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Several studies have found that exposure to traffic-generated air pollution is associated with several adverse health effects. Field studies, laboratory experiments, and numerical simulations indicate that roadside barriers represent a practical method of mitigating the impact of vehicle emissions because near road concentrations are significantly reduced downwind of a barrier relative to concentrations in the absence of a barrier. These studies also show that the major effects of barriers on concentrations are: 1) the concentration is well mixed over a height roughly proportional to the barrier height, and this effect persists over several barrier heights downwind, 2) the turbulence that spreads the plume vertically is increased downwind of the barrier, 3) the pollutant is lofted above the top of the barrier. This paper ties these effects together using two semi-empirical dispersion models. These models provide good descriptions of concentrations measured in a wind tunnel study and a tracer field study. Their performance is best during neutral and stable conditions. The models overestimate concentrations near the barrier during unstable conditions. We illustrate an application of these models by estimating the effect of barrier height on concentrations during neutral, stable, and unstable conditions. (C) 2014 Elsevier Ltd. All rights reserved.