Stationary sources have fixed positions, so it is possible to define the listeners and therefore "immission" controls are appropriate. Motor vehicles are moving sources so it is not possible to define any specific listeners so "emission" controls are appropriate. There are exceptions to this distinction. Construction equipment, and some recreation vehicles, move within a bounded area and can be considered to be time varying fixed sources. Standing motor vehicles can radiate sufficient sound to create noise disturbance. These must be treated by specific provisions.