From Sonic Retro
The signpost, also known as the Goal Post, Goal Plate or Clear Panel (クリアパネル), is one of the staples of the Sonic the Hedgehog series. Marking the end of a non-boss level in the 2D games, the mechanics for the sign differ slightly from title to title. In the modern 3D titles, Goal Rings or other similar indicators are used instead.
In the original Sonic the Hedgehog, Sonic the Hedgehog 2 and Sonic the Hedgehog CD, at the end of the acts preceding the final a signpost sits to greet you, adorned with the image of Dr. Eggman. Once touched, the sign will begin to spin, resting on an image of the character you are playing as, signifying your victory over Eggman. Unique to Sonic CD is an additional sign signifying you are close to the end of the level, giving the player enough warning if they want to go back and explore the level before being locked onto the final screen. Once the player passes this additional sign, they will not be able to backtrack and any active time warp will be cancelled.
In Sonic the Hedgehog 3 and Sonic & Knuckles, the signpost appears after defeating the first Act's sub-boss, twirling as it falls from the sky. Jumping into the sign causes it to bounce back up into the air, adding 100 points to the score with each jump. If it happens to fall on the ground on one of three specific spots, it will cause a secret item box to appear, also causing the sign to bounce back up into the air. When it finally lands on solid ground, the face of the character being played will appear. Also, while it is possible to catch a glimpse of Dr. Eggman on the sign during this process, it is impossible to have the sign land on his visage.
In addition to marking the end of an Act, the signposts in the Game Gear and (when applicable) their Master System counterparts have a reward system built in, giving the player a prize based on what image appears on the sign post. In such games, the signpost is called the Bonus Panel, or the Bonus Plate (ボーナスプレート) in Japanese. In the first two 8-bit games, the signpost Sonic spots begins with a question mark, touching it causing it to reveal its secrets. Common to both is an image of a ring which gives Sonic an additional ten rings, a portrait of Sonic which grants an additional life and a portrait of Eggman which offers no reward. In the first Sonic the Hedgehog, the only other image possible is an exclamation point, which allows Sonic to enter the Special Stage. In the numbered sequel, the exclamation point was removed, being replaced with a sign featuring Tails, which provides a Continue.
In Sonic Chaos and Sonic Triple Trouble, the question mark is replaced with Eggman, adhering to the tradition in the Mega Drive titles. However, the prize system remains, the standard image now being that of a Flicky. The images of Sonic, Tails, and (in the second game) Knuckles appears, the first two providing different results based on the character being used (getting a portrait of the playing character grants a life, getting a portrait of the other playable character grants a Continue in Chaos or 10,000 points in Triple Trouble, and getting Knuckles grants a Continue). A Chaos Emerald may show on the signpost, giving the player 50 rings at the start of the next Act. Also, on occasion, the panel would land incorrectly, and its back would be shown, in which case it had to be spun again. Sonic Blast removes the Flicky, returning the Eggman sign to the "no-prize" status, the untouched sign being blank. The prize given for getting the opposite playable character's face (e.g. getting Sonic's face as Knuckles) has been changed to thirty rings. The game also introduces a Super Sonic image to the mix, which gives the player an extra life and thirty rings.
The Sega 32X title Chaotix, though using a sign to mark the end of a non-boss Act, decided to simplify matters, using only a silhouette of Eggman on one side and the word "Clear" on the other, deciding against drawing up multiple signs for the seven playable characters in the game. The Neo Geo Pocket Color title Sonic Pocket Adventure gave a nod to the signposts from Sonic the Hedgehog 3 by having the sign fall from the sky, but in the processing removing the interactivity and making Sonic's face the only possible outcome. The Game Boy Advance titles make it even simpler, as the sign at the end of Sonic Advance has only the Eggman logo on one side, the character running past and never seeing if anything else may come up; though in VS. Mode, the player's character stops by the signpost as it reveals a two-fingered "peace" sign.
The classic style of the sign would not return until Sonic the Hedgehog 4: Episode I, which emulates the way the sign worked in Sonic the Hedgehog and Sonic 2. In Episode II, the sign will always stop on Sonic in single player, but it can stop on Tails if he is the one to pass the signpost first in Multiplayer Mode. In Episode Metal, the signpost will initially display Sonic instead of Eggman, but once the player spins it, it will stop on Metal Sonic's face.
While most 3D games in the series have neglected the signpost, Sonic and the Black Knight decided to use one to mark the end of certain levels, though only providing the word "goal" at the end with no images of any of the characters adorning either side. The signpost does return in Sonic Generations and Sonic Forces at the end of stages as Classic Sonic, in conjunction with the Goal Ring for Modern Sonic. Crisis City and Planet Wisp in Generations even feature Challenge Acts that require Classic Sonic to juggle the signpost until it lands in the designated goal area within the time limit. If the signpost lands anywhere else, it will stop on Eggman's face and give the player a 5-second time penalty.
Sonic Mania has the signpost return to its behaviour from Sonic 3 & Knuckles where it will fall from the sky after a mini-boss is defeated, allowing the player to juggle it for points and hidden Item Boxes; though in some Zones such as Studiopolis Zone, the signpost will be stationed on the ground to the right of where the sub-boss was fought, showing Eggman's face, and will be sent flying when run into at a high-enough speed like in 8-bit Sonic titles. In Competition mode, the signpost is always grounded, and will display the face of the first character to reach it once it has been spun.