Next, we estimate emission densities (in g/km²) for each U.S. census block. This process begins by creating a 250 m spatial buffer around each census block using GIS software. The buffer is intended to capture nearby roadways that might contribute to emissions exposure, even if they do not directly intersect the census block. Roadway segments that intersect the buffer are divided into two categories: those entirely within the buffered area and those partially crossing the buffer. For segments fully within the buffer, we aggregate the emissions data. We then compute the emission density for each type of vehicle traffic and pollutant by dividing the total emissions by the area of the census block. The overall emission density for all vehicle traffic, known as the total emission density, is calculated by summing the emission densities for LDV, MDV, and HDV traffic.