Cycling, also called bicycling or biking, is the use of bicycles for transport, recreation, or for sport. Persons engaged in cycling are referred to as "cyclists", "bikers", or less commonly, as "bicyclists". Apart from two-wheeled bicycles, "cycling" also includes the riding of unicycles, tricycles, quadracycles, and similar human-powered vehicles (HPVs).Bicycles were introduced in the 19th century and now number about one billion worldwide. They are the principal means of transportation in many parts of the world.Cycling is widely regarded as a very effective and efficient mode of transportation optimal for short to moderate distances. Bicycles provide numerous benefits by comparison with motor vehicles, including the sustained physical exercise necessarily involved in cycling, that cycling involves a reduced consumption of fossil fuels, less air or noise pollution, much reduced traffic congestion, easier parking, greater maneuverability, and access to both roads and paths. The advantages also include reduced financial cost to the user as well as to society at large (negligible damage to roads, less road area required). By fitting bicycle racks on the front of busses, transit agencies can significantly increase the areas they can serve.Among the disadvantages of cycling are the inherent instability of the bicycle, the immensely reduced protection in crashes (especially in collisions with motor vehicles), longer travel time (except in densely populated areas), vulnerability to weather conditions, difficulty in transporting passengers, competition (and interference) with socially beneficial forms of mass public transit,[citation needed] and the particular levels of skill and fitness required by cycling.