In Geography, accessibility of a place is generally defined as the more or less great ease with which this place can be reached from one or several other places, by one or several individuals able to travel by means of all or part of existing transport modes.
Thus, accessibility does not only refer to the sole possibility of reaching or not a given place, it also expresses the hardness of travel, the difficulty to establish the relation, most of the time estimated by measuring space and time constraints.
For this reason, accessibility will depend not only on the respective geographical position of origin and destination places, but also on the level of service offered by the transport system(s) used to make the travel.
In the case of individual transports, into which we include walking, the level of service provided is essentially conditioned by :
structure of the network (winding and configuration of ways).
quality of the infrastructures, assessed through their technical characteristics (number and width of lanes, laying-out of verges, existence of a central separator between lanes, etc.).
topographical constraints (slope).
in force regulations.
technical features of the vehicle in use (or, to make the parallel with walking, physical condition of the person).
and congestion that disrupts the working of the system and thus makes the quality of service vary according to the days of the year and to the hours of the day.
For what regards collective modes, to the six previously mentioned explanatory factors, should be added :
service organisation in use, taking security rules into account (served nodes, frequency of services and circulation schedules),
and occupation ratio of the vehicle, which may prohibit its use when its maximal capacity is attained, in case of transport systems with obligatory reservation.
Definition of accessibility as formulated above refers to space and time assessment criteria. Other criteria might however be envisaged : economic, aesthetic, relative to landscape, to environment, to tourism, etc. The observed accessibility level is thus closely linked with the chosen measurement criteria. Besides, there are many - often complementary - indicators allowing to assess the relative importance of the different explanatory components of accessibility. Many of these indicators share as a common characteristic the preliminary computation of shortest paths following a given logic (minimisation of time, of length, of cost of travel, etc.).
Finally, in a more general acceptation, accessibility is not limited to the sole travel of individuals from one place to another. For example, in a telecommunication network, accessibility to a node refers to availability and quality of acquisition or even exchange of information from one or several other nodes.
See also : distance