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Changes in VMT forecasting


Earlier this year, in a lengthy post about the disastrously stupid plan to replace the Alaskan Way viaduct with a risky, costly deep bore tunnel with no access to downtown Seattle (update: still not looking good!), I expanded my complaint to WSDOT’s planning process, mirrored by DOTs around the country. They exhibited a stubborn refusal to adjust their future projections about vehicle miles traveled based on a new information; up through the 2013 projection, they were insisting that the steady increase in VMT that characterized the second half of the 20th century is likely to return immediately.  The September 2014 WSDOT projections are now available, and something appeared to get through to them this time:

Washington State Transportation Revenue Forecast Council - Peak Traffic

Looking at the data, the change here is really striking: they went from assuming the 80’s are coming back immediately to assuming the modest declines in VMT per capita will not just continue but accelerate, such that VMT per capita is now projected to drop by over 1% per year, at a slightly increasing rate, throughout the 2020’s and 2030’s, resulting in a 2041 VMT per capita a full 33% below the peak rate (see pg 28 here). I would have expected formulas and assumptions to be tweaked and nudged, not radically revised. I think these projections are more sensible, and I obviously certainly hope they’re correct, but it’s striking to see such a dramatic change. One possible reason might be political–forecasts like this make extravagant highway projects funded by assumptions about the future more politically difficult. (That WSDOT might now recognize this is a good thing is a happy possibility to contemplate). Whatever the reason, this shift in forecasting is good political news regardless of whether it’s accurate or not, as Clark Williams-Derry explains:

Second, even if the forecast is wrong, assuming that traffic won’t grow much is the most fiscally prudent way to plan a transportation budget.

For far too long, “build now, pay later” has been the transportation budgeter’s mantra. In the 2000s, for example, Washington committed itself to massive road projects that it didn’t have the money to build. So the state floated bonds, assuming that revenue from gas taxes would show up to pay them off.

That hasn’t worked out so well. Traffic didn’t grow as expected, and gasoline and tolling revenue has gone AWOL as a result. Gradually, planners have come to realize that debt service will swallow up most of the state’s gas tax receipts, crowding out everything else. As the chart below shows, WSDOT predicts that within a few years three-quarters of the state’s gas tax receipts will pay for old projects.

And because so much of the state’s gas revenue is going to pay off old debts, state and local governments simply don’t have the money to keep existing streets and roads in good repair—let alone complete projects, such as the SR-520 bridge, that we’ve already started. And there’s even less money left for the transportation priorities where demand is actually growing, such as walking, transit, and biking.

The irony here is that one reason we might see future VMT decline is declining investment in transportation infrastructure maintenance, caused by foolishly overspending in the 00’s based on budgeting on future VMT increases. It will be interesting to see (and when I have more time I may do some digging) if this is part of a national trend. USDOT’s last VMT forecast appeared to be straight from the late 20th century, even as the US Energy Information Agency projected little to no growth. It’ll be interesting to see what they’ll do with the 2014 report.


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