Transport impact indicators for monitoring the sustainability of urban growth: Findings from a Delphi study

The selection of a right transport impact indicator of urban growth is challenging because the impacts vary both geographically and temporally. This research contributes to addressing this challenge. First, the study systematically selects 23 indicators from a pool of 62 indicators initially identified from the literature. Second, it evaluates and classifies their suitability into four groups (suitable at a particular spatial scale, a particular temporal scale, multiple spatial scales, and multiple temporal scales) through a two-round of Delphi survey involving 29 experts drawn from related sectors and from across the world. In Round 1, experts were requested to indicate the suitability of the indicators for a particular spatial/temporal scale on a 5-point Likert scale. Four summary statistics were derived using the responses to assess the consensus level among the experts: median, expected probability of occurrence, interquartile range, and standard deviation. In Round 2, experts were requested to indicate the extent they agree with the achieved consensus level and an increment of agreement between the two rounds was calculated. Results show that experts reached a consensus on 12 (52%) indicators. ‘Travel time’ was identified as a suitable indicator for all three spatial (local, city, region) and two temporal (short and long) scales. The remaining 11 indicators are suitable for only specific spatio-temporal scales. The findings serve as a guide for decision-makers, transport modelers and planners to adopt indicators according to their scale of operation.