Access measures must not only include physical mobility, but also a person’s ability to interpret, recognize, and understand key landmarks and choice points and the layout and function of environments (Golledge, Loomis, & Klatzky, 1997) . Since individuals with disability might conceive and use objective space in subjective ways, standard distance or network accessibility models do not capture the world as used by many in this group.
Many measures of accessibility were reviewed in Chapter 2. Throughout this paper, a variety of access measurements have been used to document limitations on travel caused by vision loss. Since network and distance-based measures do not capture individual differences, it is important to look at group or person-based measures and use methods to explain various constraints and barriers to access that are not revealed in conventional measures. Chapter 4 dealt with measures and models comparing accessibility of blind people to a base-line sighted person’s performance, and also between the control group and those in the test condition. To better understand the access limitations of the blind, or other disabled groups, it is necessary to know more than the fact that it takes them longer to travel; one should compare those travel times and efforts to the typical sighted and ambulatory user to empirically determine what and where these limitations are. In addition, measuring results from mitigating techniques can be done to determine any positive changes to accessibility. In this report, comparing the actions of the blind to the sighted revealed locations that served to block or delay travel, revealing that there was not an overall “penalty” to blind travel, but, rather, that specific areas of the environment caused more restrictions than others. In addition, comparing regular blind travel to technology-aided travel also demonstrated which areas of travel are most restricted by lack of vision and how technologies could mitigate these limitations. Whether travel times, activity participation rates, independent travel, ability to make transfers, or reported difficulty of tasks were examined, this method of comparison revealed specific areas for further research and mitigation. Techniques that reveal access differences, whether between conditions (Clark-Carter et al., 1986; Golledge & Marston, 1999) , or between the typical user and a person or group with a disability (Church & Marston, in press; Golledge et al., 1999; Okunuki et al., 1998) , define accessibility much better than conventional measures.
The use of a distance decay formulation helps identify specific problems faced by people with particular disabilities. Comparisons of the impedance coefficients can empirically measure the degree of restrictions and the effect of any corrective improvements. For the blind, the path distance might be the same, but the travel effort can be increased by the lack of cues. For those using wheelchairs, the distance is often longer, due to barriers in the environment. By using impedance coefficients, quantitative measures can be produced for comparison and possible remediation purposes. This technique was used to produce a “penalty” measurement to empirically determine the restrictions to access and how limitations directly affect travel time and effort.
In addition to impedance, the distance decay model also considers the magnitude of attractions. Higher levels of a location’s attraction theoretically induce more travel. Since some blind people report very few trips, this could imply that the blind give different salience to locations than the typical user, and a lower attraction coefficient must therefore be used to model their trip-making frequencies. On the other hand, prior knowledge of a location might well make that place much more attractive as compared to an equal and closer alternative that is unfamiliar. Ease of orientation and navigation, safety concerns, closeness to known transit, or even a familiar layout or menu can affect the attractiveness, and these differences should be considered when using this type of model.
Accessibility can also be studied by looking at constraints put on the task of participating and traveling (Hägerstrand, 1970) . Hägerstrand offered three such constraints, those of coupling, capacity, and authority . These have been used to explain disability access problems such as vision loss (Marston et al., 1997) and ambulatory limitation caused by MS (Thapar, 1999; Thapar, Bhardwaj, & Bhardwaj, 2001) . The loss of independence associated with many disabilities increases many kinds of capacity and coupling constraints. Waiting for travel assistance or helpers affects access, and, for those who cannot drive a car, the dependence on transit adds further to these constraints. Missing a transit vehicle by a few seconds might delay access and travel for an hour or more, and the coupling required at the other end of the trip might also be constrained.
Typical utility measures can also be used to evaluate access for the disabled, but, again, these are not always the same for this population. It has been noted that the desire to save time is overshadowed by other concerns. For the many in this group who are unemployed, the standard measures of money and time valuations might need to be adjusted. Utilities such as safety, comfort, and familiarity appear to be much stronger for this group.
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