Metrics of Urban Form and the Modifiable Areal Unit Problem

There is growing interest in incorporating urban form indicators into transportation planning and travel analysis. These indicators typically are measured at a certain level of spatial aggregation (e.g., traffic analysis zone) and therefore are subject to the modifiable areal unit problem (MAUP) known primarily in the statistical and geographic literature but generally overlooked by transportation researchers. The presence of the MAUP can cause serious inconsistency in analytical results and consequently misinform policy making. This study diagnoses the MAUP in measuring urban form through empirical modeling of travel mode choice in the Boston, Massachusetts, region. Using data aggregated in grids with five cell sizes and at the transportation analysis zone, the census block group, and the block level, the study explores the sensitivity of coefficient estimates for population density, network pattern, and land use balance to data aggregation in predicting mode choice decisions. Having confirmed the presence of the MAUP, the study discusses three approaches for dealing with it. Using a grid with a cell size of 1/2 mi appears to be the most desirable method of data aggregation among the eight methods studied. The suggested improvements in methodology will help advance the inquiry on the link between urban form and travel.