Defining accessibility

Accessibility is concerned with both the land use and the transport system. As discussed in the introduction to this report, the exact definition of what is accessibility is elusive. A selection of definitions of accessibility from various studies, listed from earliest to latest, is:

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)

...it is some generalised measure of ease of interaction. (Harris 1966)

Accessibility denotes the ease with which any land use activity can be reached from a location using a particular transport system. (Koenig 1980)

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 and Pretty 1990)

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, or the ease with which locations of interest can be reached for desired interaction. (Helling 1995)

Or put another way, and as deduced overall from Part 1, ‘accessibility’ is defined by the authors as:

The ease with which activities, either economic or social, can be reached or accessed by people.

Therefore, accessibility assessment is the measurement of how easy it is for an individual to participate in desired activities, based on a set of measurable factors, including mode and destination choice.

Accessibility using transport systems includes three interrelated components: ‘Capability’, ‘Opportunity’ and ‘Mobility’:

1      ‘Capability’ represents the ability of people to use the transportation network, for example a bus with a low floor provides a capability for mobility-impaired people’s ease of boarding and access to the public transport network. Similarly being licensed to drive and having access to a vehicle enables people to use the road network.

2      ‘Opportunity’ represents the availability of a land use activity or service, for example the presence of a supermarket provides an opportunity for shopping, and a school or college provides an opportunity for education.

3      ‘Mobility’ represents the ease of moving through the various transportation networks, for example congestion on a highway often represents the level of mobility for vehicles. The amount of delay when crossing the street often represents the level of mobility for pedestrians. Terms such as ‘level of service’, ‘average network speed’ and ‘vehicle operating capacities’ are frequently used to describe mobility.

It is this last term ‘mobility’ that transportation professionals will be the most familiar with (especially from the perspective of motorised vehicles). A significant amount of study has been undertaken in this area and mobility continues to remain a major area of research. Unfortunately mobility is often the only way the quality of the transportation system can be measured as accessibility is generally ill-defined. As a result, the interaction between capability, opportunity and mobility is not well understood.

Accessibility depends on the relationship between these three components and is concerned with both the land use and the transport system. The achievement of accessibility is ‘access’ but access by itself doesn’t describe the quality of choice or ease of being able to reach the destination. For example access to a medical practitioner could be achieved in a number of ways, such as walking to a bus stop, travelling to a rail station, waiting for a train, travelling on the train, alighting at the nearest station and travelling by taxi to the intended destination. It could also mean a house call by the medical practitioner. In both cases access is achieved, but clearly one provides a higher level of accessibility or ease by which the activity is reached. Accessibility provides an integrated way of measuring changes in either the land use or transport system. All practical approaches for New Zealand therefore include capability, opportunity and mobility as mentioned earlier.