Exploring the Impact of Different Cut-off Times on Isochrone Measurements of Accessibility

Abstract

Isochrone analysis and assessments of cumulative opportunities are a common way to quantify accessibility. However, different time cut-offs have been used by different researchers, with little investigation into what is the ‘best’ cut-off time. Outstanding questions remain concerning the most effective or predictive cut-off time and the potential implications of choosing one time limit over another. The primary objective of this paper is to explore how different cut-off times affect the calculation of isochrone-based accessibility measurements and their potential to predict travel-mode choice. Fifty dissemination areas (DAs) within the Greater Toronto and Hamilton Area (GTHA) are selected to test the impact of different isochrone cut-off times in 5-minute intervals for public transit, automobile, and walking accessibility. The relative predictive power of 30- and 45-minute isochrones in modeling mode choice is also examined. This paper finds that different cut-off times do impact the interpretability of accessibility measurements in the isochrone approach, but a defined cut-off time for general use cannot be determined based on the analysis.

How transit scaling shapes cities

“…The isochrone accessibility measure specifies a travel time threshold beyond which the accessibility values are discounted 56 , job opportunities within the threshold are counted with equal weights. The selection of travel time threshold affects accessibility measurements and influence the predicted mode share 57 . Equation 3 describes the isochrone accessibility (A T ) with travel time threshold T , and its impedance function f C i j .…”

Towards a general theory of access

“…e use of various travel cost thresholds inherently produce different cumulative opportunity measures, and more importantly may change the rankings of alternatives of places in a relative sense (Miller 2018;Owen and Levinson 2015;Xi et al 2018). is presents a major challenge with the isochrone measure, since the selection of travel cost threshold significantly impacts its interpretation.…”

“…In practice, isochrone access is oen measured in minute-by-minute increments, each a time annulus, e.g., the number of opportunities available between 13 and 14 minutes, and models using access as input to predict mode share, population density, etc. oen take the travel time cut-off value that produces the best model fit (Owen and Levinson 2015;Xi et al 2018). Isochrone access permits the calculation of a weighted sum of opportunities within incremental travel cost thresholds (Kwan 1998), and the best fit weighing function can be applied, which is similar to the idea of the distance-decay function.…”

Accessibility: measurement and application in transportation planning

“…Reasonable correlations between observed location and travel choices and an accessibility measure may be achieved, but these usually do not provide strong 'hypothesis tests' of the assumed specifications. Isochrone-based measures are particularly arbitrary and behaviourally suspect (Xi, et al, 2018), but gravity/logit model-based measures have their own issues, as discussed below.…”