track showed a 10% fuel savings when super singles were used. These savings are realized because less energy is wasted flexing fewer tire sidewalls. Second, the lighter overall tire weight allows a truck to be loaded with more freight. The third advantage is that the single wheel encloses less of the brake unit, which allows faster cooling and reduces brake fade. One of the major disadvantages of the super singles is that they are currently not as widely available as a standard tire. In addition, if a tire should become deflated or be destroyed, there is not another tire attached to the same hub to maintain the dynamic stability of the vehicle, as would be the case with dual wheels. With dual wheels, the remaining tire may be overloaded, but it will typically allow the vehicle to be safely stopped or driven to a repair facility. In Europe, super singles became popular when the allowed weight of semitrailer rigs was increased from 38 to 40 tonnes. In this reform the trailer industry replaced two axles with dual wheels, with three axles on wide-base single wheels. The significantly lower axle weight on super singles must be considered when comparing road wear from single versus dual wheels. The majority of super singles sold in Europe have a width of . The standard 385 tires have a legal load limit of . (Note that expensive, specially reinforced 385 tires approved for do exist. Their market share is tiny, except for mounting on the steer axle.) Skirted trailers An innovation rapidly growing in popularity is the skirted trailer. The space between the road and the bottom of the trailer frame was traditionally left open until it was realized that the turbulent air swirling under the trailer is a major source of aerodynamic drag. Three split skirt concepts were verified by the United States Environmental Protection Agency (EPA) to provide fuel savings greater than 5%, and four split skirt concepts had EPA-verified fuel savings between 4% and 5%. Skirted trailers are often combined with Underrun Protection Systems (underride guards), greatly improving safety for passenger vehicles sharing the road. Underride guard Underride protection systems can be installed at the rear, front and sides of a truck and the rear and sides of a trailer. A Rear Underrun Protection System (RUPS) is a rigid assembly hanging down from trailer's chassis, which is intended to provide some protection