"cyclists", "bicyclists", or "bikers". Apart from two-wheeled bicycles, "cycling" also includes the riding of unicycles, tricycles, quadricycles, recumbent and similar human-powered vehicles (HPVs).They are the principal means of transportation in many parts of the world. Cycling is widely regarded as an effective and efficient mode of transportation optimal for short to moderate distances. Bicycles provide numerous possible benefits in comparison with motor vehicles, including the sustained physical exercise involved in cycling, easier parking, increased maneuverability, and access to roads, bike paths and rural trails. Cycling also offers a reduced consumption of fossil fuels, less air or noise pollution, reduced greenhouse gas emissions, and greatly reduced traffic congestion.In addition, cycling provides a variety of health benefits. Cycling on stationary bikes have also been used as part of rehabilitation for lower limb injuries, particularly after hip surgery. Individuals who cycle regularly have also reported mental health improvements, including less perceived stress and better vitality. Among the disadvantages of cycling are the requirement of bicycles (excepting tricycles or quadricycles) to be balanced by the rider in order to remain upright, the reduced protection in crashes in comparison to motor vehicles, often longer travel time (except in densely populated areas), vulnerability to weather conditions, difficulty in transporting passengers, and the fact that a basic level of fitness is required for cycling moderate to long distances.