Access envelopes: a new accessibility mapping technique for transport and settlement planning

The article describes the application of a GIS-based accessibility measurement technique suited to assessing the impact of both transport and spatial development strategies on the location-specific affordability of job access for poor households. The access envelope methodology extends existing accessibility measures by: explicitly accounting for public transport service patterns; including transport costs as a dimension of accessibility; and deriving a single intuitive measure of access reflecting the potential income earnable by a person living in a certain location, after paying for transport. Several case studies from the City of Tshwane are presented, illustrating its use for assessing spatial integration and transport initiatives. The cases demonstrate how Tshwane’s emerging Bus Rapid Transit (BRT) system selectively enhances accessibility to jobs, although its marginal accessibility benefit is reduced by the part-duplication of existing rail lines to core employment areas. While the BRT improves the net earning potential of low-income workers in certain areas, its ultimate benefits will significantly depend on its achievement of network effects – especially via the reduction of first/last-kilometer trip costs – and its ability to leverage higher density development within walking distance of the route. Accordingly, results obtained with the access-envelopes method carry significant implications for current transport planning in the main metro cities.