The theory and practice of accessibility analysis has been a long standing interest for TransPosition, that started with work done by Ken Davidson in the 1960s, and continued through the work done by Peter Davidson on random utility models and accessibility.
TransPosition offers accessibility analysis services - more information can be found here.
Definitions of Accessibility
Accessibility is a term that has been applied to various different characteristics of the land use and/or transport system. Accessibility is always understood to be the ease of getting to something, and the various uses of the term differ in the types of things that can be reached. At the most operational level, accessibility can refer to the physical ease with which various elements of the transport system can be reached. At this level, accessibility is primarily a design issue (see for example Ryan and McNally(1995) - “Accessibility of Neotraditional Neighbourhoods”), and often there is a focus on people with disabilities, or particular emphasis on public transport. In order to prevent confusion, many people refer to this as access rather than accessibility (for example Replogle (1992) – “Bicycle Access to Public Transportation: Learning from Abroad”).
Accessibility can also be used to describe these same sorts of access issues, but at a system-wide or policy level. Tyler (1993) uses the term accessibility to refer both to the physical ease of getting onto vehicles, and also the availability of transport system as a whole:
“In recent years, the term accessibility has been concerned with the access to elements of the transport system (for example, to vehicles), with an emphasis on certain sectors of the population (for example, the elderly or the mobility impaired). Accessibility, however, is also a matter of accessibility to the transport system as a whole. In this context, accessibility includes not only the consideration of access to vehicles - step heights, hand rails, seating arrangements and so on – but also the consideration of transport system in terms of time and space from the perspective of the user. The term accessibility can be considered in a wider context than is often accepted, and should include matters of frequency, network design, interchange policy, safety and cost. It is important to stress that this perception of accessibility includes all the aspects of the current understanding of the term.”
This interpretation of accessibility is still concerned with obtaining access to the transport system, not to the activities that can be reached by the transport system. But the transport system is only a means to an end, and only really exists to let people reach other activities, as this is the reason they travel. It is possible to have a transport system that is highly accessible, but which does not allow people to easily reach the places to which they want to travel. The only way of measuring how well the transport system serves people’s needs is to consider not only the transport system, but also the distribution of activities that can be reached by the transport system.
To avoid confusion, we will refer to the lower level meaning as access, and reserve the term accessibility for the combined land use/transport description. Much of the literature seems to adopt this practice, with only a small number of papers using the term accessibility to describe what we would call access, and only one paper using access to describe what we would call accessibility, Martinez(1995) – “Access: The transport-land use economic link”.
Since 1959, there have been a number of papers published on accessibility, each with a slightly different focus, and many with alternative measures and definitions of accessibility. Although there is some variety in the measures of accessibility, there is broad agreement as to its meaning. Following is a selection of definitions or descriptions of accessibility taken from the literature.
..accessibility is a measurement of the spatial distribution of activities about a point, adjusted for the ability and the desire of people or firms to overcome spatial separation.
-- Hansen, 1959
Accessibility, according to a definition proposed by Dalvi(1978), denotes the ease with which any land-use activity can be reached from a location using a particular transport system.
-- Koenig, 1980
..accessibility, or the ease with which locations of interest can be reached for desired interaction.
-- Helling, 1995
.. it is “some generalised measure of ease of interaction
-- Harris, 1966
Accessibility is a characteristic which can be possessed by both a point in space, or a region (i.e. it can be point specific or integral, the latter being a summary measure of the individual accessibilities of all points in a region); which can be considered at various levels of aggregation (e.g. accessibility to a particular activity or to all activities; by one mode or all modes); which may be measured in terms of a number of different attributes (i.e. time, money and other level of service characteristics such as comfort, frequency, safety etc.); and which is perceived differently by different individuals (for example, travel time is valued more highly by some people than by others).
-- Peacock, 1993
Accessibility typically refers to the ease with which desired destinations may be reached and is frequently measured as a function of the available opportunities, moderated by some level of impedance.
-- Niemeier, 1997
A powerful aspect of the accessibility concept is that is combines in a single, simple measure the relevant characteristic both of the land use and the transport system. Thus any change in either system will, in general, lead to a change in accessibility at every point within the are of the system.
-- Davidson,K , 1977
Accessibility is concerned with the opportunity that an individual has to partake of a particular activity or set of activities. It is not concerned with behaviour, but with the opportunity, or potential that people at a particular location have of interacting with different types of land use.
-- Davidson,P and Pretty, 1990
From these, and other papers, it is possible to distill the following properties of accessibility
Accessibility is concerned with both the land use and the transport system, and provides an integrated way of measuring changes to either system.
Accessibility considers the desirability of travel, not the actual travel that occurs. It is thus concerned more with the potential of travel, and accessibility can be determined for areas even when no one lives there.
Accessibility is calculated with respect to a particular set of activities and a particular set of travel costs. Accessibility is TO an activity set and BY a transport system.
Different individuals may experience different accessibility because their choice set may be different, their perception of the network costs may be different, and their preferences may be different.
Accessibility may be improved by decreasing the costs of interaction, or by providing new activities. Either of these changes will have a positive (or zero) effect on accessibility.