A fixed-gear bicycle does not have a freewheel mechanism, which allows the fixed gear to move both forward and backwards. A fixed-gear bicycle also cannot coast, as the bike pedals will move whenever the bike is in motion. Fixed-gear bicycles also have only one gear, so there is no shifting. Fixed-gear bicycles are popular in urban areas--particularly with bike messengers--and require less maintenance because fewer mechanical parts are used on the bike. The most inexpensive method of building a fixed-gear bike is converting a freewheeled model. Almost any bike can be converted to fixed-gear, although road bikes with a horizontal dropout are best suited to the process.