En model for godstransportens udvikling - DCE - Nationalt Center for ...

stability of the regression results a secondary regression were made,
where the (subjectively chosen) most deviating observation is removed
beforehand. If the sign of the estimated parameters remains
the same and if the size of the parameters remains at the same level, it
is taken as an indicator of a reasonable accuracy. On the other hand
signs of misspecification shows up. This problem arise in three cases;
potatoes (3), fatty substances (9) and semi-products from metals (15). In all
three cases a discussion is provided on whether the original estimate
should be used. In all cases the first estimate is chosen because of the
very few observations, but also because the sign in all these cases are
the same as the overall trend.
A secondary element in the analysis of the handling factor is the distribution
between the size of the vehicles, and on own and haulage
transport. It is possible to calculate this directly from the production
in tonnes, but the uncertainty is so large that it has been chosen to
split the calculations into two parts: the general handling factor and
the “modal split”. The modal split is explained by technological
trends .
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. It has not been possible to explain the split using relative
prices. However, looking at the aggregate the prices do seem to have
some explanatory power. The prices point at one of the main developments
in the freight transport. Higher prices leads to less transport
in small vehicles and less transport using own vehicles. This is due to
a shift into more efficient transports, and a demand for more flexible
transports.
However, the relative prices and the trend have the same explanatory
power. It is therefore chosen to use the trends as the explanations in
the model. The calculation of the handling factor in these two steps
does not result in less precision of the predictions of the model.
Average length and load
The average trip length for small company owned vehicles has in the
period 1980 to 1997 risen from approximately 37 km per trip to 66 km
per trip. The average trip length for the large company owned vehicles
has risen from 56 to 89 km per trip, whereas the average trip
length for the small haulage vehicles has risen from 35 to 71 km per
trip and for the large haulage vehicles from 65 to 90 km per trip (a
similar picture arise when looking at the average load, which we
have left out here). This is the overall picture, but significant differences
arise when splitting the information on different types of
goods. It is therefore necessary to retain the high level of detail also
for these calculations.
The development is in most categories (goods, size and mode) nice.
However, there still does not seem to be any kind of relation between
the development of relative prices and the average figures. On the
other hand it is possible to achieve very good results using technological
trends. The main problem in relation to the prices mentioned
is that we have only got one single price. It is an enormous task for
this price to be able to describe the differences between the many
different types of goods. For instance the price on transport of building
material is very different from the price of transport of chemicals
(high risk goods). These prices will furthermore differ individually,