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4.1.3 Disaggregation of Local Information
The default VMT distribution of vehicle classes used in MOBILE6 can be used to separate the available local combined VMT information into the sixteen vehicle classifications needed for use in MOBILE6. The MOBILE6 distribution is used to determine the proportions of each subgroup in a more aggregate classification and these proportions are used to split the VMT for the aggregate class into the needed subgroup VMT estimates.

For example, an area determines that 20% of the total VMT in calendar year 2000 is from heavy duty vehicles (HDV) and the remainder (80%) is light duty vehicles (LDV), including motorcycles. The national average default value is 11.80% for HDV (vehicle classes 6 through 15) and 88.2% for LDV (vehicle classes 1 through 5 and 16). Each default LDV fraction is multiplied by the ratio of the local total LDV VMT fraction estimate divided by the national total LDV VMT fraction estimate. Each default HDV fraction is multiplied by the ratio of the local total HDV VMT fraction estimate divided by the national total HDV VMT fraction estimate. The calculations needed for this hypothetical example follow:

1. LDV = 0.4858 * (0.80/0.882) = 0.4406

2. LDT1 = 0.0671 * (0.80/0.882) = 0.0608

3. LDT2 = 0.2230 * (0.80/0.882) = 0.2022

4. LDT3 = 0.0690 * (0.80/0.882) = 0.0626

5. LDT4 = 0.0321 * (0.80/0.882) = 0.0291

6. HDV2b = 0.0383 * (0.20/0.118) = 0.0649

7. HDV3 = 0.0038 * (0.20/0.118) = 0.0065

8. HDV4 = 0.0029 * (0.20/0.118) = 0.0049

9. HDV5 = 0.0022 * (0.20/0.118) = 0.0037

10. HDV6 = 0.0083 * (0.20/0.118) = 0.0141

11. HDV7 = 0.0099 * (0.20/0.118) = 0.0168

12. HDV8a = 0.0109 * (0.20/0.118) = 0.0185

13. HDV8b = 0.0389 * (0.20/0.118) = 0.0659

14. HDBS = 0.0019 * (0.20/0.118) = 0.0033

15. HDBT = 0.0009 * (0.20/0.118) = 0.0015

16. MC = 0.0051 * (0.80/0.882) = 0.0046

The new set of sixteen values for the VMT fractions must still add to one. If a more detailed breakout of vehicle class VMT is available, such as a breakout of light-duty truck VMT from passenger car and motorcycle VMT, then only the portions of the local VMT estimate that needs to be disaggregated will need to be adjusted. In this example, the VMT fractions for each of the four light-duty truck classes would be estimated by using the default VMT fraction times the local VMT fraction for all light-duty trucks divided by the sum of the default VMT fractions for the four light-duty truck classes. The local fractions for passenger cars and motorcycles would be used directly.

The VMT by vehicle class in MOBILE6 varies from calendar year to calendar year due to uneven growth in the vehicle sales by vehicle class. The default VMT distribution used in the calculation must be taken from the calendar year that coincides with the calendar year of the local data used. Table 4.1.2 shows the default national average VMT by vehicle class calculated by MOBILE6 for all calendar years.