Analysis of the Vehicle Inventory and Use Survey for Trucks ... - About

trailer. Removal of these vehicles creates a data set referred to in this analysis as the “1992/1997
Total Fleet.”
For most of this analysis, the 5-axles or more truck/tractor-trailer combinations were evaluated
which are a subset of the “Total Fleet.” The “5-Axles or More Fleet” contains data for various
types of truck/tractor-trailer combinations whose total number of axles is greater than or equal
to 5 and are of primary interest from a truck size and weight perspective. In general, there are
four types of truck/tractor-trailer combinations evaluated: (1) straight truck with trailer; (2)
tractor truck with semitrailer; (3) tractor truck with two trailing units; and (4) tractor truck with
three trailing units.
1.4 Cautionary Notes
The values presented in the tables throughout this report are the direct result of the analysis of the
VIUS databases. There has been limited judgement as to their appropriateness; that task is the
responsibility of the reader. It should be noted that the VIUS is based on survey data which
assumes that the respondents will devote some time and effort to giving accurate estimates and
responses about their vehicle. The Bureau of the Census did do some quality checking of the
data.
State of Registration. The data presented in this report has been analyzed on the basis of the state
of registration. In drawing inferences from the data, the potential for a registration-bias must be
recognized. However, the correlation between state of registration and state of home base
location is approximately 90% for the 1997 VIUS. Home base state is defined as the location
where the vehicle is parked when not in use. If state level detail is of interest, the correlation
should be examined for that state and the vehicles of interest.
Population Estimates. In the analysis of the VIUS database, each record does not reflect one
vehicle, but instead it represents a number of vehicles in the population. To make interpretations
about the total truck population from this small sample of the population, a weighting factor was
applied to each record. This weighting factor differs for vehicles registered in different states and
for vehicles belonging to different vehicle type groups. The reason that the weighting factor is
not a constant number across all records is due to the method of sampling the truck population
which was conducted at the state level, not the national level.
Readers must be cognizant of the fact that this weighting process can create odd results in
selected situations—particularly when the sample size relating to a particular group in the
population is small. This is particularly true when analyzing the VIUS at the state level where
many states have small sample sizes. For example, our analysis of the 1997 VIUS database
estimated that New York has 65 vehicles operating as tractor + triple trailer combinations. New
York does not permit such combinations it is likely that these tractors are registered in New York
but operate in another State. Alternatively, the 65 vehicle population estimate could have arisen
from an incorrect answer on as little as 1 or 2 survey records—which were subsequently
factored-up to create the population estimate.
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