Metropolitan Accessibility and Transportation Sustainability: Comparative Indicators for Policy Reform

Abstract

Accessibility is most commonly studied and measured within the context of a single metropolitan region. By contrast, this study applies metrics of accessibility (for work, non-work, by auto and transit) that incorporate both mobility and proximity to 38 of the largest 50 U.S. metropolitan areas. This cross-sectional analysis allows both intermetropolitan comparison (of accessibility overall and of the equity of its distribution) and assessment of the determinants of metropolitan accessibility. The two components of accessibility analyzed here—mobility and proximity—exist in tension with each other: places with rapid surface travel are usually places where origins and destinations are far apart; places with many origins and destinations in close proximity are places where travel tends to be slow. For this reason, it is not apparent which urban forms offers greater accessibility: those with spread-out land uses and more rapid travel, or more compact arrangements in which travel is slower. There are good theoretical reasons to expect that surface travel speeds are all-important in determining accessibility outcomes and that anything that interferes with surface travel speeds—including denser metropolitan development—might degrade accessibility. Empirical results presented here suggest the opposite: more compact metropolitan regions offer greater auto accessibility even if their travel speeds are somewhat slower. In other words, the proximity effect of density dominates any associated degradation in travel speeds. This suggests that reform of policies that spur low-density, auto-oriented development can yield transportation benefits in terms of increased metropolitan accessibility. The report also develops indicators for assessing the equity of the distribution of accessibility between individuals within a region. Indicators developed here capture accessibility distributions across dimensions of income, race, and car ownership. Even with a given accessibility distribution by auto and by transit, the equity of the accessibility distribution also depends on the location of carless households within a metropolitan region; indicators are also developed to capture this effect.