Uncertainties
Methodology uncertainty
All data should be based on movements on national territory, regardless of the nationality of the vehicle. However, data collection methodology is not harmonised at the EU level. Sources are Eurostat, National Statistical Offices, ECMT, UNECE, UIC, DG TREN.
To answer the question of whether passenger demand is being decoupled from economic growth we need to look at the intensity of passenger transport relative to changes in real GDP. A reduction in intensity should signal relative decoupling. This has some implications on the interpretation one makes of the observed intensity values. GDP in constant prices simply takes away the effect of price increases from year X to year Y but it does not guarantee that GDP in year X for country A is comparable to GDP in country B (as year X is the result of price increases from previous years etc). Therefore, cross-country comparisons of transport intensities based on real GDP may be relevant for trends (i.e. growth/changes over time) but not for comparing intensity values in specific years. If we are interested in knowing whether passenger transport intensity is higher in one country than in another, GDP should ideally be measured in purchasing power parities. These are currency conversion rates that both convert to a common currency and equalise the purchasing power of different currencies (i.e. they eliminate the differences in price levels between countries).
It is arguable, however, whether purchasing power parities are the best currency unit for time-series analysis. One way to avoid such problems is to use population instead of GDP. This would in principle be appropriate for the comparison of intensities between countries as well as for looking at trends over time. It seems also more equitable. To respond to the question of whether or not we are decoupling transport demand from economic activity (i.e. looking at growth rates over time) we would still need to use GDP.
There is one important difference between passenger transport demand from the structural indicators and passenger transport demand according to the core-set indicator definition, in that the latter includes air transport demand. In 2004, Eurostat launched a project to produce a set of modal split indicators. The aim of the project is to establish consistent and comparable datasets for the transport performance (in tonne-kilometres, passenger-kilometres and vehicle-kilometres) of road, rail, inland waterways, maritime and air transport on the national territory ('national territory' principle) of the countries and to use those series in a regular annual production of modal split indicators. With regards to the air transport data, Eurostat does not collect data on transport performance on the national territory of the countries where this performance takes place, as would be required by the "national territory principle". Recognising the increasing importance of air transport demand, Eurostat is working on methods regarding the calculation and territorial attribution of transport performance data for air transport. Until such data becomes available, the aggregate for the core set indicator "passenger transport demand" will include estimates of air transport demand based on the European Commission's Directorate General for Transport and Energy. In particular, their annual publication "Pocket Book Energy and Transport in Figures"
We are awaiting the results from Eurostat's project in order to apply their methodology as soon as it is validated.
Data sets uncertainty
The unit used to measure the volume of passenger transport is, as defined in the indicator, passenger-kilometre (pkm). It represents one passenger travelling a distance of one kilometre. Load factors for car passenger transport (i.e. the average number of passengers per car) are not obligatory variables as data on passenger transport performance are collected through the Eurostat/ECMT/UNECE Common Questionnaire on Transport Statistics.
Detailed information about road transport (including data quality) is available from Eurostat
Loading of the vehicle is a key factor playing a key role in the assessment of whether or not there is decoupling of passenger transport demand from GDP growth. Since load factors are not available a sound assessment of passenger transport trends becomes very difficult. One could not, for instance, properly determine what share of the trend observed for passenger-kilometres belongs to changes in the average number of passengers in the vehicle. For a complete picture of transport demand and the related environmental problems, it would therefore be valuable to complement the data on the number of passenger-kilometres with vehicle-kilometres.
The 2 indicators "volume of passenger transport relative to GDP" and "modal split of passenger transport" are part of the European Commission's structural indicators. As such, their components are already calculated and downloadable in their final form from Eurostat's database. Where data is not available, estimations have been made or data has been taken from other sources: national statistical institutes, ECMT, UNECE, UIC or DG for Energy and Transport. Estimates are not always included in Eurostat's Newcronos database but are used for the purpose of calculating the structural indicators. Data are kindly provided by Eurostat's Transport Statistics Unit in the form of an Excel Spreadsheet for internal use. The final calculations (i.e. the indicators) are published on the structural indicator's homepage.
Eurostat is in the process of reviewing/completing the quality section of the metadata part for the 2 structural indicators as well as on the possibility to include the estimates in the Newcronos database.
All data should be based on movements on national territory, regardless of the nationality of the vehicle. However, data collection methodology is not harmonised at the EU level. The coverage of passenger transport for many countries is incomplete, mainly due to lack of data on transport by passenger car.
Rationale uncertainty
The main policy question relates to whether passenger demand is being decoupled from economic growth. Thus, one needs to monitor trends in the intensity of passenger transport demand relative to changes in GDP at constant prices. The ratio of inland passenger transport to GDP could increase even though the actual passenger transport volume falls. Similarly, the indicator could fall despite of a possible increase in the volume of passenger transport. What makes the ratio increase or decrease is the relative change in the volume of passenger transport (numerator) to gross domestic product (denominator). As long as the numerator increases more (or falls less) than the denominator, the indicator "passenger transport demand" will increase. The indicator does indeed summarise "passenger transport intensity". From an environmental point of view, it is important not to overlook trends in the total volume of passenger transport. The actual absolute values are key to understand environmental pressures originating from more passenger transport demand.
Intensity can be also explained using the concepts of relative and absolute decoupling. Relative decoupling in passenger transport demand occurs when its volume grows at a rate below that of gross domestic product. In this case, however, the volume of passenger transport may well increase as long as this increase is less rapid/strong than the one observed in economic activity. Absolute decoupling in passenger transport demand occurs when the volume of passenger transport falls. This is the necessary condition. Absolute decoupling is present if GDP increases or remains unchanged. If GDP falls, there is absolute decoupling only if the fall in passenger volumes is stronger than the contraction in GDP. This is important since from a purely statistical point of view one could imagine a situation where no absolute decoupling is observed and yet this may be good for the environment. For example, both GDP and passenger volumes could fall, with the latter falling less than the former. The fact that the volume of passenger transport goes down is good for the environment but the hypothetical situation just described does not strictly correspond to absolute decoupling.
Even if two countries have the same passenger transport intensity or show the same trend over time there could be important environmental differences between them. The link to the environmental pressure has to be made on the basis of the energy fuels used to satisfy passenger demand. The primary fuels today are gasoline and diesel but other options may be available in the future.
In relation to the modal split indicator (i.e. percentage share of transport by passenger car in total inland passenger transport), what makes a share increase or decrease for a particular mode depends on the change in the volume of transport for that specific mode relative to the total volume of all modes vis a vis the relative changes observed for the other modes. That is, not only it depends on whether the volume of passenger car transport increases or decreases but also on how the increase or decrease in the total volume of inland passenger transport is distributed across the different modes. From an environmental point of view, the relative contribution of each mode to the total volume of passenger transport has to be put in the wider context. Absolute (as opposed to relative) values of transport volumes for each mode are key to understand the environmental pressures.
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