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60 I N N O VAT I O N S I N T R AV E L D E M A N D M O D E L I N G , V O L U M E 1 single households, an increase in dual-worker house- with relatives. Cluster 3 included individuals who made holds, and more women entering the workforce. about three trips each day, spent about 1.9 hours each day From 1980 to 1992, vehicle miles of travel (VMT) out of the home, and made 0.7 of the trips with family in the region increased by approximately 80%, while members. Cluster 4 included individuals who made 3.9 employment grew by 40%, and population increased by trips on Day 1, but no trips on Day 2. These individuals less than 5%. Thus, growth in VMT far outpaced spent 3.7 hours out of the house on Day 1 and 1.6 trips growth in population and employment. Part of the were made with family members. Individuals in Cluster 5 increase in VMT was due to changing demographics and made no trips on Day 1 and 3.9 trips on Day 2, with 3.4 part was probably due to the available capacity on many hours spent out of the house and 1.6 of the trips made freeways and roadways in the Puget Sound Region in the with family members. Cluster 6 included individuals who early 1980s. Much of the Interstate system in the area made no trips on Day 1 or Day 2. was constructed in the 1960s and 1970s. By the late Clusters 4, 5, and 6, which were the clusters with 1980s and early 1990s, traffic congestion was becoming infrequent trip makers and no trip makers, were exam- a problem, however. ined for possible weekly patterns. Cluster transitions At the time, some policy makers and other groups from 1999 to 2000 were also explored. Approximately focused on using this percentage growth in VMT in plan- 70% of the individuals in Clusters 1 and 2 in 1999 stayed ning future project needs. If 1992 is used as the base year in the same clusters in 2000. In comparison, only 32% for VMT increases, however, a different picture emerges. of the individuals in Cluster 3 in 1999 stayed in Cluster From 1992 to 2004, VMT increased by approximately 3 in 2000 and 47% of the individuals in Clusters 4, 5, 22%, while employment grew by 20% and population and 6 in 1999 stayed in the same cluster in 2000. These increased by about 19%. Over the same period, lane results indicate that 5-day or 2-week travel diaries are miles increased by approximately 12%. needed to capture the behavior patterns of individuals Tracking the population in Washington State by gen- 50 years of age and older. der and age since the late 1800s highlights some interesting The most important factor correlating to a transi- trends. In 1880, the population was fairly small. Washing- tion among clusters was a change in employment status. ton became a state in 1889 and the gold rush in Alaska, This change might include retirement, working part-time, which contributed to the growth in Seattle, occurred in the and volunteering. Other factors that correlated with clus- 1890s. By 1890, the population of Washington was ter transitions were a change in driver's license status, a increasing due to immigration. The increase was primarily change in available vehicles, and a change in the number in males between the ages of 24 to 39. These growth trends of children in the household. Factors that did not corre- continued to 1910, along with an increase in females in the late well with a transition in clusters were gender, land same age groups. From the 1920s through the 1940s, the use mix, and a change in home location. One implication population remained relatively stable in number with an for travel demand modeling is that it is important to first aging trend. In the 1940s, Washington experienced popu- forecast the employment status of the baby boom gener- lation increases due to the military bases in the state. In ation before forecasting their trip making. 1950, the number of births increased significantly, reflect- The number of trips and miles traveled per day cor- ing the start of the baby boom. The population in 1960 related with changes in the number of automobiles in the reflected a further increase in births, as well as continued household, the number of drivers in the household, chil- immigration. The trends from 1960 through 2000 reflected dren becoming adults, older adults moving into the the aging of the baby boom generation, the continued household, and household location. These results indi- immigration, and more people living longer. cate that to forecast baby boomers at the household The aging of the baby boom generation raises inter- level, information on available vehicles, number of driv- esting questions concerning potential similarities to and ers, demographic shifts with different travel require- differences from the travel behavior of individuals cur- ments, and the relocation of households is needed. rently in older age brackets. The panel survey data were examined to provide insights into the travel behavior of baby boomers as they retire or near retirement. NEW SURVEY ITEMS FOR A FULLER A cluster analysis was conducted using the 2-day DESCRIPTION OF TRAVELER BEHAVIOR: travel diaries of individuals 50 years of age and older. BIOGRAPHIES AND SOCIAL NETWORKS Cluster 1 comprised individuals who made close to 5.6 trips on each of the 2 days, with approximately 7 hours a Kay Axhausen day out of the home. About half of these trips were made with family members. Cluster 2 included individuals who Kay Axhausen discussed research related to travel sur- made a little over four trips each day, with 8.4 hours a day veys in Switzerland. He described elements that may spent out of the home and only 0.02 of the trips made influence travel behavior, factors to consider in defining