Personal Accessibility Analysis
B1. Personal accessibility can be affected by a number of factors including:
- An individual's own mobility;
- By the physical disposition of destinations relative to the individual;
- By the availability of means of transport;
- Or by a combination of the three.
B2. There are 3 overlapping types of approach that can be used in accessibility analysis. All accessibility approaches relate to a specific location - an origin or a destination - and include zoning aspects and opportunity and deterrent features. All three approaches have a role to play in policy and project appraisal since different decisions require information to be presented in different ways.
B3. The choice of appraisal technique for any individual decision needs to be of an accuracy appropriate to the particular situation, with the resources devoted to the analysis being commensurate with the scale of the circumstances. Examples are given in Table 2 overleaf. Analysis to support practical decision making will usually benefit from a more rigorous multi-criteria framework approach. Local authorities may wish to develop supplementary planning guidance relating to accessibility.
B4. Clear policies relating to a group of people for a particular purpose can be analysed in a straightforward manner. Accessibility analysis can though become complex and confusing if the question being asked is not identified at the outset. It is important therefore for all analyses to define problems clearly, gather the required supporting information and involve the relevant stakeholders. Consistent and rigorous techniques can assist in building consensus between various stakeholders.
B5. Isochrones are used to demonstrate the geographical distribution of impacts. This is the most commonly used approach internationally. It simplifies a given problem by defining thresholds. For the approach to be useful the choice of thresholds must accurately reflect some aspect of travel behaviour which is specific to local characteristics. This is because measures of accessibility have different values in different areas.
B6. This sums all the available opportunities and takes into account a measure of deterrence related to how easily opportunities can be reached. Opportunity indices are relatively easy to interpret.
B7. This measure considers the attractiveness of the available opportunities to represent their value as a transport choice in terms of time or cost. These indices measure efficiency but are less descriptive.
B8. This is a variable element. The level of detail is dependent on the policy / proposal being examined. Strategic issues will utilise wider geographical areas resulting in a more coarse zoning system. Larger zones provide a valuable overview of areas. Local issues will utilise more detailed localised representation. This will be much more time consuming and in some cases uneconomic.
- Population: Total number of people; employment status; and age;
- Employment:Number of employees by location;
- Health:Presence or absence of a facility;
- Supermarkets:Floorspace in square footage;
- Banks/ Building Societies: Numbers of branches up to a maximum of 5 representing the availability of choice;
- Chemists: Presence or absence of a facility;
- Petrol stations:Presence or absence of a facility;
- Post officesPresence or absence of a facility.
B9. Origin accessibility considers opportunities available to an individual or business. The opportunity is therefore based on the land uses at alternative destinations.
B10. Destination accessibility considers catchments for a destination. The opportunity is therefore based on land uses and types of traveller at alternative origins.
- The need to carry goods and/or other people.
B11. These features affect the perceptions of travel and therefore influence real behavioural patterns. It is recommended that deterrence features of car available and non-car available trips is considered as many trips involve a number of modes and for non-car available trips the car options are excluded from the calculation.
Table 2. Types of Accessibility Analysis
Description and Uses
Catchment / Contour Indices
These count the number of jobs, shops etc within a thresholds travel cost (distance/time etc) from a defined location. They are used for a whole variety of planning purposes for both land use and transport infrastructure and are often used by developers to consider the commercial viability of a potential development location.
Access to Public Transport
These measure walking access time to the public transport services. Walking time or distance thresholds to the public transport services are set and summed across all the available services. The quality of public transport being accessed is categorised on a scale which takes account of service frequency, mode and reliability. (Simple but limited scope).
Peripherality Indices / Rural Accessibility
These identify thresholds in terms of cost, distance, time etc from defined types of opportunity. They are usually calculated from major centres of population (incl. towns or services such as hospitals)
Time Space Geographic Measures
These measures simplify travel behaviour and choice in terms of the opportunities available within a limited travel budget time. The threshold is therefore travel time available for an individual or group.
The simple measures above are all special forms of Hansen indices incorporating thresholds to simplify data or analysis requirements.
Similar to above but here all specified opportunities are assumed to have the same weighting. The measure is simply the sum of the cost (time etc) to each of the opportunities.
'Economic Potential' Measures
Where the opportunities being considered in the Hansen index are regional incomes, and the deterrence function is measured in distance, then the accessibility index is sometimes describes as the economic potential of a location.
Utility Based Measures
These measure the value to an individual or group of the choices available to them. The main difference to those above is that additional opportunities only provide an increase in accessibility if they provide some additional value. If there is already a surfeit of opportunities adding more will result in little change in the index. The normal units of measurement are generalised cost or time.
B12. In considering the results of the accessibility analysis it should be remembered that the measures are intended to give only a general indication of levels of accessibility. They are though of assistance in identifying practical solutions and delivering schemes that will be of real benefit.
B13. Planning authorities should establish 'accessibility profiles' for sites taking into account the elements below. The profiles should reflect the catchment area served, likely quality of service and result in relative indicators of accessibility for different sites.
- Transport system accessibility by population sector to an opportunity:
- A transport improvement or an increase in the number of opportunities will increase accessibility.
- The scale of change is not easily assessed through qualitative comparisons.
- Accessibility to public transport services:
- This is particularly useful if public transport services can be classified accurately in terms of their frequency or destination.
- Obtaining detailed data on origins, destinations and routes of services can be a major exercise.
- For accessibility of housing to public transport the recommended guidelines are less than 400m to bus services and up to 800m to rail services.
- Accessibility can be categorised, for example weak or strong.
- Accessibility to local facilities by walking and cycling:
- A maximum threshold of 1600m for walking is broadly in line with observed travel behaviour.
- If there is a significant population within 800m then improvements to the quality of walking and cycling networks will increase accessibility.
- Ratios comparing accessibility for different mobility groups:
- One of the most useful measures is the ratio of accessibility for car available to non-car available people.
- These ratios allow consistent comparisons to be made between locations.
- Accessibility for freight:
- This is best undertaken using logistics management software.
- It is usually undertaken at an individual company level.