The State of Washington is looking for 2,000 people willing to test a road usage tax.

It's exactly what it sounds like – a tax you pay depending on how many miles you drive.

Here's the problem: Right now, people in Washington pay a tax of 49.4 cents per gallon of gas. The revenue pays to fix roads and bridges and maintain ferries, but as cars get more efficient, people buy less gas, and that means less revenue for the state.

Right now, the average car here gets 20 miles per gallon. In 20 years, it's expected to be 35 miles per gallon. And gas tax revenue would drop by 50%.

Related: Road use charge pilot project

How would the state track road use?

You could take your odometer reading every time you renew your tabs, but you wouldn't know how many miles were driven out-of-state. Alternatively you could install a GPS in your car that the state can track, or you could use a smartphone app.

Washington is one of seven states that received a federal grant to test Road Use Charges, so there's no cost to participants.