Washington transportation policy makers are faced with a “dangerous curve ahead” signpost due to the state’s overreliance on a gas tax. Unless an alternate route is mapped out, we’re headed toward a fiscal cliff in the next few decades.

Shifting to a pay-per-mile formula is the best idea on the table right now. A trial project involving 2,000 volunteers who tracked their mileage for a year was recently completed; the state Transportation Commission will review a report and may vote on a proposal in December; and legislators could be debating bills in Olympia next year.

A degree of urgency is warranted, given that Washington’s 49.5-cent-per-gallon gas tax ranks fourth-highest in the U.S. and public patience for further hikes may be reaching a limit. Meanwhile, gas tax revenue is projected to decline by 45 percent between now and 2035 as the number of fuel-efficient and electric-charged vehicles grows exponentially.

At risk is our system of roads, highways and bridges, which don’t sustain any less pavement pounding just because a vehicle drinks less petroleum. (Unless there’s some new technology coming quickly that will turn your eco-friendly car into a hovercraft.)

And don’t forget the gas tax keeps the state ferry system afloat. Those boats hold the same number of SUVs regardless of whether they’re gas-guzzlers, hybrids or fully electric.

So yes, phasing in a road mileage tax is a pragmatic option that lawmakers need to explore seriously.

At the same time, however, they need to study changes to Washington law to ensure the revenue is spent on traditional transportation infrastructure, not diverted to the state general fund or to legislators’ pet projects.

Specifically, that will require a close look at the 18th Amendment to the Washington Constitution.

The amendment sets forth that all gas tax and license fees deposited into the state motor vehicle fund be spent on “highway purposes.” So far, so good.

The Supreme Court has reinforced those restrictions several times, including in 1961, when it ruled that using motor vehicle revenues to relocate utilities doesn’t benefit the highway system. Likewise, in 1969 it said financing a public transportation system doesn’t qualify as a highway purpose. (That’s why Sound Transit cooked up a hodgepodge of car tab fees and other taxes to pay for its vast mass-transit network.)

The problem with the 18th Amendment is that its language centers on collecting tax “on the sale, distribution or use of motor vehicle fuel.” Ratified in 1944 when the internal-combustion engine was king, it didn’t anticipate a switch to a mile-based tax or other non-petroleum revenue source.

State lawmakers, therefore, shouldn’t mess around with a mileage tax unless they have a parallel discussion about preserving the original intent of the gas tax.

Washington clearly needs a more reliable way to pay for transportation in the 21st century. But as long as there are highways — whether filled with gas or electric cars, driverless vehicles or cars piloted by brainwaves — we’ll also need a reliable stream of money dedicated to highway purposes.

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