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~~i give a 25 percent reduction in per capita VMT as density is doubled.s Similarly, a travel survey in the Greater Toronto Area suggested that doubling density results in a decrease in per capita VMT of about 25 percent.' A comparison of cities in Washington state found housing density, population density, jobs-housing balance and retail-housing balance to co-vary and to be associated with reduced driving.g But the cost of such surveys limits them to too few households to provide a statistical analysis of all the important variables at neighborhood level. The above studies were for much larger areas than neighborhoods, limiting their ability to measure the effects of neighborhood characteristics like density, transit service and pedestrian and bicycle friendliness. A 1990 study of five communities, ranging from traditional to suburban, in the San Francisco region used actual odometer readings.9 The study found that high residential density, nearby shopping, good transit and a good walking environment go together, while low density zones lacked all these. The co-variance of these variables increases the difficulty of disentangling their effects, but does allow density to capture much of the effects of the others. The residents of higher density communities drove 20 to 30 percent per household less every time neighborhood density doubled. Nob Hill, at 32 times higher household density as San Ramon, had 1/4 its household auto ownership and VMT. However, the limited number of .communities studied prevented disentangling the effects of residential density, nearby shopping, transit and the walking environment. Using a household travel survey in the Seattle area, Frank and Pivo found that employment density, population density, land-use mix and jobs-housing balance are associated with less auto use.'° These relationships held up even when household demographics, car ownership and transit are controlled. A study of 27 neighborhoods in San Francisco, Los Angeles, San Diego and Sacramento, using odometer readings, found that doubling residential density cut auto ownership 16%, while doubling public transit service reduced VMT an additional 5%." With density as a surrogate for all the variables, doubling residential density cut VMT 20%. Kara Kockelman, in a study of over 1000 travel analysis zones and 1,200 census tracts in the San Francisco Bay Area, found that the following influence household VMT: household size, auto ownership, income, weighted jobs within 30 minutes, dissimilarity of the zone's major land use from its neighbors, and the balance of land uses within each zone within a half mile.12 Robert Dunphy and Kimberly Fisher calculated the average VMT from the 1990 National Personal Transportation Survey (NPTS) for households whose ZIP codes had the same population density.13 For the five density ranges above 4000 persons/square mile (about 5 households/residential acre), their Table 4 shows a decrease of 38% in daily driving as density doubles. These studies have shown a strong relationship between urban form and driving. Of course it makes intuitive sense that people living in neighborhoods originally built for the convenience of residents without private automobiles would have lower auto ownership and driving. But which 3