Mileage-Based User Fee Winners and Losers - RAND Corporation

Ratio of adults to vehicles
As discussed above, the number of vehicles and workers in a household are each important
predictors of demand. However, it is also important to account for the ratio of vehicles to adults.
VMT is increasing with both vehicles, as we have discussed above, and with adults, which follows
logic. This variable categorizes each household into 9 types, 10 if households without any vehicles
are included, based on the number of vehicles owned (1, 2 or more than 2) in different household
sizes (1, 2 or more than 2 adults).
Table 3.6 displays the proportion of households in each vehicle to adult category type. In
addition, the coefficients for these variables are included, along with their robust standard errors
because these were omitted from Table 3.2 for space constraints. All are significant at the 1 percent
level of statistical significance with pooled data in the model with price interactions. 34 The
coefficients, along with the mean VMT for each category, clearly show that VMT is increasing jointly
with the number of vehicles and the number of adults.
Table 3.6 Coefficients For the Vehicles to Adults Household Type Variable
Number Number Percent of Average
Robust
Coefficient
of Vehicles Of Adults Population VMT
St. Error
0 all 8.6 0 - -
1 1 20.6 10,196 0 (1)
1 2 10.0 11,633 0.16 0.02
1 3 or more 1.7 11,527 0.20 0.05
2 1 3.7 19,050 0.98 0.08
2 2 29.0 24,482 1.15 0.07
2 3 or more 3.6 25,224 1.22 0.08
3 or more 1 1.9 31,500 1.27 0.09
3 or more 2 12.9 36,389 1.33 0.08
3 or more 3 or more 8.8 44,266 1.40 0.08
Note: (1) The model would be over-specified if this coefficient was estimated.
Life cycle groups
These two categorical variables (condensed from ten groups of life cycle type) control for
households with children and households comprised entirely of retired adults. 35 Households with
children, comprising 35 percent of households, have a much greater demand for VMT than other
households. Retired households, in contrast, have a much lower demand for VMT. Retired
households comprise 27 percent of the population in 2008-09. This is an increase from 2001-02
when retired households comprised 24 percent of the population. The differences in the travel
behavior, and vehicle characteristics, between households in different lifecycle stages have important
equity implications as well. It is also significant that these statistics reflect the aging of the American
population as well as the increasing mobility of older Americans. The average VMT of retired
households has increased from 13,164 in 2001-02 to 13,798 in 2008-09. This was during a period
when, as previously noted, other households were reducing VMT due to higher travel costs and
lower incomes.
34 The variables are not all individually significant at this level in the model without interactions but they are all jointly significant at the
1 percent level of statistical significance.
35 More information on the ten original life cycle groups and the details on the construction of this variable are available in the
Technical Appendix.
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