IT’S COMPLICATED: Examining Claims About Induced Demand, Adding Road Capacity and Traffic Congestion. “The ‘iron law of roadway congestion’ isn’t.”

Here are some of the limitations with what Duranton and Turner did. First, their detailed analysis deals only with urban Interstates. Analytically, they treat parallel arterials and all other roadways in the metro area as a large blob. That leaves them unable to analyze (as opposed to only speculating about) the extent to which drivers shift trips from parallel arterials to the newly expanded urban Interstate (which would not represent new driving, but simply a re-allocation of existing driving).

Second, the authors’ premise that all such freeways have a “natural level of saturation” implies that all urban Interstates should fill up to the point of serious peak-period congestion. But in fact, actual congestion levels (in both extent and duration) vary considerably among the set of a metro area’s freeways. And the extent of congestion also varies enormously among large metro areas overall. In 2014, Randal O’Toole’s Antiplanner blog provided data on daily VMT per lane-mile on urban freeways around the country, finding a range from as low as 9,000 to a high of 22,000.

Third, since most metro areas in recent decades have added relatively little freeway capacity, the authors’ analytical results, even if correct, would tell us only that making marginal increases in freeway capacity produces little in the way of freeway congestion reduction.

We have a similar real-word experiment going on right where I live. The I-25 “gap” of two-lane highway between Monument (a Colorado Springs exurb) and Castle Rock (a Denver exurb) should have been widened 20 years ago, when traffic first became a noticeable problem. Instead, the decision wasn’t made until about two years ago, and only after two highway patrol officers were killed by oncoming traffic (the shoulders are quite narrow in most places) during routine ticket stops. And instead of two new lanes north and south, we’ll get just one new toll lane in each direction. In other words, the expanded interstate was obsolete at the planning stage, forget about 2022 when it’s finally completed.

Yet our “addiction to cars” or some such will get the blame for the unimproved traffic snarls, accidents, and deaths.

UPDATE: Link was missing. Fixed now — sorry!