and commercial developments that will be built between now and the end of the forecast period. How many new housing units are likely to be built, and how many of those new units will be in compact, mixed-use developments? The demographic, economic, and political factors that affect the quantity and character of new development were the subject of Chapter 4 of this report.
The second set of assumptions concerns the number of vehicle miles driven by households in different types of developments. Will the number of vehicle miles driven by the average household in existing types of developments continue to increase as it has in the past, or will it slow down or stop? And how many fewer vehicle miles will households in the new compact, mixed-use developments travel? The empirical evidence on the reduction in VMT attributable to compact, mixed-use development was summarized in Chapter 3 of this report. The committee did not account for any behavioral feedback effects, but the sensitivity of key assumptions is tested.
If estimates of reductions in VMT are to be translated into savings in energy use and CO2 emissions, one must make a further set of assumptions about the fuels and fuel economy of future vehicles. Will cars continue to be powered by internal combustion engines or hybrids running on fossil fuels, or will all-electric, hydrogen, or other more novel forms of propulsion emerge to play a significant role? And whatever fuels are used, what will be their carbon content and CO2 emissions per VMT? The savings in energy and CO2 emissions from the use of vehicles that do not use fossil fuels will depend in part on the energy source. For example, the electricity to run an electric vehicle may be generated by a CO2-emitting, coal-fired electric plant. Ideally, the full life-cycle costs of alternative energy sources should be considered in computing energy and emissions savings.
Two previous studies attempt to estimate the reduction in VMT that might result from more compact development. In Costs of Sprawl—2000,