stations.2 Similar threshold information and data are needed to determine what development densities and land use patterns are optimal to support walking and bicycling. In contrast to transit use, encouraging more pedestrian and bicycle travel appears to depend more on neighborhood land use design and the presence of local shopping (TRB 2005).
Studies of changing housing and travel preferences: Studies of the housing preferences and travel patterns of an aging population, new immigrant groups, and young adults are needed to help determine whether future trends will differ from those of the past. Part of the difficulty of estimating reductions in VMT, energy use, and CO2 emissions from more compact development stems from the uncertainties involved in forecasting the residential location preferences and travel patterns of the population by 2050. For example, the baby boom generation will begin to sell off its large supply of low-density suburban housing within the next decade, but how will they downsize—to smaller-lot single-family units in suburban retirement communities or to apartments in more central walkable locations? And to what extent will they drive less relative to preceding cohorts of retirees? Monitoring the location preferences and travel behavior of this large group is critical in identifying opportunities for more compact development and alternatives to automobile travel. Similar monitoring of the residential preferences and travel behavior of immigrant populations and young adults, about which little is known, is also important. Finally, it would be useful to collect more data on how residents trade off travel and housing costs in making residential location decisions, particularly in the effort to find affordable housing.
If ambitious goals to reduce energy use and CO2 emissions necessitate changes in land development patterns, the research outlined above should provide a more precise understanding of future household