Light-speed circuits are a gimmick that appear in Shadow the Hedgehog. They are circuits within cyberspace which can be used for transportation.
In cyberspace, light-speed circuits appear as straight red lines, often with orange energy circuits running next to them. When coming into contact with one in cyberspace, the users will be transmitted as electron particles, allowing them to traverse at high speeds. According to Shadow, using the light-speed circuit gives a sensation of weightlessness.
Light-speed circuits appear exclusively in Digital Circuit and Mad Matrix. Using them in gameplay allows the players to take several alternate paths to other sections of the Stages that will provide them with Rings and power-ups. There are also often firewalls on these circuits that will try to block the players' paths.
In Mad Matrix, there are several Black Arms bombs set up across a maze-like light-speed circuit section which the player must detonate by passing by them in order to complete the Dark Mission.
↑ 1.01.1Sonic Team (November 18, 2005). Shadow the Hedgehog. PlayStation 2. Sega. Area/Level: Digital Circuit. "Rouge the Bat: This is a light-speed circuit. Touch it, and your body is transmitted as electron particles."
↑Sonic Team (November 18, 2005). Shadow the Hedgehog. PlayStation 2. Sega. Area/Level: Digital Circuit. "Shadow the Hedgehog: What is this sensation?! I feel weightless atop this light..."