This standard establishes performance and equipment requirements for braking systems on vehicles equipped with air brake systems. Purpose. The purpose of this standard is to insure safe braking performance under normal and emergency conditions. Application. This standard applies to trucks@ buses@ and trailers equipped with air brake systems. However@ it does not apply to: (a) Any trailer that has a width of more than 102.36 inches with extendable equipment in the fully retracted position and is equipped with two short track axles in a line across the width of the trailer. (b) Any vehicle equipped with an axle that has a gross axle weight rating (GAWR) of 29@000 pounds or more; (c) Any truck or bus that has a speed attainable in 2 miles of not more than 33 mph; (d) Any truck that has a speed attainable in 2 miles of not more than 45 mph@ an unloaded vehicle weight that is not less than 95 percent of its gross vehicle weight rating (GVWR)@ and no capacity to carry occupants other than the driver and operating crew; (e) Any trailer that has a GVWR of more than 120@000 pounds and whose body conforms to that described in the definition of heavy hauler trailer set forth in S4; (f) Any trailer that has an unloaded vehicle weight which is not less than 95 percent of its GVWR; and (g) Any load divider dolly.