2012 Toyota Prius Owners Manual - Page 607

2-4. Using other driving systems Disabling pre-collision braking Pre-collision braking enabled Pre-collision braking disabled The "PCS" warning light will turn on when pre-collision braking is disabled. Radar sensor The radar sensor detects vehicles or other obstacles on or near the road ahead and determines whether a collision is imminent based on the position, speed, and heading of the obstacles. 304