Vehicle Miles Traveled (VMT)
Two-Step Adjustment


STEP 1:

This step adjusted the odometer-span VMT to a standard annualized VMT covering a full year, regardless of whether the span of odometer readings covered approximately 1 year or only a shorter span of time. Annualized VMT for vehicle i were computed as:


equa1

where:

Fj = Monthly VMT fractions from the standard distribution in Table A

soi = Month of starting odometer readings for vehicle i

eoi = Month of ending odometer readings for vehicle i.


The starting and ending Fj were prorated according to the exact day of the month for the odometer readings. For example, if a final odometer reading was taken on September 15, then (14/30)xFSEP was used.

Table A. Distribution of Average Monthly Vehicle Miles Traveled Fractions

Month j
Average VMT
per Vehicle
Fj
January
688
0.0728
February
697
0.0738
March
771
0.0816
April
783
0.0829
May
832
0.0880
June
847
0.0896
July
868
0.0919
August
872
0.0923
September
800
0.0847
October
802
0.0849
November
756
0.0800
December
734
0.0777
Total
9,450
1.0000

  Source: 1984 Petroleum Marketing Index (PMI) Survey. The survey is a demographically and geographically balanced sample of 4,100 households. Respondents maintained fuel purchase diaries for an average of 10 months. As part of the survey, information was collected on the characteristics of trips taken in vehicles during a designated day. Trip lengths were recorded as respondent perception rather than from odometer readings. The distribution of monthly mileage fractions has been obtained from this survey.

STEP 2:

Once an annualized VMT was obtained from STEP 1 as described earlier, it was adjusted to correspond to the time period that vehicle i was in possession by the sample household as:

equa2

where:

Fj = Monthly VMT fractions from standard distribution in Table A

spi = Month starting possession of vehicle i by the household, or January 1991, whichever is later.

epi = Month ending possession of vehicle i by the household, or December 1991, whichever is earlier.

If vehicle i was in the household for the entire year then spi = JAN and epi = DEC. If a vehicle was acquired or disposed of during the survey, the starting or ending Fj was prorated according to the date of acquisition or disposal.

The distribution of average monthly VMT fractions used by RTECS was given in Table A. To ensure that these fractions reflected current patterns, a study of Federal Highway Administration (FHWA) Traffic Volume data for 1991 was conducted. This study resulted in FHWA VMT fractions being constructed for 1984 and 1990. FHWA 1990 data was used since the 1991 data for the entire year was unavailable. Annual VMT calculations were completed using both the Petroleum Marketing Index (PMI) Survey and FHWA fractions. The differences in average annual VMT per vehicle between using the PMI and FHWA VMT fractions ranged between 1 and 18 miles and were less than the standard errors of the average annual VMT. Therefore, in 1994 the PMI VMT fractions given in Table A were used to compute annualized VMT since the differences in annual VMT between using the PMI and FHWA fractions were minor (RTECS Technote 5). 1


End Notes

1.Harrison, I. M. "VMT 1994 Patterns," Residential Transportation Energy Consumption Survey Technical Note 5, unpublished document. (Washington, DC , 1994).

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Release Date: August 3, 2000
File Last Modified: July 24, 2000

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Ivy Harrison
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