Guide dogs (colloquially known in the US as seeing-eye dogs) are assistance dogs trained to lead blind or visually impaired people around obstacles. Although dogs can be trained to navigate various obstacles, they are red–green colour blind and incapable 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. In several countries guide dogs, along with most other service and hearing dogs, are exempt from regulations against the presence of animals in places such as restaurants and public transportation.