This paper discusses in depth the relationship between accessibility and usability in product design. It presents a definition of accessibility and introduces the concept of ‘usable accessibility.’
Recent regulations have brought accessibility to the forefront of interface design for many organizations producing websites, software, and hardware. While accessibility considerations cross into design, development, and testing, it is often human factors engineers and usability specialists who hold the primary responsibility for accessibility. Indeed, accessibility is most effectively implemented when considered early and throughout a user-centered design process.
Many usability specialists are given the responsibility for accessibility with little prior knowledge in the area. As such, many ‘myths’ and misunderstandings complicate the usability specialists’ jobs. Additionally, usability specialists responsible for accessibility are faced with new legal and technical issues. This paper explores the relationship between usability and accessibility. The resources section at the end lists sources for information on other aspects of accessibility, such as legal issues.
At the most basic level, accessibility is about people being able to access and use a product. For example, web accessibility is about designing web pages that people can present and interact with according to their needs and preferences. A primary focus of accessibility is access by people with disabilities. The larger scope of accessibility includes benefits to people without disabilities.
While accessibility is presented here in its relationship to usability, it is important to remember that the fundamental point is the ability to use the product at all. What is nice to have for some people is required by other people to be able to use products. Usability often assumes accessibility.
Accessibility can be approached is a subset of usability. Put simply, usability means designing a user interface that is effective, efficient, and satisfying. In this context of usability, accessibility means designing a user interface to be effective, efficient, and satisfying for more people in more situations. However, satisfaction is much less an issue with accessibility. Accessibility is more concerned with making interfaces perceivable, operable, and understandable. That interaction is effective and efficient becomes more important when products are part of education and employment.
While many would agree with the concept of accessibility as related to usability, that is often not how accessibility is approached in practice. Many designers and developers were recently introduced to accessibility because of regulations such as Section 508 of the Rehabilitation Act in the US. In such cases, the motivation for accessibility is often limited to meeting standards and guidelines for accessibility. Many times this puts the focus on the technical aspects of accessibility, and the human interaction aspect is lost.
This problem can be avoided by adopting the broader definition of accessibility as a guiding principle. Instead of focusing only on the technical aspects of accessibility, it is important to recognize that usability is also an important aspect of accessibility. Consciously addressing 'usable accessibility' helps clarify the difference between what meets minimum accessibility standards and what is usable by people with disabilities.
Setting a goal of usable accessibility will impact how you develop and evaluate solutions. For example, it has been said that you can meet Section 508 web accessibility standards by focusing only on the markup. However, to evaluate for usable accessibility, you need to actually interact with the rendered web pages in various configurations, preferably including usability testing that involves participants with disabilities. Usability testing for accessibility is one aspect of accessibility considerations that fit into UCD.
User-centered design (UCD) is an established and proven (although unfortunately not universally followed) process for designing mainstream hardware, software, and web interfaces that considers usability goals and the users' characteristics, environment, tasks, and workflow in the design of an interface.
While usability and UCD have been topics of books, conference presentations, and education programs over the last decade, design that considers the needs of people with disabilities is still relatively uncommon in education and practice. As a result, the range of users who can use products and the situations in which products can be used is less inclusive than if the needs of people with disabilities were considered in design. At issue is the range of user characteristics and enviroments that designers define in the design process, consciously or unconsciously.
Without a formal process to consider others, it is common to design for ourselves. Therefore, many products are designed based on the individual designer's preferences, abilites, and environment. Especially in web development, large percentage of designers are young, without disabilites, experienced with computers, and operating with the latest technologies. Thus, all too often, that is the user profile they tend to design for.
Even when specific user analysis is conducted, the range of users considered is often too narrow. Primarily because of a simple lack or awareness, designers tend not to include people with disabilites and people operating in more unusual environments in their user analysis.
In order to design inclusively, designers need to consider the widest range of possible users and enviroments. For example, the Web Content Accessibility Guidelines (WCAG) notes that many users may be operating in contexts very different from the designers' context:
When all possible users and environments are considered in product design, the process can be called inclusive design. (This does not mean that all user circumstances should be given equal weight in design. Certainly it is best that designs are optimized for the most common configurations, characteristics, and situations and are flexible to accommodate other configurations.) Inclusive design as the integration of accessibility into a UCD process is similar to "design for all" (commonly used in Europe) and "universal design" (used more broadly in the US).
