, the share of environmentally friendly buses would increase more significantly than in the application of variant 1, where it is possible to expect the number of registrations of new environmentally friendly buses copying the forecast state even without the application of the Act on the Promotion of Environmentally Friendly Buses. In the period 2023–2025, when applying variant 2 according to the prepared forecast and data, the number of registrations of clean buses in suburban and urban bus transport would be assumed at the level of 554 vehicles and in the period 2026–2032 at the level of 894 vehicles. To ensure air protection is one of the important attributes of legislation and its application, in the Slovak Republic, at the end of January 2023, the Ministry of the Environment submitted to the interdepartmental comment procedure the Act on Climate Change and Low Carbon Transition of the Slovak Republic, which sets climate targets also for road transport and, in case of non-compliance, a lawsuit may be filed against the entities responsible for their fulfilment. For example, the road transport sector aims for a 29% increase in emissions by 2030 compared to the 2005 reference year. The Ministry of Transport of the Slovak Republic and the Ministry of Economy of the Slovak Republic [46] are responsible for meeting this objective. The environmental impact of transport is a complex issue that cannot be tackled in isolation, but through a set of measures that can help achieve a common goal. The following measures can also mitigate the environmental impacts of road transport:

• investing in cost-effective and collective transport
• promote clean vehicles and public transport vehicles
• reduce vehicle downtime in traffic, including with the help of intelligent transport systems
• focus on transport in cities, where measurable results can first be achieved, including through the establishment of low emission zones
• promote training and qualification of drivers in environmentally friendly and safe driving

With legislation, incentives and public investment support, road transport solutions can decarbonise transport. There are many quick and suitable transport solutions that can limit CO₂ emissions, requiring a certain level of financial investment and time: modernising and promoting public transport and improving the level of service provided, and promoting clean trucks and buses with higher capacity. These are all steps that can be taken globally in the short term.

There is currently a strong trend towards reducing carbon dioxide and other harmful emissions, such as nitrogen oxides and particulate matter, in Europe. As a result, many countries and cities are taking measures to improve air quality, including limiting the operation of high-emission vehicles and promoting cleaner transportation. In some European countries, there are even plans to completely eliminate diesel buses from cities in the coming years. In addition to switching to alternative fuel vehicles, fleet renewal also focuses on increasing fuel efficiency. In the coming years, an important part of fleets may be the use of autonomous vehicles, which could improve transport efficiency and safety.

Considering the size of the fleet and its mileage, transitioning to a more environmentally friendly fleet nationwide can also contribute significantly to achieving global climate change objectives. Moreover, the fleet renewal initiative serves multifaceted purposes beyond merely mitigating transport emissions. It also aims to enhance the accessibility of public transit for individuals with mobility impairments, elevate the quality of provided public transportation services, and bolster safety standards.

Overall, the renewal of Europe’s bus fleet should focus on environmentally friendly and energy-efficient solutions that should help improve air quality. However, fleet renewal is not just about reducing emissions. New vehicles can also be more efficient and safer. For example, modern technologies can help reduce fuel consumption, improve vehicle safety, and attract new drivers to the profession. The renewal of the bus fleet is an important step towards sustainable transport and reducing greenhouse gas emissions. This can have a positive impact on the environment and air quality, benefiting the health of Europe’s citizens.

The paper did not address all aspects of how to increase the share of environmentally friendly buses in the fleet, e.g., the impact of subsidies on the procurement of clean vehicles, the impact of the number and performance of charging and refuelling stations on alternative fuels, and the effect of expenses on operations of environmentally friendly buses, etc. For efficient operation of electric vehicles, it is necessary to monitor energy consumption and charge status, which depends on various factors, such as weather conditions, route profile, driving conditions, energy recovery, etc. For these reasons, it is advisable to develop and apply software and simulators to plan the operation of electric buses, to obtain an overview, e.g., whether electric buses are suitable for the lines and whether they can complete the planned routes and when it is necessary to charge them.

The simulator should recommend the optimal bus model for operation in the given conditions of the city or region, simulate the actual range, state of charge, and plan the charging strategy and charging infrastructure and its requirements accordingly. The renewal of the vehicle fleet with environmentally friendly buses requires project preparation and setting up of technological processes before the procurement begins.

It would also be interesting to see how alternative propulsion buses are used in the operation of public transport or suburban transport and to try this out with diesel-powered buses. We already have examples, especially from urban public transport, where most buses are powered by an alternative propulsion or the entire fleet of buses is powered by an alternative propulsion system. For example, public transport in the city of Martin (Slovakia) with a population of over 50,000 uses only 35 CNG buses; in the city of Prievidza (Slovakia), CNG buses are in operation and will be supplemented by 10 electric buses, etc.

There are also great possibilities for the operation of buses with alternative propulsion systems in suburban bus transport in the vicinity of regional cities.