With their powerful results, the studies coming out of the Equality of Opportunity Project, led by Raj Chetty and Nathaniel Hendren, have become an important touchstone for journalists and transportation policy advisers. In their 2014 [PDF] and 2015 [PDF] studies, Chetty and Hendren show that place matters for low-income families. When low-income families have the opportunity to raise their children in better environments, their children do better as adults. And with their use of “big data,” Chetty and Hendren can show that these better environments are not just correlated with improved incomes, but actually cause them.
The Equality of Opportunity Project did not set out to be a study of transportation policy. Only one of the 40 variables that they tested speaks directly to transportation. This variable could have easily disappeared, like most of the other tested variables that did not make the final model.
Instead, transportation turned out to be extremely important. References to the Equality of Opportunity Project’s findings have found their way into numerous newspaper articles, policy reports, grant applications, and prominent public discussions of transportation policy that continue to this day.
The project’s transportation variable involves commutes. I say “involves commutes” because in an unfortunate bit of nomenclature, Chetty and Hendren call this variable “commute time.” This mis-naming has led to continuing confusion among journalists and policy advisers, who make the intuitive, but inaccurate, leap to describing what happens to families when a parent spends a long time commuting.
Instead of measuring “time” in the conventional sense of “minutes,” Chetty and Hendren do something quite different. Their commute variable is defined by the percent of commuting workers who can get to their job in less than 15 minutes. It’s a measure of people, not time.
Moreover, it’s a measure of the relative size of a very select group of people: workers with really short commutes. Nationally, this group is a shrinking minority. In 2000, 29.4 percent of commuters got to work in less than 15 minutes. In 2015, this percentage had fallen to 26.2 percent.
Chetty and Hendren find that for counties and multi-county commute zones, the higher the percentage of workers with really short commutes, the better it is for the children of low-income families.
To humanize what I am rechristening the “short commutes” variable, a journalist or policy adviser could talk about the probability of a parent in a low-income family having a really short commute. The short commutes variable, however, says very little about the impact of lengthy commutes. Nor does it say much of anything about the importance of transit service: Only 3.5 percent of workers who commute by bus or rail enjoy a trip of less than 15 minutes.