It’s hard to tell this President’s serious proposals from his idle speculations sometimes. It can be particularly difficult when he is giving a kickoff interview to Rupert Murdoch’s newest newspaper. But that is exactly what happened this week, when the subject of gas taxes came up in President Trump’s interview with the California Post (a new Golden State offshoot of the New York Post, the venerable tabloid owned by Murdoch.

Trump has often made mention of his desire to drive down the high cost of driving, both by driving down the cost of gasoline (through increasing domestic oil production, SPR releases, invading Venezuela, etc.) and the price of car ownership (by canceling costly NHTSA safety mandates and making car loan interest tax-deductible in some cases).

In an interview with the Post released on January 26, he talked about California’s highest-in-the-nation gasoline tax rates (70.9 cents per gallon, including sales taxes, per the Tax Foundation’s reference map) and how that whenever the price of the commodity itself goes down, “they jsut raise their taxes.” The Post article quotes Trump as saying “I would cap it [the state gas tax rate]. It’s unfair.”

There are an extremely wide variety of state gasoline tax levels in the U.S. (all in addition to the 18.4 cent per gallon federal excise), ranging from 70.9 cents per gallon in California all the way down to just 9.0 cents per gallon in Alaska, as this map from the Tax Foundation shows.

The remainder of the Post article speculates as to how the President might go about capping state gas tax rates (with no further input from the President, as far as readers can tell) – through legislation, executive order, etc. (In this author’s opinion, none of that would be constitutional, for good reason. If the federal government can order a state to repeal a specific tax, then the federal government also has the power to order a state to increase a specific tax, and I am not at all sure that most Trump voters want future Administrations having that power.)

But what the Post article never mentions is that California, like most other states, deposits the entirety of its gasoline tax receipts into its transportation trust fund, in order to pay for transportation projects. If you lower the tax rate, states – which are required to run balanced budgets, because they can’t print money – have to cut back on transportation project spending. (In some states, like Pennsylvania, the currently high tax rate was set by the legislature as part of a deal to spend the money on specific projects. Lowering the gas tax rate would blow up the deal and unfund some of those projects.

So the real question is this. When he gave this interview, was President Trump aware that state gasoline tax money goes straight to transportation spending projects via the user-pay trust fund model, and that cutting the tax rate guarantees lower transportation spending by the state?

If he was unaware of the connection, then this whole news cycle doesn’t mean much, because it is extremely unlikely that there will ever be any substantive follow-through, since there does not seem to be a constitutional path for the President to impose such a ceiling on state fuel taxes.

But if he was aware of the connection between gas taxes and transportation spending, then his willingness to reduce state transportation revenues and spending bodes poorly for the upcoming surface transportation reauthorization process. Because maintaining the Highway Trust Fund (the federal user-pay linkage between highway user taxes and surface transportation spending) in reauthorization will involve someone having to take political leadership on reconciling tax revenues with spending levels, and a President who actively wants to blow up that linkage at the state level probably won’t spend any political capital supporting the linkage at the federal level, either.