than foot traffic alone, and ORV operators who attempt to test their vehicles against natural obstacles can do significantly more damage than those who follow legal trails. Illegal use of off-road vehicles has been identified as a serious land management problem ranked with dumping garbage and other forms of vandalism. Many user organizations, such as Tread Lightly! and the Sierra Club, publish and encourage appropriate trail ethics. ORVs have also been criticized for producing more pollution in areas that might normally have none. In addition to noise pollution that can cause hearing impairment and stress in wildlife, according to the U.S. Forest Service, old-style two-stroke engines (no longer a component of new off-road vehicles, although some are still in use) "emit about 20 to 33 percent of the consumed fuel through the exhaust" and "discharge from two-stroke snowmobile engines can lead to indirect pollutant deposition into the top layer of snow and subsequently into the associated surface and groundwater." In 2002, the United States Environmental Protection Agency adopted emissions standards for all-terrain vehicles that "when fully implemented in 2012... are expected to prevent the release of more than two million tons of air pollution each year—the equivalent of removing the pollution from more than 32 million cars every year." Common off-road vehicles Common commercial off-road vehicles include four-wheel-drive pickup trucks like the Ford F-Series, Chevrolet C/K, Dodge Ram, and Toyota Tacoma. A number of military vehicles have also seen civilian use, including the Jeep CJ and the AM General Hummer. Some, like the early Land Rovers, were adapted to military use from civilian specifications. Specialized commonly available off-road vehicles include Utility terrain vehicles (UTVs), All-terrain vehicles (ATVs), dirt bikes, dune buggies, rock crawlers, and sandrails. All-terrain vehicle Off-road motorcycle There are various types of off-road motorcycles, also known as dirt bikes, specially designed for off-road events. Compared to road-going motorcycles, off-road machines are simpler and lighter, having long suspension travel, high ground clearance, and rugged construction with little bodywork and no fairings for less damage in spills. Wheels (usually 21" front, 18" rear) have knobby tires, often clamped to the rim with a rim lock. Commercial, military and less common off-road vehicles European militaries and utilities have used Land Rover Defenders, Haflingers, Pinzgauers, Volvo L3314, and Mercedes-Benz Unimogs for all-terrain transportation. The Portuguese UMM Alter is less common. The military market for off-road vehicles used to be large, but, since