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Section 1: GBFM User Guide
Page 13
Looking back from 2004 to 1996, CSRGT reports traffic as 6.1% lower in 1996 than in 2004
– i.e. a scale factor of 93.9%.
Therefore this GBFM modelled scale factor of 81.3% represents rather too much of an
impact. This implies that while employment is an important determinant of freight generated
at any time, changes in industrial structure has led to employment (in manufacturing
particularly) generating less freight over time. It follows that a scaling factor needs to be
incorporated over time to reduce the relative impact of employment as a determinant of
freight generated. This appears to be intuitively reasonable given recent structural changes
in the economy.
International traffics are controlled by trade figures. Although changes in employment can
change the distribution of international traffic within a county, the totals are always scaled
back up to the known trade figures. This results in no overall response in tonnes to changes
in employment levels.
1.16
Floor space
GBFM also responds to changes in floorspace disaggregated by:
• Warehouses
• Factories
• Retail space
• Office space
It is therefore possible to input future levels and locations of planned new floorspace by type
to model the extra traffic generated.
Again a representation of 1996 has been approximated by scaling all floorspaces down to
92.7%
of
their
2004
values
(derived
from
http://www.communities.gov.uk/publications/planningandbuilding/commercialfloorspace )
Traffic between zones is proportional both to the floorspace in the origin zone and to the
floorspace in the destination zone. The relationships are scale factors:
(zone floorspace) / (average zone floorspace).
Therefore doubling the floorspace in both the origin and destination zones in isolation would
roughly quadruple the traffic because changing 2 zones in isolation has little impact on the
average zone floorspace. However any blanket changes have no impact because any
individual floorspace is changed by the same proportion as the average
Therefore there is no change to GBFM’s tonnage after blanket-changing the floorspace
areas because the total demand for freight cargo in Great Britain is itself assumed fixed by
consumer demand; it is not generated by the addition of warehousing space.
Printed on 31/03/08
Our Ref:
16:28