In transport planning accessibility refers to a measure of the ease of reaching (and interacting with) destinations or activities distributed in space, e.g. around a city or country. Accessibility is generally associated with a place (or places) of origin. A place with "high accessibility" is one from which many destinations can be reached, or destinations can be reached with relative ease. "Low accessibility" implies that relatively few destinations can be reached for a given amount of time/effort/cost or that reaching destinations is more difficult or costly from that place.
The concept can also be defined in the other direction, and we can speak of a place having accessibility from some set of surrounding places. For example, one could measure the accessibility of a store to customers as well as the accessibility of a potential customer to some set of stores.
In time geography, accessibility has also been defined as "person based" rather than "place based", were one would consider a person's access to some type of amenity through the course of their day as they move through space. For example, a person might live in a food desert but have easy access to a grocery store from their place of work.
In general, accessibility is defined as:
- = index of origin locations
- = index of destination locations
- = a set of weights associated with destinations e.g. the number of jobs in a traffic analysis zone
- is a cost of travel from to and
- is an impedance function on the travel cost giving the utility of a destination.
Travel cost metrics ( in the equation above) can take a variety of forms such as:
- Euclidean Distance
- Network distance
- Travel time
- Monetary cost or fare
- Comfort or subjective ease of travel
- Internal and/or external costs
Cost metrics may also be defined using any combination of these or other metrics. For a non-motorized mode of transport, such as walking or cycling, the generalized travel cost may include additional factors such as safety or gradient. The essential idea is to define a function that describes the ease of travelling from any origin to any destination .
The function on the travel cost determines how accessible a destination is based on the travel cost associated with reaching that destination. Two common impedance functions are "cumulative opportunities" and a negative exponential function. Cumulative opportunities is a binary function yielding 1 if an opportunity can be reached within some threshold and 0 otherwise. It is defined as:
where is the threshold parameter.
A negative exponential impedance function can be defined as:
where is a parameter defining how quickly the function decays with distance.
Relation to land use
Accessibility has long been associated with land-use; as accessibility increases in a given place, the utility of developing the land increases. This association is often used in integrated transport and landuse forecasting models.
Transport for London utilize a calculated approach known as Public Transport Accessibility Level (PTAL) that uses the distance from any point to the nearest public transport stops, and service frequency at those stops, to assess the accessibility of a site to public transport services.
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