Why commute times don't change much even as a city grows

For as much as U.S. metros have grown over the past few decades, commute times have remained oddly stable. One-way commutes averaged 21.7 minutes in 1980 and 25.3 minutes in 2010. That's not nothing — it's 30 hours more a year assuming 250 work days — but it's also not nearly as much as decade upon decade of urban sprawl might suggest. As bad as our daily commutes are, in many ways we're fortunate they aren't worse.

So why aren't they worse? Among the most compelling ideas to explain this phenomenon is that people have daily "travel time budgets" of about an hour that they refuse to exceed. (This budget is often called "Marchetti's Constant,"though it's better attributed to Yacov Zahavi.) But that alone isn't enough to explain what's happening. After all, if traffic increases and jobs stay in the same place, then our hour won't get us as far as it once did, whether we like it or not.

Then what else is going on? Two new papers target this classic question in a fresh way. Their findings suggest that much of what's going on is more people relying on public transit.





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