Factors affecting blind mobility

This thesis contains a survey of the mobility problems of blind people, experimental analysis and evaluation of these problems and suggestions for ways in which the evaluation of mobility performance and the design of mobility aids may be improved. The survey revealed a low level of mobility among blind people, with no significant improvement since a comparable survey in 1967. A group of self taught cane users were identified and their mobility was shown to be poor or potentially dangerous. Existing measures of mobility were unable to detect improvements in performance above that achieved by competent long cane users. By using newly devised measures of environmental awareness and of gait, the advantages of the Sonic Pathfinder were demonstrated. Existing measures of psychological stress were unsatisfactory. Heart rate is affected by physical effort and has been shown to be a poor indicator of moment-to-moment stress in blind mobility. Analysis of secondary task errors showed that they occurred while obstacles were being negotiated. They did not measure stress due to anticipation of obstacles or of danger. In contrast, step length, stride time and particularly speed all show significant anticipatory effects. The energy expended in walking a given distance is least at the walker's preferred speed. When guided, blind people walk at this most efficient pace. It is therefore suggested that the ratio of actual to preferred speed is the best measure of efficiency in mobility. Both guide dogs and aids which enhance preview allow pedestrians to walk at, or close to, their preferred speed. Further experiments are needed to establish the extent to which psychological stress is present during blind mobility, since none of the conventional measures, such as heart rate and mood checklists show consistent effects. Walking speed may well prove to be the most useful measure of such stress