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In this paper the influences of road and traffic conditions on fuel consumption and fuel costs of conventional diesel, parallel hybrid, and stoichiometric compressed natural gas buses in intercity bus service are analysed. Calculation of fuel consumption and fuel costs for these three different bus technologies was conducted for road network of the Republic of Serbia. Three scenarios were considered. The first scenario includes bus traffic volume carried out on the road network in 2014. The other two scenarios are characterized by the decrease i. e. increase of traffic volume by 20% with unchanged state of road infrastructure in comparison to the year 2014. Obtained results show that in intercity bus service the greatest influence on the fuel consumption of buses has operating speed of the bus, followed by terrain type on which buses operate. The impact of other factors (international roughness index, fluctuation of traffic volume by 20%, and correction factors of fuel consumption) is less pronounced. In various operating conditions the fuel cost savings per 100 km of compressed natural gas buses compared to diesel buses are in the range of €8.84-12.16. These cost saving for hybrid buses compared to diesel buses are in the range of €3.33-7.27.

The paper gives special attention on long distance passenger transport and specific emissions related to different transport modes, particularly road and air transport sector. The goal of this research is creation and selection of appropriate methodology for modelling the cost estimation of GHG emissions in road and air transport sector for Republic of Serbia as well as the application of the methodology regarding to detailed calculation by transport mode and sub modes. Input data for road transport sector refer to the 2013 and include road and traffic conditions on the road network. Input data for air transport sector are related to the 2014 and international airport ‘Nikola Tesla’ Belgrade as the main hub point with the highest recorded number of aircraft operations in the Western Balkan countries. The obtained results reveal that, due to realized transport volume, diesel cars have the largest share of the costs of GreenHouse Gas (GHG) emissions within the passenger long distance road transport. Cost estimates of CO2 emissions in the air transport sector shows that A319 aircraft type have the major share in total costs. The reasons are twofold: first, a high level of Landing and Take-Off (LTO) emission factor for CO2 and second, largest number of LTO cycles.

The paper's goal is to unify practical and theoretical aspects of internalisation of external costs, in line with the "polluter pays" and "user pays" principles. Due to the impossibility of applying an ideal economic solution for internalisation of external costs, alternative solutions have to be developed and implemented. One of the possible solutions for internalisation of external costs of CO 2 and pollutant emissions from passenger cars is presented in this paper. It is a new methodology for calculating annual circulation taxes on passenger cars. This methodology, besides CO 2 , also takes into account the pollutants whose emissions are regulated by the Euro standards (CO, HC, NO x and PM), as well as the vehicle age and kilometres driven. The proposed methodology has been tested on some of the best-selling passenger cars in Europe. The results of analysis show significant differences between our methodology and the methodologies that are used in five European countries (Ireland, the United Kingdom, Malta, Luxembourg and Sweden), which use the CO 2 emissions as a reference value for their calculation. Also, we have proved that the annual circulation tax, calculated using our methodology, provide better internalisation of external costs compared to the fuel tax.

In this paper, the calculation of exhaust emission costs originating from aircraft and road vehicles in the base year 2017 and in the forecasting year 2032, in the Republic of Serbia, was carried out. The presented methodology includes a number of influential factors for air transport (airport capacity, number of operations, aircraft type, relevant engine, range) and for road transport (changing of traffic volumes, design and operating speeds, the quality of the pavement structure, type of terrain and category of road sections, dependence of exhaust emission from changes in vehicle speed). It was found that in the current operating conditions, the dominant costs in the total exhaust emission costs are the costs of nitrogen oxides (61%) in road transport, whilst carbon dioxide costs are dominant in air transport (52%). In the future, carbon dioxide costs will have a share of over 80% in the road transport sector and over 58% in the air transport sector in total exhaust emission costs. The average exhaust emission costs per one aircraft operation (international flights) will range from 141 to 145€. In road transport, the average exhaust emission costs at 100 km in 2032 will range from 1.8 to 2.2€.

Within the transport sector, road transport is the largest source of Carbon Dioxide (CO2) emissions. Greater use of vehicles that run on clean alternative fuels can contribute to reduce CO2 emissions. This paper gives special attention to the Compressed Natural Gas (CNG) buses and their comparison with conventional diesel buses, which in countries such as Serbia have a dominant share. Justification of using CNG buses in order to mitigate climate changes is measured by realised annual and average external costs of CO2 emissions. These external costs provide a basis for future use of economic instruments by which negative impacts of transport on the environment can be limited. Research of CO2 emissions and external costs of CO2 emissions in intercity bus service was conducted for three technical-technological concepts of CNG buses in comparison to the two types of conventional diesel buses. Analysis was carried out according to four various scenarios that define different operating conditions on the road network of the Serbia. Obtained results show that CNG buses reduce annual external costs of CO2 emissions by 2…24% compared to conventional diesel buses. Obtained average external costs of CO2 emissions per 100 bus-kms show to what extent their changes are a result of changes of external costs of CO2 emissions and to what extent they are due to changes of operating conditions on the road network.

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The fi rst part of the paper researches and states the reasons why tax on motor fuels is considered one of the most important economic instruments in transport sector. In addition, the paper presents possible ways of differentiating motor fuels taxes and analyses the justifi cation for imposing such taxes from the ecological viewpoint. The second part of the paper devotes special attention to the analysis of this economic instrument's application in the European Union Member States, as well as its place and role in the environmental protection policy.