New York, Hawaii, and California have made the most progress in improving their energy efficiency over the last 20 years, while Alaska and North Dakota have made the least, according to a study released this week by the American Council for an Energy-Efficient Economy. Overall, the U.S. cut its energy intensity — or energy used per unit of gross domestic product (GDP) — by 42 percent between 1970 and 1999, while also cutting its carbon emissions per unit of GDP by 47 percent. Still, much more progress needs to be made, and the ACEEE recommends that states create incentives for the use of renewable power and efficient technologies, phase out old coal-fired power plants, and tax gas-guzzling light trucks and cars, using the money to give financial breaks to buyers of fuel-efficient vehicles.