internal combustion engine coupled to a standard gearbox often has an output speed too high. The vehicle often has one of two things, either a very low ("granny") first gear (like the all-wheel drive Volkswagen Transporter versions) or an additional gearbox in line with the first, called a reduction drive. Some vehicles, like the Bv206 in the picture on the right, also have torque converters to further reduce the gearing. Many wheeled off-road vehicles provide power to all wheels to keep traction on slippery surfaces. For a typical four-wheel vehicle this is known as four-wheel drive. Vehicles designed for use both on and off-road may be designed to be switched between two-wheel drive and four-wheel drive so that the vehicle uses fewer driven wheels when driven on the road. Criticism of off-road vehicles Safety SUVs are built with higher ground clearance for off-road use and thus have a higher centre of gravity, therefore increasing the risk of rollover. When an SUV turns, the vehicle's mass resists the turn and carries the weight forward, thus allowing the traction from the tires to create a lateral centripetal force as the vehicle continues through the turn. The conflict between the top weight of the SUV's desire to go straight while the friction of the tires on the road causes the bottom of the vehicle to move away and out from under the vehicle during a turn. SUVs are more likely to be in rollover accidents than passenger cars. According to a study conducted in the United States, SUVs have twice the fatality rate of cars and have nearly triple the fatality rate in rollover accidents. Of vehicles in the United States, light trucks (including SUVs) represent 36 percent of all registered vehicles. They are involved in about half of the fatal two-vehicle crashes with passenger cars, and 80 percent of these fatalities are to occupants of the passenger cars. Environment In the United States, the number of ORV users since 1972 has climbed sevenfold—from five million to 36 million in 2000. Government policies that protect wilderness but also allow recreational ORV use have been the subject of some debate within the United States and other countries. All trail and off-trail activities impact natural vegetation and wildlife, which can lead to erosion, invasive species, habitat loss, and ultimately species loss decreasing an ecosystem's ability to maintain homeostasis. ORVs cause greater stress to the environment