Noise barriers can be applied to existing or planned surface transportation projects. They are one of the most effective actions taken in retrofitting existing roadways and commonly can reduce adjacent land-use sound levels by up to ten decibels. A computer model is required to design the barrier since terrain, micrometeorology and other locale-specific factors make the endeavor a very complex undertaking. For example, a roadway in cut or strong prevailing winds can produce a setting where atmospheric sound propagation is unfavorable to any noise barrier.