So if we really are driving less than we used to — or, at the very least, no more than we used to — when will we stop increasing road capacity? Traffic growth or decline is a notoriously difficult trend to forecast accurately. But given vehicle-mile trends, it stands to reason that sooner or later states and cities will warm to the possibility that maintaining existing roads is a wiser public investment than building new ones.
The link in the piece about traffic projections is a good one, and I would also recommend reading Chuck Marohn's take on it here. I would add from my professional experience that the business of traffic models and projections leaves a great deal to be desired. We all know the saying, "garbage in, garbage out" and it's no different when it comes to projections of traffic. Traffic projections almost universally overstate what will happen on most major streets, long after millions and millions of dollars have been spent to widen roads and build intersections.
About ten years ago I worked on a project where a city's traffic engineer was using future projections that had a street in our project with higher traffic than all but two in an entire region of 2 million people. The model was absurd on its face, and based on highly dubious math and assumptions. And yet, it was taken at face value as a matter of near scientific fact. Sadly, this sort of work has been going on for years, to the detriment of all of our communities. Here's hoping that with the obvious downward trend in driving we can shake the professional world into shape and start to look at infrastructure with a different mindset and different tools.