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Rh the explanation is undoubtedly that the difficulties of obtaining passenger automobiles, and their high cost, caused many owners of trucks to use these vehicles in lieu of passenger cars. This explanation is borne out by the fact that it was the lighter vehicles that accounted for most of the increase in the percentage of total travel by empty vehicles. The percentage for empty truck combinations did not vary much from 30 percent throughout the period from 1941 to 1947, whereas that for single-unit trucks increased from 34.6 percent in 1941 to 53.6 percent in 1946. Since the beginning of the war, approximately half of the travel by single-unit trucks on the highways has been by empty vehicles compared to about one-third in the prewar period.

For the year 1946, when the greatest proportion of the vehicle-mileage was by empty vehicles, the percentages for single-unit trucks in each region were as follows: It was in the South Central regions, and the Mountain region, therefore, where the greatest use of trucks for passenger-car purposes was made.

Vehicle-mileage of trucks

Because of the increased proportion of empty vehicles, the vehicle-miles traveled by loaded vehicles, of course, declined somewhat more than the total vehicle-mileage of trucks and combinations. Offsetting this, however, was the increase in the proportion of combinations and the heavier loading of that type of vehicle so that the ton- mileage hauled decreased less than the vehicle-mileage. Table 7 gives the average carried load of loaded trucks and combinations by years from 1936 to 1947, inclusive. There was little change in the average loading of single-unit trucks during that period, but a continued upward trend in that of truck combinations from 1941 to 1946. The average load carried by all types of vehicles increased somewhat faster, because of the increasing proportion of combinations.