Traffic on I-35 is the same as it was 20 years ago. But we’re expanding anyway.

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I-35 expansion divides public opinion during economic downturn - Austin  MonitorAustin Monitor
A rendering of the planned expansion of I-35 downtown. The overhaul adds two HOV lanes in each direction.

Wanna hear something incredible? Traffic on I-35 isn’t any worse now than it was 20 years ago.

That can’t be right, you say. Austin’s most prominent traffic pundit certainly didn’t believe it:

But check out this article in the Austin Chronicle from 2002:

Each day, more than 200,000 cars and trucks cross Town Lake on I-35, making that road the busiest six-lane highway in Texas. By 2020, TxDOT consultants predict, 330,000 cars and trucks will cross Town Lake each day — the transportation equivalent of trying to shove the Colorado River through a garden hose.

This article, by the way, described the plan, which had been in the works since the late 80’s, to expand I-35. Back in 2002, the plan was for the expansion to be complete in 2020.

Twelve years later, in 2014, traffic levels were exactly the same. In a report, TxDOT conceded that traffic hadn’t grown on I-35 but attributed it to a national decline in vehicle activity from the recession as well as the opening of SH 130 and SH 45. However, the report warned that traffic had begun to pick up again and would eventually balloon to more than 300,000 by 2035.

Alas, five years later, in 2019, daily traffic was still hovering at around … 200,000. Here’s the daily traffic count at Lady Bird Lake & I-35, year by year:

Just as it did before, TxDOT has simply moved the projection up by a few years. Now instead of talking about 300k vehicles by 2035, they’re talking about 300k by 2045. This is from the Statesman article last month:

Pre-pandemic, more than 200,000 vehicles traversed the highway each day. TxDOT estimates that number will grow to more than 300,000 by 2045.

I have not been able to get in touch with someone from TxDOT to talk about this today but I feel compelled to share this ASAP because the public comment period on the proposed I-35 expansion ends on Dec. 31.

There may be some nuances I am not accounting for. It’s not clear to me, for instance, whether the projected traffic takes into account the proposed expansion or not. If it does, then it aptly illustrates why expanding will simply induce demand and not relieve congestion. However, what is clear is that the projections 20 years ago assumed that the road would roughly stay the same but that traffic would explode. That did not happen. Nor has traffic increased in the past decade –– it’s actually declined.

This is not an uncommon issue in highway planning. The U.S. Department of Transportation systematically exaggerates future highway use when justifying expansions.

It appears that U.S. DOT, which gets its data largely from local agencies, like TxDOT, always assumes that traffic will be higher than it actually is.

For instance, in 2012 Eric Sundquist, a transportation policy wonk at the University of Wisconsin, took a look at the U.S. DOT’s annual traffic projections and found that they always overestimated future traffic:

When the U.S. DOT’s most recent Conditions and Performance Report to Congress hit the streets in 2012, it forecast that national vehicle-miles traveled would reach 3.3 trillion that year. A few months later we learned that their estimate was almost 11 percent too high.

Enough time has passed by now that 61 yearly projections can be compared to the reported VMT. And in 61 cases out of 61, the C&P estimates were too high. For example, the 1999 C&P overshot 2012 reported VMT by more that 22 percent—almost 11 extra states’ worth of driving.

In fact, though the national VMT trend line began flattening in the 1990s and actually turned down in the 2000s, the slope of the C&P projections has remained nearly constant.

State and local agencies, like TxDOT, have an incentive to overestimate future traffic. The more traffic they can project, the more likely they are to convince the feds that they need more money.

As is the case with so many other highway expansions, the proposed expansion of I-35 is paved with bad data.

You’ve got two days left to tell TxDOT what you think about expanding I-35. You can tell them by submitting a comment here.

This is just a small sample of what you get EVERY weekday if you subscribe to the Austin Politics Newsletter

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