Q2 Z2,(Q2) Z2(Q2) - Institute for Water Resources - U.S. Army

the quantity of commodity k transported from SMSA
i to SMSA j by mode 1,
Mk the distance in miles traveled by goods shipped between
ij 1 area i and area j,
P k the production of commodity k in area i, and
i Ck the consumption of commodity k in area i.
(i, j • 1, 2, ... 25)
(k • 26, 29, 30, 32)
(1 ■ rail, truck)
The quantity figures were available directly from the census tape,
whereas some processing of the data was necessary to obtain the other
variables. The mileage values were obtained by dividing reported ton-
miles by reported tons. Area production is the sum of an area's total
exports and its reported internal shipments. Consumption is an area's
total imports plus its reported internal shipments.
The data were then divided into two groups according to whether
the shipments traveled less than or greater than two hundred miles.
This was to ascertain whether motor transport actually was dominant in
the shorter shipments with rail transport becoming competitive at ap-
proximately this distance. 17 For the aggregate regressions a dummy
variable was utilized to identify the two groups. A second-degree
term in distance was also added to the equation. This was an attempt
to restrict the dummy variable to reporting definite breaks in the
function as opposed to reflecting mild nonlinearities.
Quantities by mode were first regressed on M, M2 , iP and j C for
each distance group and commodity. The groups were than combined and
. 17. This is a common assertion. See, for example, Meyer, op. cit.
132