Development Department Research Programme Research Findings No. 89Accessibility: Review of Measuring Techniques and their Application
Derek Halden, David McGuigan, Andrew Nisbet and Alan McKinnon Derek Halden Consultancy
The primary aim of transport is to enable people and businesses to gain access to jobs, shops, friends, family, and many other activities. The ease with which these activities can be accessed depends not just upon transport systems but upon patterns of land use. Accessibility measures seek to define the level of opportunity and choice taking account of both the existence of opportunities, and the transport options available to reach them. This research project examined the current use of accessibility analysis in Scotland and, through the use of a number of case studies, demonstrated the potential for developing their use in the future.
- Current use of Accessibility Analysis is largely qualitative.
- For the purposes of practical application, the various ways of measuring accessibility can be classified generically as: Simple Measures; Opportunity Measures; and Value Measures.
- All of these measures have a role to play in different circumstances, for example:
- Accessibility to local facilities by walking and cycling;
- Accessibility to public transport services;
- Transport system accessibility to opportunities such as jobs, education, etc.
- Comparison of accessibility by alternative modes
- Accessibility for freight
- Use of Accessibility Measures could aid public policy decision making at national, regional, and local levels.
- At a national level, accessibility analysis could be used to define overall performance levels and targets for improvement, including for the trunk road network and the strategic rail network, in addition to checking that local and regional strategies are consistent with national guidelines.
- At a regional level, accessibility analysis could be used to check the impacts of joint plans or specific schemes (such as congestion charging) geographically or on types of traveller. In addition, Structure Plans could include targets for accessibility.
- At a local level, accessibility analysis can demonstrate the extent to which transport proposals support economic, social and environmental policy objectives.
- Accessibility analysis techniques are evolving and offer an opportunity to develop a framework for a more robust economic analysis of transport policy where motorised and non-motorised modes are treated consistently, and which is consistent with recent SACTRA recommendations for more rigorous analysis of the rationale for transport investment within economic analysis.
If the aim of national policy is to improve travel choices, then accessibility measurements have the potential to provide as direct an indicator as is theoretically possible of whether these aims are being achieved. Accessibility measures seek to define the level of opportunity and choice, taking account of both the existence of opportunities, and the transport options available to reach them. Good accessibility as a transport objective also carries very broad support from every strand of opinion within society. There are therefore very practical advantages in demonstrating the effects of transport projects and plans in terms of accessibility.
Research for the DETR 1 provides a comprehensive theoretical structure for the development of accessibility analysis. This earlier research demonstrates how measures of accessibility could underpin analysis under three of the Government's new appraisal criteria - integration, economy and accessibility.
New national guidance on transport and planning also suggests the need for a more integrated approach in assessing how different modes are affected by land use changes.
This project considers the need for accessibility analysis in land use and transport appraisal, summarises current practice for such analysis in Scotland, demonstrates the use of such techniques through case studies, and suggests how guidance could widen the use of the techniques.
Current land use and transport appraisal guidance requires accessibility to be considered in the following circumstances:
- To local facilities by walking and cycling
- To public transport services
- To opportunities such as jobs, education, shops etc.
- In comparing accessibility by alternative modes
- For the planning of freight transport
Figure 1 shows how accessibility measures build upon demand assessments within land use and transport planning to guide scheme implementation towards achieving policy objectives.
Figure 1 - Accessibility Measures with Land Use and Transport Planning
The study comprised a review of accessibility analysis techniques, a telephone survey of public and private sector practitioners, and four case studies to demonstrate the use of accessibility measures.
The review identified how the available techniques could be used to meet current land use and transport planning needs.
The telephone survey sought views on accessibility analysis in Scotland from 29 relevant organisations representing developers, consultants, local authorities and transport operators. These surveys identified that accessibility issues were viewed as an important part of integrated transport appraisal, but that detailed guidance would be needed if new quantitative techniques were to become an established part of the decision making process.
The case studies illustrated how accessibility techniques could be applied to policy and scheme appraisal in practical situations.
Four case studies were undertaken covering a variety of geographical locations, demographic issues, and type of land use and transport change. Practical approaches to data collection and analysis were identified, tailored to the policy needs of each case study. In each case origin and destination accessibility was considered separately. Various trip purposes and population sectors were examined using the three main forms of index.
