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自行車，或稱腳踏車，單車，自動車，鐵馬，是一種以人力踩踏腳踏板驅動的小型陸上車輛。自行車通常是雙輪的，但也有三輪的（主要用於貨運，相對少見）。在台灣和中國大陸，通常稱為“自行車”，“鐵馬”，台灣也稱“孔明車”，“腳踏車”;江浙，上海等
地亦有「腳踏車」一稱;在香港，澳門，廣東，廣西， “中國南方地區則更常稱為為”單車“;在新加坡，馬來西亞，廣東潮汕地區則稱為”腳車“，江西贛語更稱之為”線車（甩）“，”鋼絲車“ “腳踏車嘚”。
一般人日常騎車（按步行出力）的速度約15〜25公里每小時（9.3〜15.5英里每小時），經過訓練的人騎行專用車可以達到45公里每小時（28英里每小時）的速度速度增加空氣阻力）;騎車能源效率很高，消耗能量和二氧化碳，平均每公里不到汽車的十分之一。
A bicycle, often called a bike or cycle, is a human-powered, pedal-driven, single-track vehicle, having two wheels attached to a frame, one behind the other. A bicycle rider is called a cyclist, or bicyclist.
Bicycles were introduced in the 19th century in Europe and as of 2003, more than 1 billion have been produced worldwide, twice as many as the number of automobiles that have been produced. They are the principal means of
transportation in many regions. They also provide a popular form of recreation, and have been adapted for use as children's toys, general fitness, military and police applications, courier services, and bicycle racing.
The basic shape and configuration of a typical upright or "safety bicycle", has changed little since the first chain-driven model was developed around 1885. But many details have been improved, especially since the advent of modern
materials and computer-aided design. These have allowed for a proliferation of specialized designs for many types of cycling.
