It’s Time to Rethink Disparity Analyses in Transportation

Triggered by the vast inequalities among population groups, urban and transportation planners are increasingly conducting "disparity analysis" of accessibility. These analyses seek to assess how proposed transportation projects may have differential effects on distinct population groups.

For example, such analyses may show how investment leads to large accessibility increases among suburban residents while generating little to no accessibility benefits for inner-city residents.

Equity Redefined: Sufficiency Over Disparity Analyses

In "Equity in Accessibility: Moving from Disparity to Insufficiency Analyses" (Journal of the American Planning Association, Vol. 88, No. 4), Karel Martens, Matan E. Singer, and Aviv Lee Cohen-Zada challenge these disparity analyses and call on researchers and practitioners alike to conduct sufficiency analyses instead.

As Martens et al. argue, disparity analyses are not a neutral framework because they implicitly assume that the status quo is fair. Moreover, these analyses do not answer the fundamental question of whether observed accessibility levels are sufficient to accomplish daily routines. Higher accessibility does not necessarily mean sufficient accessibility.

In addition, disparity analyses encourage planners to look at transport equity at the level of broadly defined population groups. But there is often variation within the same group, which these disparity approaches overlook and unintentionally hide. For example, low accessibility experienced by group members living at the edge of a city gets averaged out by the high accessibility enjoyed by members of the same group living closer to urban centers.

Martens et al.'s sufficiency approach seeks to address "in-group variation" by centering studies on individuals rather than groups. Their approach also focuses on answering what level of access is good enough for people to accomplish daily routines.

Figure 1. Sufficiency analysis: Share of the poor population in each region facing insufficient accessibility when employing the 1% and 10% sufficiency threshold. The number behind each bar represents the regional Accessibility Sufficiency Index (ASI) score for the 10%-sufficiency threshold; the higher the number, the more inequitable the region.

Transit Disparity Harms Equitable Access

For the study, Martens et al. compared job accessibility (by public transportation, from home to work) within 49 of the 50 largest U.S. metropolitan areas. They limited the analysis to populations with incomes below the official U.S. poverty line, assuming that poorer residents have more limited access to car transportation and will thus rely more heavily on public transit than more advantaged populations.

By employing the same data to conduct both a disparity and a sufficiency analysis, the authors demonstrated how the sufficiency analysis revealed an otherwise overlooked inequitable pattern across the 49 metropolitan areas. While the disparity analysis showed that disadvantaged minorities often enjoyed higher levels of transit accessibility than their advantaged counterparts, the authors found that a large number of disadvantaged people did not have sufficient access.

Even when applying a very low sufficiency threshold, set as equal to 1 percent of the jobs accessible by car, they found that nearly 11 million people below the poverty line did not have sufficient access. When a higher sufficiency standard of 10 percent of jobs accessible by car was employed, Martens et al. found that 90 percent of the below-poverty population across all the cities had an insufficient level of accessibility to destinations.

This study — specifically the comparison between sufficiency and disparity analysis — helps demonstrate how disparity analyses may do more harm than good if planners do not contextualize their findings. For example, in their disparity analysis, the authors found that in most metros disadvantaged groups experience higher transit accessibility than more advantaged population groups. This finding seems to suggest that disadvantaged groups are better off. Yet, 'better' may not be 'enough', as the authors' sufficiency analysis reveals: the transit service that disadvantaged groups receive is often incomparable to the car accessibility enjoyed by most advantaged groups.

Marten et al.'s article is a timely call for planners to determine (and then base) transit analyses on an explicit and agreed-upon sufficiency standard. This way, planners can understand how accessibility relates to day-to-day routines and whether current transit systems truly allow all people to take part in opportunities afforded to individuals who have access to a private vehicle.

Top image: Bus stop in Utah. E+/RichVintage