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above four scenarios (percentage increases and decreases in IVTT) and compared. The impacts of changes in IVTT on the aggregate activity travel patterns were examined by using several measures, including (a) trip frequency, (b) person- miles of travel (PMT) and vehicle- miles of travel (VMT), and (c) person- hours of travel (PHT). Changes to Trip Frequency A 10% increase in IVTT decreases the total number of trips by 1%, and a 25% IVTT increase decreases the total number of trips by 2.4% (Table 1). A 10% decrease in IVTT increases the total trips by 1.16%, and a 25% IVTT decrease increases the number of trips by 3.1%. In addition, the frequency of home- based work trips is least sensitive to IVTT changes. Further disaggregating the trip by destination activity purpose shows that the frequency of trips for socialârecreation, shopping, and personal business is the most sensitive to changes in IVTT. (IVTT affects genera- tion of activities for these purposes via the accessibility measure.) Changes to PMT and VMT An increase in IVTT decreases the overall PMT (Table 2) and VMT (Table 3), whereas a decrease in IVTT increases the overall PMT and VMT. As would be expected, the PMT and VMT for home- based work trips show the least sensitivity to changes in IVTT. In contrast, the distances traveled for nonwork purposes (especially shopping, socialârecreation, and personal business) are affected by transportation level of service. These changes are consistent with intuitive expectations, as the home and work locations are fixed in the short term. Changes to PHT The impacts of changes in IVTT on total PHT are pre- sented in Table 4. An increase in IVTT increases the PHT for work and decreases the PHT for nonwork purposes (especially shopping and socialârecreation), resulting in an overall increase in PHT. A decrease in IVTT reduces work PHT and increases nonwork PHT, resulting in an overall decrease in PHT. 80 INNOVATIONS IN TRAVEL DEMAND MODELING, VOLUME 2 TABLE 1 Impact of IVTT on Trip Frequency Home- Based Work Home- Based Other Non- Home- Based Overall No. of Trips* % Diff. No. of Trips* % Diff. No. of Trips* % Diff. No. of Trips* % Diff. Base case 3.70 â 5.58 â 2.35 â 11.62 â 10% increase 3.70 0.08 5.51 1.27 2.29 2.30 11.50 1.05 25% increase 3.71 0.24 5.41 3.05 2.23 5.08 11.34 2.42 10% decrease 3.69 0.22 5.66 1.49 2.41 2.52 11.76 1.16 25% decrease 3.68 0.59 5.80 4.06 2.51 6.83 11.99 3.14 *Number of trips is in millions. TABLE 2 Impact of IVTT on PMT Home- Based Work Home- Based Other Non- Home- Based Overall PMT* % Diff. PMT* % Diff. PMT* % Diff. PMT* % Diff. Base case 54.03 â 83.34 â 27.53 â 164.91 â 10% increase 54.03 0.01 77.21 7.35 24.76 10.05 156.01 5.40 25% increase 54.03 0.00 69.21 16.95 21.56 21.69 144.81 12.19 10% decrease 53.96 0.13 90.65 8.77 30.91 12.26 175.52 6.43 25% decrease 53.80 0.44 104.02 24.81 37.53 36.30 195.34 18.46 *PMT is in millions of miles. TABLE 3 Impact of IVTT on VMT Home- Based Work Home- Based Other Non- Home- Based Overall VMT* % Diff. VMT* % Diff. VMT* % Diff. VMT* % Diff. Base case 44.84 â 58.45 â 22.00 â 125.30 â 10% increase 44.85 0.02 54.07 7.50 19.80 10.03 118.71 5.26 25% increase 44.83 0.02 48.40 17.19 17.24 21.64 110.48 11.83 10% decrease 44.81 0.08 63.65 8.88 24.70 12.25 133.15 6.27 25% decrease 44.65 0.43 73.25 25.32 30.04 36.53 147.95 18.07 *VMT is in millions of miles.
