Auto Running, or Auto Screen-Scrolling, is a recurring game mechanic in the Sonic the Hedgehog series.
Auto Running involves the character moving forward automatically, using an "on rails" style movement, across an open path while the player controls the character's left-to-right movement and attacks. Auto Screen-Scrolling is when the screen is automatically moving and the player must keep up with it to advance in the stage. This mechanic has been used to help create a feeling of speed for the player as Sonic runs.
In Angel Island Zone Act 2, one auto screen-scrolling section occurs when Sonic is being chased by the Flying Battery which lasts until the battleship's ammo is lowered. It flies away before the boss battle with Flame Mobile.
In Mushroom Hill Zone, during the second act fight with the Jet Mobile, there's an auto screen-scrolling section. Similarly to the battleship from Sonic 3, the player has to chase Robotnik down and hit him while avoiding the spiked obstacles.
Auto Running is used in Sonic Adventure during the second phase of Speed Highway where Sonic runs down a tall skyscraper and the player must control him by maneuvering him to the left and right to avoid obstacles. There is also a very brief but similar section in Windy Valley.
Auto Running was used in the "Super-Speed Stages" in Sonic's story. This was done to create a feeling of excitement and sense of Sonic's maximum speed for the player. Here, the player controls only left and right movement, and jumping; what makes these levels difficult is that hitting a wall or obstacle will cause Sonic to lose his rings, as if hit by an enemy, and it will leave Sonic open for another hit until he regains his balance. This was also not helped with the wonky controls and the sometimes-poor collision detection.
Auto Running was used in all the Adventure levels in the game. In this game, you can customize the speed of Sonic to fit the level. This variation was more enjoyable to some, but for others it was an annoyance, since you can never freely control Sonic in the game.
Auto Running did not play as large a role in Sonic Unleashed, but it was used throughout all the daytime mini-bosses and bosses of the game. Egg Chaser and Interceptor sections were done with Auto Running, and were featured in several Acts, such as Windmill Isle Act 2 and Eggmanland (Wii/PS2 versions only), and Rooftop Run Act 1, Skyscraper Scamper Act 1 and Jungle Joyride Act 1 (360/PS3 versions only). Some downloadable content featured Auto Running levels, such as Rooftop Run Act 5 (along the whole aqueduct) and Skyscraper Scamper Act 3 (along an entire highway).
In the Console/PC version, Auto Running is used in five Modern Sonic stages, during Speed Highway while running down the building, while being chased by the G.U.N. Truck in City Escape, during the Egg Chaser sections in Rooftop Run, during the Silver Rival battle, and during the Egg Dragoon boss battle.
In the 3DS version, Auto Running takes place at the beginning of five Modern Sonic stages and at the start of two rival battles, at the end of Green Hill while escaping from the giant totem pole, in Emerald Coast while running away from the Orca, and during the Egg Emperor boss battle.
Auto Running is used in the White Park Zone boss, when fighting against Metal Sonic. Sonic and Tails begin to Auto Run after Metal Sonic ambushes them on Tails' rocket. Auto Running is also used in Death Egg Mark II Act 1, when fighting Metal Sonic and Dr. Eggman.
Auto Running is only accessible through Golden Cannons which bring Sonic into the Mach-Speed Sections. However, they were used as full Zones in Desert Ruins Zone 2 and Lava Mountain Zone 4. During the Auto Run, if Sonic were to hit a wall or an obstacle that wasn't an enemy or hazard, he would lose a life.
Auto Running occurs during certain boss battles throughout the game, like in the boss battles with Heavy Gunner and Metal Sonic. During Auto Run, the player still has access to all the playable characters' maneuvers.