Additional stratified analyses showed that in zip code areas where annual average concentrations of CO, N[O.sub.2], and P[M.sub.10] were above the 75th percentile, the DWTD effects were higher for preterm birth (results not shown).
Stratification on median values for census block-group-level SES indicators showed that the effects observed for DWTD were stronger for women residing in lower SES areas (results not shown), with the greatest differences in effect estimates observed when stratifying on the median proportion of children in poverty.
Using existing data and applying GIS methods, we estimated the DWTD measures without knowledge of disease status.
Although the DWTD model may be less accurate than complex air dispersion models, its interpretation is quite straightforward, allowing us to evaluate the relative importance of residential proximity to vehicular emissions.
There are several potential sources of exposure misclassification related to address mapping and estimation of DWTD values.
Therefore, our estimated DWTD values based on these counts reflect annual average measures only.
The DWTD values take into account only the total number of vehicles passing by a residence and do not differentiate among gasoline and diesel-fueled vehicles, vehicle speeds, and the typical age of vehicles that frequent a given street.
There is some suggestion of such an effect in our study because the relationship between DWTD and adverse birth outcomes was greater in lower SES areas.
Although DWTD values were only weakly correlated with census blockgroup--level SES indicators (Pearson correlation coefficients ranged from -0.06 to 0.05) and with years of maternal education (r = -0.03), background air pollution concentrations were related to the census SES variables (correlation coefficients ranged from -0.42 to 0.46 depending on pollutant) and maternal education (r = 0.20).
For example, for smoking to confound the observed relationships between DWTD and adverse birth outcomes, parental smoking would not only have to increase as one moves closer to heavy-traffic roadways, but this smoking pattern would also have to affect a woman's pregnancy mostly during fall/winter months.
This was the first study to evaluate the relationship between exposure to motor vehicle exhaust (measured by DWTD) and adverse birth outcomes in a large urbanized area.