We all extoll the abilities of assistance dogs when seeing, hearing or reading about their near heroic capacity for taking care of their charge.  This leads many to wonder how one trains such dogs without realizing the function of an assistance dog as well as the capabilities and limitations they bring to their jobs.  Take, for example, the guide dogs for the blind.
Dogs selected for assistance need intelligence, willingness to learn, ability to concentrate for long periods of time, and a controllable activity level.  Signs of aggression, nervous temperament, fearfulness, or aggression toward other dogs or animals will get them removed from the program.
All guide dogs are reared in a puppy raising home until they are 14 to 16 months old so they are partially “pre-trained” and socialized upon entering the program.  They stay at the training facility for 4 to 6 months where they go through intensive training using praise, petting, play, and non-food-based rewards.  Clicker training with food rewards is used in the early stages of the training program but then both clicker and food are phased out as training progresses.
Before training even begins, the needs of the handicapped person is assessed to determine which specific actions a guide dog can perform to fill the voids.  Precise, trainable, behaviors must be specified as you can’t tell the dog to help out where needed.  First the dog must learn the basic commands for navigating the environment such as forward, left, right, stop, and backup.  The dog must be able to lead his charge in a straight line from point A to point B, and stop for all changes in elevation like curbs and stairs where he must stop and wait for his companion to feel for the edge and respond.  These stops are important, not only for safety, but to also orient the person as counting curbs is the way blind people know where they are and how close they are to their intended destination.  The assistant dog is required to check overhead for obstructions like low hanging tree limbs, to lead their charge around obstacles and avoid narrow spaces where the dog and his companion cannot walk side by side.
Assistance dogs cannot determine the route to a new destination nor can they read traffic signals.  The blind person must determine when it is safe to cross a street by listening to the sound of traffic.  Every guide dog is trained in “intelligent disobedience” which means that if the dog feels that any command given will result in exposing his companion and himself to danger, he will refuse the command and wait until it is safe to proceed.
In conclusion, as with the guide dog for the blind, the dog is not a substitute for vision but rather to assist the visually impaired person to move safely through the world much like normally sighted people do.