Pokémon Ruby, Sapphire, Emerald, Omega Ruby, and Alpha Sapphire
The Mach Bike allows players to move a lot faster than regular Bicycles and the Acro Bike (16 steps per second in Generation VI). If the player is already riding the Mach Bike, then using this item causes the player to resume walking.
Riding the Mach Bike or Acro Bike is required on the Seaside Cycling Road. On the Seaside Cycling Road, a player's time and number of collisions while riding this Bicycle from the northern entrance to the southern exit are timed and recorded. This is part of the data sent over to other players via record mixing.
When this item is used, the player simply starts riding the regular Bicycle (unless the player is any place where riding the Bicycle is not allowed). If the player is already riding the Bicycle, then using this item causes the player to resume walking (unless the player is in the Cycling Road, where riding the Bicycle is required).
When the player attempts to enter the Cycling Road, the game specifically checks for the event flag indicating that the player has received the Bicycle from the Miracle Cycle shop in Cerulean City. As a result, if the player has the Mach Bike but has never received the regular Bicycle from Miracle Cycle, the player will still be unable to enter the Cycling Road.