istock trafficA nationwide traffic study just released by traffic data firm INRIX found that Seattle has the 10th worst congestion in the nation.  That’s the second time in the last few months that Seattle has ranked in the top 10 in a national congestion ranking. 

But more interesting than the city vs. city rankings is this:  if you believe INRIX’s national ranking tables, Seattle-area congestion fell by nearly one-third between 2006 and 2010!  That’s huge!  Meanwhile, greater Portland’s rush hour backups eased by about 20 percent.  Looking at absolute declines, rather than percentages, greater Seattle’s congestion declines ranked third in the nation, trailing only the recession-racked Miami and Ft. Myers, FL metro areas.  Portland clocked in at a respectable 22nd place for its more modest congestion reductions.

I have no particular insight as to why Seattle’s congestion fell so quickly in INRIX’s rankings.  The metro area’s economy hasn’t been hit as hard as some other parts of the country, so it’s not just the economic downturn that’s in play.  Seattle’s transit system is good, and attracted lots of rush hour drivers from the roads; but surely other metro areas have decent transit.  And while economist Ed Glaeser correctly pointed out that Seattle’s compact, transit-oriented neighborhoods have grown faster than the nation as a whole, the growth of transit-friendly neighborhoods can’t have been responsible for more than a fraction of greater Seattle’s congestion decline in just 4 years.  Maybe it’s just a lucky combination of all the above factors.  Or maybe—heaven forbid!—it’s just a data anomaly or an outright error.

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  • And more generally, I don’t think we should assume that researchers have fully taken the measure of Seattle traffic.  It bears repeating:  both the new study, and the Texas Transportation Institute (TTI) congestion study released in January share a severe methodological flaw:   they give sprawling cities with long commutes a boost in their congestion rankings, compared with more compact cities.

    Portland-area consulting firm Impresa took a look at the bias in TTI’s congestion measures last fall.  And while INRIX uses somewhat different methods, as far as I can tell their “Travel Time Tax” rankings have the same basic flaw.  By ignoring the effect of commute distance, they make sprawling cities with long commutes look less congested.  (In a nutshell:  the longer the commute, the less a minute of delay matters to the “Travel Time Tax.”  So a 10 minute delay in a compact city might suggest “high” congestion, while  the same delay in a sprawling city might suggest “low” congestion.  If anyone wants more math, feel free to ask for it in comments, or just check out the explanation from Impresa that’s quoted in this post.)

    So to me, it’s just too soon to say that Seattle has the 10th worst congestion in the country.  That doesn’t mean that we should ignore what INRIX has to say about traffic trends; just that we should all be careful reading too much into their rankings.