The Ford Explorer is a full-size sport utility vehicle produced by the American manufacturer Ford since 1991, and since 2011 based on a crossover platform. The Ford Explorer became one of the most popular sport utility vehicles on the road. The model years 1991 through 2010 were traditional body-on-frame, mid-size SUVs. For the 2011 model year, Ford moved the Explorer to a more modern unibody, full-size crossover SUV platform, the same Volvo-derived platform the Ford Flex and Ford Taurus use although with significantly greater ground clearance. It is slotted between the traditional body-on-frame, full-size Ford Expedition and the mid-size CUV Ford Edge. Although outwardly similar, the fifth generation Explorer, Ford Edge and Ford Escape do not share platforms. The fifth generation Explorer does, however, share platforms with the Ford Flex and Lincoln MKT. The original Explorer has also been involved in controversy, after a spate of fatal rollover accidents in the 1990s involving Explorers fitted with Firestone tires. Both two-door Explorer Sport and four-door models of Explorer have been sold. Part-time four-wheel drive is an available option, and since 1995 this has been a 'shift on the fly' system with full protection against being engaged at high speed. A specially modified Special Service Vehicle version is also available from Ford Fleet for law enforcement agencies, fire departments, and EMS agencies. Explorer was also the name of a trim package offered on the Ford F-Series trucks from 1968 to 1986. The 2011 Ford Explorer was named North American Truck of the Year.