Defining and measuring the impact of community severance on local accessibility

In: Presented at: 14th International Conference on Mobility and Transport for Elderly and Disabled Persons (TRANSED 2015), Lisbon, Portugal. (2015)

Abstract

Community severance (or the "barrier effect") arises when transport infrastructure (e.g. railways and motorways) or roads bearing high volume or speed of motorized traffic cut through communities, disrupting access to goods, services, and people. The impact is especially severe on older people, who are more vulnerable to losses in walking mobility, and spend more time in their home area. Despite the growing awareness of its potential impact on the health and social wellbeing of local communities, severance is not usually assessed quantitatively or assigned a monetary value in transport scheme appraisal, which tends to rely on subjective qualitative scales. There is a growing need for objective indicators that can be used in a consistent way for understanding the nature, incidence and intensity of the problem. This paper reviews critically the existing methods for quantifying severance , exploring the reasons for the slow integration of these methods into actual transport plans. A conceptual framework is proposed for decomposing the severance problem into a series of questions: a) what are the barriers restricting mobility? b) how do people move? c) where do they go? and d) who is affected? These questions are then used as a base to review the existing literature. The objective was to consolidate the knowledge that has been produced since the 1970s in several countries and which is scattered in governmental guidance documents, technical reports, and academic studies which in many cases have had limited dissemination and applications. The review revealed a tendency for the simplification over time of the methods in official guidance documents for transport appraisal. However, the concept of severance used by researchers has become wider. Several methodological issues are also unresolved. Indicators are sensitive to inputs such as the set of pedestrian destinations, the traffic volumes and speed thresholds that define severance, the walking distances and speeds, and the measurement of pedestrian routes. Severance also depends on the spatial context (urban, suburban or rural), the distance to the barriers and on how long the barrier has been present in a community. The valuation of severance using methods based on people's stated or revealed preferences poses additional challenges, as it is also difficult to disentangle the value of severance from those of other nuisances of transport such as noise and collision risk. Despite the challenges, there is great potential for the integration of quantitative measures of severance in appraisal methods such as cost-benefit analysis, multi-criteria analysis and environment and community impact assessments. However, this potential can only be realized if the development of tools by researchers adopts a multidisciplinary approach and involves the stakeholders on the problem.