More than 13 percent of people ages 16 or older drove under the influence of alcohol in the United States last year, and more than 4 percent of people in that age group drove under the influence of illicit drugs, according to a new government study.

Compared with previous years, the rates of driving under the influence of alcohol and drugs have decreased in the past decade, the study said. When comparing data from 2002 through 2005 with data from 2006 to 2009, the average yearly rate of drunken driving declined from 14.6 percent to 13.2 percent, and the rate of driving under the influence of drugs declined from 4.8 percent to 4.3 percent, the study said.

"Some progress has been made in reducing the levels of drunk and drugged driving through education, enhanced law enforcement and public outreach efforts. However, the nation must continue to work to prevent this menace and confront these dangerous drivers in an aggressive way," said Pamela S. Hyde, administrator of the U.S. Substance Abuse and Mental Health Services Administration (SAMHSA). The study tallied responses from 423,000 people ages 16 or older.

The study also broke down drunken-driving rates by state. Wisconsin and North Dakota had the highest rates, with 23.7 percent of Wisconsin residents and 22.4 percent of North Dakota residents reporting that they drove drunk in the last year.

Rhode Island and Vermont had the highest rates for driving under the influence of drugs, with 7.8 percent of Rhode Island residents and 6.6 percent of Vermont residents saying they drove under the influence in the last year, according to the study.

States with the lowest rates of drunken driving in the past year included Utah, at 7.4 percent, and Mississippi, at 8.7 percent. Iowa and New Jersey had the lowest levels of drugged driving, at 2.9 percent and 3.2 percent respectively, the study said.

Levels of self-reported driving while drunk or on drugs also differed dramatically among age groups. Drivers ages 16 to 25 had a higher rate of drunken driving than those ages 26 or older (19.5 percent versus 11.8 percent).  And people ages 16 to 25 had a higher rate of driving under the influence of illicit drugs than those ages 26 or older (11.4 percent versus 2.8 percent), according to the study.

Twelve states have seen reductions in the levels of drunken driving and seven states now have lower levels of drugged driving, when data from 2002 through 2005 were compared with data from 2006 to 2009.

But one in three people who died in a motor vehicle accident tested positive for drugs in 2009, among accidents with known drug test results, according to the National Highway Traffic Safety Administration's Fatal Accident Reporting System census.

Pass it on: Be careful on the roads — 13.2 percent of people of driving age said they drove drunk in the last year.