All accessibility measures relate to a specific location, origin or destination, and include representation of defined opportunities and a separation element (usually a function of time and cost) between these opportunities and the location. The opportunity terms, separation functions and the sizes of the zones for considering accessibility need to be expressed at a level of detail appropriate for the needs of the particular assessment.
For the purposes of the practical application of these measures, there are three generic but overlapping types of indicator:
Simple measures are fairly easy to understand e.g. the number of a particular type of opportunity within a defined time threshold. They are most useful for local walking and cycling opportunities including access to public transport services. For these trips there is some basis for the time thresholds in the analysis. For motorised trips by car and public transport, thresholds founded in real travel behaviour are much harder to define. Results, therefore, need to be viewed with caution as they can be more sensitive to the choice of threshold than to changes in accessibility.
Opportunity measures have the benefit of being easy to understand since, like the Simple measures, they are expressed in terms of number of jobs, number of people etc. They have many potential uses including: the comparison of accessibility changes for different population groups, the identification of the catchment for a destination, and the comparison of accessibility for car available and non-car available trips.
Value or utility based indices are expressed in generalised time or cost, so findings can be more difficult to interpret. However by providing a direct measure of the value of transport systems they are powerful appraisal tools. They can be used in similar ways to the opportunity measures and can provide a complete measure of the benefits of a particular scenario.
Practical Application of the Measures
The case studies undertaken during this research demonstrate that quantitative accessibility analysis is both practical and useful. It has considerable potential for widespread application in transport and development planning. There are various practical issues for appraisal of land use and transport schemes as described below.
Accessibility to local facilities (and public transport) by walking and cycling
A maximum threshold of 1600 metres for walk-in catchments is broadly in line with observed travel behaviour. Within the 0-1600 metres range the deterrent effect of distance is less clear. Scottish Executive Planning Advice suggests thresholds of 400 metres and 800 metres for accessibility of housing to the public transport network. Simple indices using such thresholds should provide as clear an indication as is generally needed of the walk-in catchment for local facilities and public transport.
Quality of walk and cycle access is also important. Where there is a significant walk-in catchment population, improvements to infrastructure quality will be a major factor in improving accessibility.
Transport system accessibility to opportunities
A transport improvement or an increase in the number of opportunities will increase accessibility. Practical analysis depends upon a manageable approach to data assembly. Land use data is easier to obtain than transport data and successful analysis depends upon an appropriate choice of zoning system.
Simple measures are of less use than the Opportunity or Value measures since they require the same data but give less accurate results. Opportunity indices are easy to interpret showing, for example, how many more people can access a new development if a transport improvement is made. Value indices, whilst a more powerful measure of the efficiency of the transport network, are less descriptive of the change.
Ratios comparing accessibility for different mobility groups
A major strength of accessibility analysis is that it focuses on people rather than transport modes. The case studies compared accessibility for people who had access to a car with people who did not. In each of the case studies, car available travellers had very much better accessibility than those who were dependent upon public transport, walking and cycling. However, even for car available trips, public transport was competitive with the car for some trips.
One of the most helpful measures is the ratio of accessibility for car available to non-car available people. These allow consistent comparisons to be made between locations.
Accessibility considerations are also important in planning for freight transport. Public policy and commercial aims are to improve the economic efficiency of supply chains taking account of the social and environmental impacts of various options.
Proprietary logistics management software includes accessibility analysis and will be the preferred tool in many cases. Where a simpler approach is required, mapping can be undertaken using iso-cost and iso-chrone techniques.
It should be recognised, as this research has found, that accessibility analysis techniques are evolving as their development and use becomes more widespread amongst practitioners. The case studies consider a range of accessibility measurement techniques and these confirm that such an analysis is both practical and useful. The research report, which accompanies these findings, includes a user friendly software routine for PC, which allows the measurement techniques discussed in this research to be used in practical applications. It is hoped that this will assist the evolution and further development of accessibility analysis techniques.
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1 David Simmonds Consultancy, University of Leeds, MVA, Oxford Brookes University.1998. Accessibility as a Criterion for Project and Policy Appraisal. Unpublished Report for the Department of Environment, Transport and the Regions.