Sport utility vehicle (SUV) is a category of motor vehicles that combine elements of road-going passenger cars with features from off-road vehicles, such as raised ground clearance and four-wheel drive. There is no commonly agreed definition of an SUV, and usage varies between countries. Some definitions claim that an SUV must be built on a light truck chassis; however, broader definitions consider any vehicle with off-road design features to be an SUV. A crossover SUV is defined as an SUV built with a unibody construction (as per passenger cars), however in many cases crossovers are simply referred to as SUVs. In some countries—such as the United States—SUVs have been classified as "light trucks", resulting in more lenient regulations compared to passenger cars. The predecessors to SUVs date back to military and low-volume models from the late 1930s, and the four-wheel drive station wagons and carryalls that began to be introduced in 1949. The 1984 Jeep Cherokee (XJ) is considered to be the first SUV in the modern style. Most SUVs produced today use unibody construction (as per passenger cars); however, in the past many SUVs used body-on-frame construction. During the late 1990s and early 2000s, the popularity of SUVs greatly increased, often at the expense of the popularity of large sedans and station wagons. More recently, smaller SUVs, mid-size and crossovers have become increasingly popular. SUVs are currently the world's largest automotive segment and accounted for 36.8% of the world's passenger car market in 2017. SUVs are criticised for their impact on other road users and the environment. Elements of their design introduce safety risks both to their drivers and to others, and they are considered a leading cause of the increase in pedestrian deaths in the United States over the past decade. They are typically less fuel efficient than smaller vehicles, thus contributing more to global warming.