This study evaluates the effects of neighborhood characteristics on motor vehicle usage per household (autos/HH) and total vehicles miles travelled annually per household (VMT/HH). It first defines four neighborhood descriptors that a priori influence personal transportation costs. These are:
- Residential density, or the number of dwelling units per residential land area.
- Transit accessibility. An index of transit accessibility is defined and measured for the neighborhoods under study.
- Neighborhood shopping. An index is developed that defines the ability to perform neighborhood shopping errands with a short walking trip from a home.
- Pedestrian accessibility. Factors that encourage or discourage walking are combined into an index that is quantified for the neighborhoods under study.
In addition, mean household income and household size (people/HH) are evaluated as explanatory variables for transportation costs.
Twenty-eight communities in California, representing a diverse range of variation in the four neighborhood variables being tested, were selected for this analysis. Census data were used to estimate the mean number of autos per household for each community. Analysis of smog check data according to the zip code of auto ownership were used to compute vehicle miles travelled per automobile. Methods described in the text were used to define and evaluate the other neighborhood variables.
Once the characteristics of these neighborhoods were defined, statistical methods were employed to provide the best explanation of the observed values of autos/HH and VMT/HH.
Overview of Results
Using the variables estimated in this study, it is possible to project automobile ownership and usage, and thus average costs, with good reliability. The best statistical correlations we were able to find are based only on density and the transit accessibility index; adding other variables or replacing these variables with others did not explain usage data any better than these two alone. The results obtained were:
Autos/HH = 2.704*(density)-.25 R2 = 0.85; and
VMT/HH = 34,270*(density)-.25*TAI-.076 R2 = 0.83.
Using these two equations, and estimates for annual fixed costs of car ownership and variable costs of an additional mile of driving, allow the calculation of a matrix of average annual household auto expense as a function of density and the transit accessibility index. These are presented in Table 8 of the text.
The report also includes a Survey of Previous Studies:
- Density Based Studies
- Community Characteristics Promoting Transit and Walking
- Walking Distances To Transit
Wednesday, June 1, 1994