The contemporary model of disability explicitly recognizes that both the social and physical environment are factors in the disablement process (see, for example, World Health Organization 2001; Brandt and Pope 1997) and that the process is not a direct causal relationship but, rather, highly probabilistic, i.e. impairment may have different impacts depending on the person, the environment and the resources available. This model recognizes that the social and physical environment is an enabling context that has a great impact on the experience of disability and the process of rehabilitation. It also recognizes that the process of disablement is actually universal and highly variable. Environment, as in the case of any child who has no way of reaching a school, can create limitations on activity and participation, even without the presence of impairment. Furthermore, the impact on two people with the same impairment can be very different, depending on personal factors. For example, a family who can afford private transportation could bring their child to school if there was no accessible public transit, while a family without those means cannot.

Accessible design can be defined as design that does not discriminate against people with disabilities. Universal design, in contrast, is generally defined using the "Mace" definition, originating in the United States, as "the design of products and environments to be usable by all people, to the greatest extent possible, without the need for adaptation or specialized design" (Mace 1985, Mullick & Steinfeld 1997, Ostroff 2001).

Source:

http://www.amazon.com/Universal-Design-Creating-Inclusive-Environments/dp/0470399139