Semi-trailer aerodynamic devices are devices affixed to semi-trailers, for the purpose of reducing aerodynamic drag caused by air turbulence. The two major types of device in use are trailer skirts (or side skirts), affixed to the underside of trailers, and trailer tails (or boat tails, or rear fairings), affixed to the rear. Several such devices have been recognized by the U.S. Environmental Protection Agency's SmartWay Transport Partnership, making them eligible for funding under the Diesel Emissions Reduction Act.[1]