Traffic-related air pollutants are a significant public health concern near freeways. Previous studies have suggested that soundwall and/or vegetation barriers (defined here as any substantial installation of vegetation on either side of the sound barrier, trees or tall bushes etc.) may reduce near-freeway air pollution, but the literature is inconsistent, and data for vegetation and other conditions common in California are very limited. Here the researchers combine mobile and stationary measurement and modeling approaches to evaluate the impact of various barrier configurations at four sites in California, and make pair-wise comparisons of the following configurations: no wall, sound wall only, vegetation only, and combined vegetation-soundwall barriers (eight study locations in total, each with a perpendicular transect). If present, trees were substantially taller than the solid barriers, and were planted outside of the wall in some locations and inside on others, if any. Chosen study sites were located along major highways in Santa Monica, Encino, Sacramento and Riverside. Three of these sites were chosen as test sites for daytime conditions (Sacramento, Encino and Riverside) and one was chosen for nighttime and early morning conditions (Santa Monica).