This study examines how the relationship between active travel (AT) and the built environment is influenced by the modifiable areal unit problem (MAUP). Characteristics of the built environment are measured at 14 geographical scales. At each scale, these characteristics are related to a binary measure of AT using a logit model. The data for this study come from the Space–Time Activity Research (STAR) project, which was conducted in Halifax, Canada, between April 2007 and May 2008. The variable coefficients and their significance are examined for MAUP influences. The results suggest that the relationship between AT and the built environment is indeed impacted by the MAUP. The results are used to suggest guidelines for selecting an appropriate scale at which to measure the built environment in order to minimise MAUP effects.