The increases in mobility and in car ownership and use are consistent with the trend of declining population living in the city core in favour of the suburbs. Travel distances have increased - at European level - from the 5 to 10 km to the 10 to 15 km band (European Environment - Dobríš+3 Report). Travel demand and modal split are influenced by settlement size, urban density and structure. The relationship between settlement size and emission-efficient transport is more complex. It is however evident, that both the dispersal of population from major city centres and smaller settlement sizes are typically accompanied by higher car use. More centralised urban structures  have the effect of increasing journey length, but they also increase the use of public transport. The growth in urban mobility and the predominance of road traffic are among the principal causes of urban air pollution, congestion and noise.
 
 

Trend in vehicle number in total in the SR	Trend in vehicle number by type  in the SR
Source: ŠÚ SR	Source: ŠÚ SR

Reducing pollution from mobile sources can be achieved by changes in engine design that reduce emissions. However, most new engine designs do not reduce emissions of carbon monoxide, nitrogen oxides, and hydrocarbons to acceptable levels. This makes it necessary to pass the exhaust gases through catalytic converters. Attached to the exhaust system, these devices convert carbon monoxide and hydrocarbons into water and carbon dioxide.