Measuring the Accessibility of Public Transport: A Critical Comparison Between Methods in Helsinki

This research compares two location-based methods of evaluating public transport accessibility and applies them in Helsinki. After discussing a series of methodological aspects, the authors calculate the Structural Accessibility Layer (SAL) public transport indicator and the Public Transport and Walking Accessibility Index (PTWAI) for a grid with 8,325 zones, comparable in size to the smallest census unit. Both methods are operational for urban planners and policy makers interested in a relatively straightforward way of quantifying the accessibility of sustainable transport modes such as public transport. The results display similar accessibility patterns when moving from larger to smaller isochrones (60 to 38 min). However, the findings are inconclusive between SAL and PTWAI: SAL (38 min) displays good accessibility by public transport (more than 94 % of the population living within two-thirds of the metropolitan area has very high and high access to public transport), but PTWAI indicates that 35 % of the population, primarily households with children (43 %), experience low and very low access. The contrasting results are mainly due to the derivation of the two indicators and have considerable implications for policy making. The findings of this research imply that PTWAI is preferable to planning assessments regarding public transport, given its relatively richer content. However, for multi-mode-based accessibility categorization, SAL appears more appropriate. It is the analyst’s role to understand the objective and contents of each index and choose the tool fit for their purpose. Then, a judgement should be made on the trade-off between the detail of the measures and results and the computational burden. Given the sensitivity of the models to various input parameters and assumptions, cross-validation and replication are key for ascertaining the credibility and usefulness of the models.