In these chapters it will often appear that more attention has been given to blind pedestrians and the task they perform than to pedestrians with useful remaining vision and to the task they perform. We are well aware of the important perceptual differences between blind and visually impaired pedestrians and the important differences between the tasks they perform. Many investigators believe, however, that an initial emphasis on the problems confronting blind pedestrians is the best approach to take in seeking a basic understanding of the mobility problem in general. The task confronting blind pedestrians is the task of independently traveling from a starting place to a destination. This may appear, at first glance, to be a simple definition, but when the information and abilities needed for its performance are taken into account, the apparent simplicity vanishes.
Mobility is more than the ability to avoid obstacles while moving aimlessly through space.
Mobility worthy of serious interest is that undertaken with a purpose in mind, that of reaching a destination, and, if pedestrians are to reach destinations, they must know not only what those destinations are, but also where they are. They must be oriented, and they must be able to maintain the currency of orientation as they move through space. Relevant information must be differentiated from a background of irrelevant, often masking stimulation. The requirements of the task are not adequately specified by information acquired from the space in which the task is performed while the task is in progress, and the blind pedestrian must supplement this information with information retrieved from memory. Finally, the observation of task performance is often difficult because the behavior to be observed is embedded in highly integrated sequences of behavior, and because critical task components must often be inferred from their behavioral consequences.