Future research beyond the scope of this assessment could further corroborate our findings and build on our results. First, CTM simulations could usefully validate our core results, especially as they concern the behavior of secondary PM2.5 from vehicle emissions. Second, it would be helpful to quantify the effect of decades of vehicle emissions controls on other air pollutants that are relevant to the health of overburdened communities, including nitrogen oxides, diesel PM, and air toxics. Third, although in-situ observations of TRAPs have historically not been available at sufficiently high spatial resolution to systematically characterize changes in disparity, careful analysis of data at particular locations may be able to complement our statewide insights. Moreover, as hyperlocal measurements of traffic-related air pollutants become more widespread, these types of observational studies may be more feasible in the future.