Guide dogs, (also known as service animals or assistance animals), are assistance dogs trained to lead blind and visually impaired people around obstacles. Although the dogs can be trained to navigate various obstacles, lots of them are (red–green) color blind and are not capable of interpreting street signs. The human does the directing, based on skills acquired through previous mobility training. The handler might be likened to an aircraft's navigator, who must know how to get from one place to another, and the dog is the pilot, who gets them there safely.