SEATTLE – On the day that Washington state's attorney general is set to announce the findings of a year-long investigation into high gas prices, a Seattle-based think tank says people in the Northwest are leading the nation in cutting back on gas consumption.

The Sightline Institute says residents in Washington, Oregon and Idaho have cut back their per-capita consumption by 11 percent from 1999 to 2007. The study, released Thursday, says that translates to nearly a gallon a week on average.

"An 11 percent decline in consumption is like every driver taking an annual, five-week holiday from their cars," said Sightline research director Clark Williams-Derry.

The report, called "Braking News," also finds that a decline in CO2 emissions per capita parallels the drop in gasoline consumption.

It says the Northwest's gasoline use per person is at its lowest level since 1966.

In addition, Northwest states are outpacing the nation in gasoline reduction. Ten years ago, Northwest drivers used more gas per person than the national average. Sightline says as of 2007, they consumed about nine percent less than the U.S. average.

Williams-Derry attributed the decline to people buying more efficient vehicles, driving less and using transit more often.

"Transit ridership is at an all-time high in greater Portland and greater Seattle, and at the highest level nationally in 50 years," said Williams-Derry.

Now the bad news. Even with the decline, population growth has offset the per-capita decline in gas consumption. That means even though drivers are buying less gasoline, more people are here buying gas, period. The study says total gasoline consumption and related greenhouse gas emissions in the region have remained roughly flat since 1999.

Williams-Derry offers some suggestions in response to the report.

"Adopting smart policies such as a cap-and-trade program that includes highway fuels can extend our gas savings. It's our best shot at protecting ourselves from rising prices at the pump, while reducing climate-warming pollution."

As of Thursday, the average price for a gallon of regular unleaded is $3.59 in Washington, $3.52 in Oregon and $3.38 in Idaho. The national average is $3.42.