A new analysis says sprawl costs over $107 billion each year – and here is how they arrived at that figure:

To get to those rather staggering numbers, Hertz developed a unique methodology: He took the average commute length, in miles, for America’s 50 largest metros (as determined by the Brookings Institution), and looked at how much shorter those commutes would be if each metro were more compact. He did this by setting different commute benchmarks for clusters of comparably populated metros: six miles for areas with populations of 2.5 million or below, and 7.5 miles for those with more than 2.5 million people. These benchmarks were just below the commute length of the metro with the shortest average commute length in each category, but still 0.5 miles within the real average of the overall category.

He multiplied the difference between the benchmark and each metro’s average commute length by an estimated cost-per-mile for a mid-sized sedan, then doubled that number to represent a daily roundtrip “sprawl tax” per worker, and then multiplied that by the number of workers within a metro region to get the area’s daily “sprawl tax.” After multiplying that by the annual number of workdays, and adding up each metro, he had a rough estimate of how much sprawl costs American commuters every year.

Then Hertz calculated the time lost by all this excessive commuting, “applying average travel speed for each metropolitan area to its benchmark commute distance, as opposed to its actual commute distance,” he explains in a blog post…

Hertz’s methodology may not be perfect. It might have served his analysis to have grouped these metros into narrower buckets, or by average commute distance rather than population. While it’s true that large cities tend to have longer commutes, there are exceptions. New Orleans and Louisville are non-dense, fairly sprawling cities, but their highways are built up enough that commute distances are fairly short. To really accurately assess the “sprawl tax” in cities like those, you’d have to include the other costs of spread-out development mentioned previously—the health impacts, the pollution, the car crashes, and so on. Hertz only addresses commute lengths and time.

In other words, a number of important conceptual decisions had to be made in order to arrive at this final figure. What might be more important in this situation is to know how different the final figure would be if certain calculations along the way were changed. Is it a relatively small shift or does this new methodology lead to figures much different than other studies? If they are really different, that doesn’t necessarily mean they are wrong but it might suggest more scrutiny for the methodology.

Another thought: it is difficult to put the $107 trillion into context. It is hard to understand really big numbers. Also, how does it compare to other activities? How much do Americans lose by watching TV? Or by using their smartphones? Or by eating meals? The number sounds impressive and is likely geared toward reducing sprawl but the figure doesn’t interpret itself.