D5 Annex report WP 3: ETIS Database methodology ... - ETIS plus

D5 Annex WP 3: DATABASE METHODOLOGY AND DATABASE USER
MANUAL – FREIGHT TRANSPORT DEMAND
O 2 ΣO 2
Total ΣD 1 ΣD 2 ΣD 3 ΣD 1..3 (= ΣO 1..2 )
In this example, the sets of ‘O’s and ‘D’s belong to specific countries. Country to country totals are
known, as are regions to/from country totals for a significant proportion of EU and accession
countries.
The solution attempted within CA­SSS was to derive a set of gravity models, one for each SITC
commodity relating the volume of traffic in a given O/D cell to the distance between the origin and
destination, and the size of the regions, measured by the total volumes of the commodity produced
and consumed. Other measures of region ‘mass’ could be hypothesised, but in the CA­SSS study
most were found to be closely correlated to the total import and export volumes.
The functional form used was:
T p = d n e ­md E p I p
Where:
T p
d
E p
I p
n,m
= tonnes of product ‘p’ lifted
= distance (kms)
= Exports of product ‘p’
= Imports of product ‘p’
are the parameters to be estimated
The use of a function that combines the exponential and power­n (where n is negative) curves
provides greater control in circumstances where distance is close to zero, than would be the case if
only the power­n curve were used. This is important if the parameters are being estimated for zone
pairs where the distances are relatively high and then transferred to zone pairs where the distances
could be small.
The intention was to develop a technique that could estimate parameters for, potentially, a highly
disaggregated set of commodities, and in which a number of non­linear functions could be tested
and compared. The parameters would be estimated at the national level, for which large volumes
of data are available, using regional information where possible. The same parameters would then
be transferred to compute values at a region­region scale. So, for example, if at a national scale, the
propensity to trade ( T p /E p I p ) for Portugal and Spain is higher than for Portugal and Finland, for a
given product, it seems sensible to conclude that the propensity to trade for adjacent Portuguese and
Spanish regions may be higher than for distant Portuguese and Spanish regions.
It also appears sensible to distinguish between product groups, as, for example, any distance decay
effects may be more significant for low value, high volume goods than for higher value densities.
This is exemplified using results from the 1996/7 study, for two contrasting commodities.
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27 May 2004
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