Development of context-sensitive accessibility indicators: a GIS-based modelling approach for Cape Town

Abstract

Adequate public transport infrastructure and services are essential to facilitate access to basic opportunities, such as jobs, healthcare, education, recreation or shopping, especially in low-income cities where the majority of the low-income population have no access to the car. In the context of transport exclusion and urban poverty, access and accessibility metrics can serve as good indicators for the identification of transport-disadvantaged zones or population groups in a city. In Cape Town, accessibility-based planning is being embraced by the authority as a means of addressing the planning defects of the past apartheid regime, which created a city that is spatially fragmented by race and income levels. Among the agenda outlined in its 5-year Integrated Transport Plan of 2013-2018, is the need to develop a highly integrated public transport network in which all households would have equitable access to the public transport system, especially for the majority of the urban poor who reside in the city outskirts far from major economic centres. Although planning efforts are being made to redeem the defects of the past, there is still the need for tools and indicators to understand the current situation, as well as to further aid planning and decision making about land-use and transport. The objective of this research, therefore, is to develop suitable indicators of accessibility, identify possible spatial and socioeconomic drivers of accessibility and evaluate equity in the distribution of accessibility benefits for various population groups in Cape Town. In the study, transport network data of Cape Town are utilised to develop GIS-based indicators of network access and origin accessibility to various opportunities like jobs, healthcare and education, across various modes of travel. An Access Index measures public transport service presence within a zone, based on route and stops availability. The index is used to compare the coverage levels provided by each mode of public transport in the city. Also, an Accessibility Index is proposed, that measures the number of opportunities 'potentially reachable' within a specified 'reasonable’ travel time. A key consideration in measuring accessibility by public transport is the monetary cost of overcoming distance, based on the pricing structure that exists in Cape Town. Equity in accessibility is further evaluated both vertically and horizontally. Vertical equity is evaluated using a proposed Accessibility Loss Index, which analyses the potential implication of affordability and budget restrictions on accessibility, based on the income level of the poor households. GINI type of measures is also proposed to evaluate horizontal equity across the various population groups for various travel modes. To further understand the likely drivers of accessibility, an exploratory OLS regression technique is employed to investigate the relationship between accessibility and a combination of socioeconomic and built environment features of the study area. The study reveals among other things that potential accessibility achievable by car is far higher than that achievable by public transport. The paratransit mode provides the most extensive access coverage, and the highest level of accessibility among all the public transport modes investigated. However, this mode shows to be one of the most expensive options of travel, especially for low-income households who are likely to be restricted by travel monetary budgets. The train turns out to be the most affordable travel option, although the level of accessibility achievable with the train is much lower compared to the paratransit or regular bus. From a vertical equity perspective, the consideration of transport affordability drastically reduces the opportunity space and potential accessibility for the poorest population group compared to the higher income groups. The study further interrogates the distance-based tariff model of public transport services in Cape Town, which it considered to be detrimental to the welfare of poor households, regarding the potential to access essential opportunities. The contribution of this study to the body of research on accessibility is twofold: methodological and contextual. On the methodological dimension, it presents a GIS based approach of modelling accessibility both for the car and for a multimodal public transport system that combines four modes; bus, train, BRT and a minibus taxi (paratransit). It also builds on existing gravity-based potential accessibility measure by incorporating an affordability dimension. The consideration of affordability adds a further layer that enables vertical equity evaluation by judging the potential for destination reachability by the monetary out-of-pocket cost of travel. This approach is considered to be more sensitive to the context of low-income cities like Cape Town, where low-income household’s daily travel decisions are likely to be more guided by monetary cost.