The term universal design was originally used in association with buildings, and has more recently been used in describing an approach to accessibility for information and communication technologies (ICT). Vanderheiden has defined it as such:
"Universal design is the process of creating products (devices, environments, systems, and processes) which are usable by people with the widest possible range of abilities, operating within the widest possible range of situations (environments, conditions, and circumstances), as is commercially practical." 
Universal design refers to a process, rather than the resulting product. When implementing such a process, it becomes clear that some design elements that are merely good, nice-to-haves for usability are very important when considering access by people with disabilities.
The beginning of this chapter listed elements of usability and noted that they are good for accessibility as well. The converse is also true, as demonstrated by looking at formal accessibility guidelines. For example, consider the following web guidelines from the W3C:
It would be good if all websites followed these guidelines. They are aspects of usability, yet, these specific guidelines come from the Web Content Accessibility Guidelines (WCAG); they are web accessibility guidelines.
Certainly, in developing guidelines specifically for accessibility, it is often difficult to distinguish between usability and accessibility. It becomes clear that many design aspects that are good for general usability are required for accessibility.
Consistent navigation and presentation across a website is one example of a design aspect that is good for usability and much more important for accessibility. Imagine using a site where the navigation is inconsistent. You have to skim the page to find the navigation in a different location each time. If you have good vision and are using the site on a high-resolution monitor, inconsistent navigation presentation is a minor inconvenience. It requires only visually skimming the page, or maybe one or two mouse clicks to scroll down the page.
However, if you have no vision, the inconsistency is a much greater problem; you cannot visually skim the page. People who are blind and use a screen reader hear only one word at a time, rather than seeing the entire web page at once. People with other visual impairments see only a small portion of a web page at a time, such as people using significant screen magnification or people with extreme tunnel vision (rather like looking through a drinking straw).
People without disabilities can face similar constraints. Web phones and personal digital assistants (PDAs) – which are increasingly being used to access websites – have very small display screens. Web page navigation that may be visible in one screen on a common monitor may not fit in one screen on a different device.
In all these cases, consistent navigation impacts the usability of the website even more when the user is operating under constraints from disabilities or devices. This parallelism between constraints from disabilities and constraints from devices is one of the reasons why differentiating between usability and accessibility is difficult and usually unnecessary.
When designing products, it is rarely useful to differentiate between usability and accessibility. There are times when such a distinction is considered, such as when looking at discrimination against people with disabilities and when defining and addressing specific accessibility standards and guidelines. The following problem definitions distinguish the difference between usability and accessibility:
The distinction between usability and accessibility is especially difficult to define when considering cognitive and language disabilities. It is further blurred by the fact that features and functionality for people with disabilities benefit people without disabilities because of situational limitations.
The Differentiating Between Usability and Accessibility section distinguished accessibility as related to disability. A broader definition of accessibility, provided below, is more suitable in most circumstances.
Accessibility can be defined as the quality of a product that makes it possible for people to use the product even when they are functioning under limiting conditions or constraints. Accessibility is about designing so that more people can use your product effectively in more situations.
Although most people consider accessibility in terms of disability, that is not the whole picture. A more broad definition of accessibility covers people operating under situational limitations as well as functional limitations:
Most of the legal requirements for accessibility are concerned with meeting the needs of people with functional limitations, or disabilities. Understanding the benefits of accessibility to people operating with situational limitations clarifies some of the benefits to people without disabilities and the related business benefits of accessibility. Two seemingly different objectives – designing systems that work in a wide range of environments, and designing systems for a wide range of user characteristics – have similar solutions.
The issues of accessible design are essentially the same whether you are concerned about designing a mobile Internet appliance with input and output limitations, designing a public kiosk to be accessible to people with disabilities, or designing a web application targeted to seniors. Accessible products include those that can be used hands-free, eyes-free, or ears-free (requirements of many mobile products) and by people who are in noisy or dark environments. If we design a system that is truly universal and mobile, we will have created a system that is accessible to almost anyone with a physical or sensory disability. 
Curb cuts (the slope from a sidewalk to a street) are a common example from the physical world of how solutions for disability access are similar to solutions for situational limitation access. Curb cuts are primarily intended to accommodate people in wheelchairs. Curb cuts also benefit people with situational limitations that make it difficult to get over a curb. In fact, most curb cuts are used more often by people without physical disabilities – pushing strollers or shopping carts, riding a bicycle or roller blades, pulling luggage or appliances dollies – than by people in wheelchairs. The curb cuts example illustrates how people without disabilities benefit from a design for people with disabilities.
Understanding the relationship between accessibility and usability is helpful in integrating accessibility into design processes and business practices.