Speed and volume versus purpose

The starting point is the recognition of a predominant paradigm for transport appraisal that focuses on the volume of physical movement (e.g. vehicle or passenger kilometres travelled, or numbers of trips) or its speed as the key means of assessing the performance of transport systems. This is known as the mobility perspective. Academic researchers in the field increasingly argue it should be replaced by an accessibility perspective, which measures populations’ ability to reach potential and actual destinations, pursue activities and obtain goods and services. The accessibility perspective focuses on the underlying purpose of travel rather than the act of travelling itself.

The mobility perspective can be seen as focused on transport systems and on the quantity or speed of movement rather than the underlying needs and desires that motivate travel. It is a much-repeated axiom that travel is a derived demand, i.e. it is the means to reach desired destinations and pursue activities that are the primary demands. Using the volume of movement as the benefit indicator arguably obscures this basic insight. By contrast, adopting the accessibility perspective explicitly recognises this and refocuses analysis on people’s ability to obtain access to the services they need and want.

Adopting accessibility, rather than mobility, as the primary indicator in transport planning is intended to ensure that the focus is on the underlying needs of travellers, rather than short-term mobility demand. By the same logic, it should also more effectively take account of the indirect effects of transport infrastructure choices on the whole of society rather than simply on users of the transport system. It could also provide scope for considering a wider range of policy instruments, such as those that increase access while reducing the need for mobility. These can include various changes to land use regulation (e.g. facilitating mixed-use developments and increasing density), as well as measures that enhance virtual, rather than physical, accessibility.

The accessibility perspective is increasingly adopted in policy discussion about the strategic objectives of projects and programmes. However, it plays little part in guiding transport infrastructure planning and project appraisal decisions, with the mobility perspective continuing to dominate formal project appraisal and selection processes (ITF, 2020).

The state of the art in accessibility analysis

The dominance of the mobility perspective is underpinned by its prescription of a single, pre-eminent analytical tool: CBA (cost-benefit analysis).

CBA centres on the calculation of expected time savings and the determination of their monetised value, both of which are easily understood concepts in that they relate readily to that part of people’s daily lives spent trying to save time and money. They provide a simple narrative, which is readily understood by policy makers, stakeholders and the public: it is good to save time, and this has a value, so it is worth spending money to secure this benefit (ITF, 2020).

By contrast, there is no pre-eminent accessibility indicator that is widely accepted as the appropriate starting point for project appraisal practice. Rather, there is a wide range of indicators, each of which is more or less well suited to different uses. However, many provide metrics whose meaning is not readily or intuitively understood. Geurs (2019) identifies four distinct types of accessibility metrics: infrastructure based, location based, utility based and person based.

Widely used in engineering, infrastructure-based metrics include travel speed and congestion indices and typically measure the “sum of all impedances” (i.e. time and cost) of moving from a particular point to all other relevant points, thus giving an accessibility measure for that point.

Location-based measures can be broadly distinguished from infrastructure-based measures in that they incorporate both transport and land-use components rather than focusing specifically on the transport components. Location-based metrics can be used from an origin or destination perspective. Cumulative opportunity measures the raw number of opportunities available within a given travel time. Gravity-based opportunity measures by contrast, do not include an arbitrary cut-off point but discount accessibility benefits by time taken to reach the opportunity.

Following economic utility theory, utility-based metrics focus on analysing the welfare benefits that people derive from access to spatially-distributed activities. The best-known is the “logsum” measure of consumer surplus, which measures the difference between the cost of travel and the value of the access provided. Because it directly measures the expected utility derived from access, it can be readily integrated in a CBA framework.

Lastly, person-based metrics take into account the fact that activity has both spatial and temporal dimensions – i.e. activities occur at specific locations for finite temporal durations. They measure accessibility in terms of the space-time feasibility of opportunities available to an individual, using the volume of the three-dimensional space-time prism or the number of opportunities in its projection on planar space (i.e. potential path space [PPS]) as indicators. These indicators are person-specific. However, these measures have not found their way into planning practice most likely because they are not easily understood without graphic illustrations.

Institutional and cultural factors

Some authors have identified the institutional dynamics of transport planning as one of the factors that tends to cement continued use of mobility-based approaches and constrain the adoption of accessibility-based alternatives. Levine (2019) argues that mobility models and metrics retain their central role in transport and land-use policy in part because they represent professional norms in the transportation field: “Practitioners bound to uphold their professional norms by legal or ethical constraints, or by simple convention, would find themselves hard pressed to deviate from established standards such as these.” Similarly, Martens (2019) describes transport planning in terms of an institution, with set rules and professional norms and limited opportunities for departing from them.

Martens argues that the rules are political in nature, as they address the attention of planners to some issues over others, embodying particular perspectives on what counts as a transport problem and what possible solutions exist, or what benefits are worth pursuing. This perspective implies the need for political action to drive step changes in approach.

How accessibility measures should be deployed

Concern that the current mobility-based approaches to transport planning do not adequately weigh the need to ensure an acceptable minimum standard of accessibility underlies much of the advocacy of a shift toward an accessibility perspective. Project appraisal and selection based on CBA and related methods will frequently result in investment choices that confer accessibility gains predominantly on those who are already well served in this regard, while doing little or nothing to assist those with the lowest levels of accessibility. This will occur wherever projects demonstrating the highest level of access gain per unit of cost have this characteristic and reflects the fact that CBA is not inherently concerned with distributional questions. Accessibility-based evaluation is not immune from this weakness, but it has the advantage of readily supporting distributional analysis across individuals, groups, and locations (ITF, 2020).

Van Wee and Geurs (2011) point out that the social justice implications of inequalities can be evaluated from the differing philosophical perspectives of Utilitarianism, Egalitarianism and Sufficientarianism (broadly, Sufficientarianism as a theory of distributive justice aims to make sure each of us has enough).

Geurs (2019) argues these different perspectives can each be relevant for different policy purposes. Utilitarian framing, for example, can be useful for many areas of policy delivery, particularly where the focus is on maximising the total benefits of an investment for all members of society. Conversely, when a proposed investment has a specific focus on achieving greater equity, whether for specific groups or in relation to deprived areas, egalitarian and sufficientarian approaches are more relevant (Lucas et al., 2016). If either of the latter two approaches is adopted, the distribution of changes in accessibility that arise from a particular investment become at least as important an analytical consideration as the total size of the changes.

 Conclusions

Adopting the accessibility perspective implies a broader scope of analysis in project appraisal, including consideration of environmental, health, safety and distributional perspectives. The accessibility paradigm provides a robust approach to identifying which communities and individuals suffer most from poor accessibility, whereas standard economic analysis does not usually address these factors. By implication it also enables a focus on the size of the benefits that can be delivered to the least well served – i.e. the equity gains to be made – through adopting policies based on these metrics.

References

Geurs, K. (2019). Accessibility and transport appraisal: approaches and limitations. International Transport Forum Discussion Papers, No. 2020/22, OECD Publishing, Paris.

ITF. (2020). Accessibility and Transport Appraisal: Summary and Conclusions, ITF Roundtable Reports, No. 182, OECD Publishing, Paris.

Levine, J. (2019). The accessibility shift: conceptual obstacles and how to overcome (one of) them. International Transport Forum Discussion Papers, No. 2020/23, OECD Publishing, Paris.

Martens, K. (2019). A people-centered approach to accessibility. International Transport Forum Discussion Papers, No. 2020/24, OECD Publishing, Paris.

Van Wee, B., & Geurs, K. (2011). Discussing equity and social exclusion in accessibility evaluations. European Journal of Transport and Infrastructure Research, 11(4), 350-367.