Accessibility

With accessibility we mean how well a certain area can be reached.

Definition
Accessibility (or just access) in transportation planning refers to the ease of reaching goods, services, activities and destinations, which together are called opportunities. For example, is there sufficient road capacity and are there sufficient parking spaces? Are there good options for public transport? What is the average travel time to reach the centre? Access to these locations is the goal of mobility. The ease of reaching these locations is determined by sufficient road capacity and sufficient parking spaces. Accessibility can be defined in terms of potential  (opportunities that could be reached) or in terms of  activity  (opportunities that are  reached).

Other meanings
The words accessibility and  access  can have various meanings and implications.
 * In the physical design of urban objects, such as transport modes or buildings, accessibility is described as access and egress which is the rate or means of entry or exit of an urban object.
 * In roadway engineering, access refers to connections to adjacent properties.  Limited access roads have minimal connections to adjacent properties, while local roads provide direct access. Access management involves controlling the number of intersections and driveways on a highway.
 * In the fields of geography and urban economics, accessibility refers to the relative ease of reaching a particular location or area.
 * In pedestrian planning and facility design accessible design  (also called universal design ) refers to facilities designed to accommodate people with disabilities. For example, a pathway designed to accommodate people in wheelchairs may be called accessible.
 * In social planning, accessibility  refers to people’s ability to use services and opportunities.

Mathematical approach
A formula that is often used is to measure accessibility in a traffic zone j is :

$$ Accessibility_j = \sum_j {Opportunities_j } \times f\left( {C_{ij} } \right) $$ where: weighted more than farther or more expensive places).
 * $$i$$ = index of origin zones
 * $$j$$ = index of destination zones
 * $$f\left( {C_{ij} } \right)$$ = function of generalized travel cost (so that nearer or less expensive places are

In words, the accessibility of a certain destination equals the opportunities in that destination multiplied with the generalized travel costs from each origin to that destination. A destination with many opportunities and short travel times therefore has a higher accessibility than a desination with fewer opportunities and/or longer travel times.

In transport economics, the generalized cost is the sum of the monetary and non-monetary costs (=monetarized travel time, value of time) of a journey.

If the accessibility of a certain location is low, though there are sufficient opportunties, one should consider to improve the accessibility, e.g. by adding public transportation opportunities, lowering the costs of public transport, increasing road capacity, creating more parking spaces etc.

Also for security, accessibility is important. A city with a good accessibility has good possibilities to get emergency services to an incident location, or to get people quickly out of the area in case of any danger.

Factors that influence accessibility
Transportation demand refers to the amount of mobility and accessibility people would consume under various conditions. Transportation activity refers to the amount of mobility and accessibility people actually experience. Transportation options or modes of transport refer to the quantity and quality of transport modes and services available in a particular situation. Improving the quality and quantity of the modes of transportation means improving accessibility. Robustness of the transportation network has an influence on the accessibility.

Sometime, a particular factor significantly affects accessibility. For example, inadequate information or poor security around public transport terminals can constrain public transport use (potential riders don’t know how to use it or have exaggerated fears of discomfort and risk).

Related subjects

 * Access and egress
 * Robustness