nation constrained growth factor method where a specific growth factor is
applied to all trips terminating in zone j (see Equation 2.9 below).(2.8)(2.9)
where
Gtti¢=Expected growth in trip volumes between year tand year t¢for trips with
their origin in zone i(origins only)
Gttj¢=Expected growth in trip volumes between year tand year t¢for trips with
their destination in zone j(destinations only)
Where information exists on zone-specific growth factors for both origins and
destinations an average factor method can be applied where, for each origin-
destination pair, the overall zone-specific growth factor is obtained from the
average of the expected growth from origin iand destination j:(2.10)
To obtain a more precise answer, however, a doubly constrained growth factor
method can be used. One of the most frequently used models of this type was
devised by K.P. Furness (the Furness method).2.5.4 The Furness method (Furness, 1965)
This again is a growth factor method, but in this instance the basic assumption
is that in the future the pattern of trip making will remain substantially identi-
cal to those at present, with the trip volumes increasing in line with the growth
of both the generating andattracting zones. It is still more straightforward than
the gravity model and quite applicable to situations where substantial changes
in external factors such as land use are not expected.
The basic information required in order to initiate this procedure can be sum-
marised as:Data
Tijt– The existing trip interchange matrix (in baseline year t)
Oi– The total number of trips predicted to start from zone iin the future fore-
casted year
Dj– The total number of trips predicted to terminate in zone jin the future
forecasted year.
To be computed
Ttij¢– The revised trip interchange matrix (in forecasted year t¢)
Gtti¢– Origin growth factor for row i(growth between year tand year t¢)
Gttj¢– Destination growth factor for column j(growth between year tand year t¢).TT
GG
ijt ijt
¢=¥È itt¢¢+ ttj
ÎÍ̆
(^2) ̊ ̇
TTGijt¢¢=¥ijt ttj
TTGijt¢¢=¥ijt itt
Forecasting Future Traffic Flows 27