Arrest rates for both adults and juveniles across San Diego County decreased by double digits in 2011, with the decreases holding true for all age and ethnic groups, as well as both felonies and misdemeanors, according to an annual report released today by SANDAG.

In a striking trend revealed by the data, the region experienced a 90 percent drop in marijuana-related arrests due to a change in state law, according to the report, Arrests 2011: Law Enforcement Response to Crime in the San Diego Region.

California Senate Bill 1449, which took effect on Jan. 1, 2011, reduced the charge for possession of up to one ounce of marijuana from a misdemeanor to an infraction. Following this change, the number of misdemeanor-level arrests for marijuana offenses in the region plummeted to 703 last year, down from 6,786 in 2010.

Countywide, 36.7 adults were arrested in 2011 per 1,000 population. That represented an 11 percent decrease from 2010 and a new ten-year low. The juvenile arrest rate in 2011 was 41.1 per 1,000, 18 percent lower than in 2010. The juvenile rate is traditionally higher than the adult rate, but the difference has declined over time. In terms of numbers, many more adults than juveniles are actually arrested each year.

“With the recent upswing in crime rates and the state’s shift of offenders to local jurisdictions, we expect the arrest trend to change to some degree next year when new statistics become available,” SANDAG Director of Criminal Justice Research Dr. Cynthia Burke said.

In 2011, regional law enforcement agencies arrested a total of 87,781 adults and 13,629 juveniles. An average of 240 adults and 37 juveniles were arrested each day in the region.

The report contains a complete breakdown of arrest rates by jurisdiction, as well as arrests by category of crime.

Other notable facts included in the report:

• Arrests for violent crimes, such as homicides, rapes, and aggravated assaults, were down for both adults and juveniles.

• Approximately half of adult arrests were for alcohol and drug-related offenses. Nearly 1,300 adults on average were arrested each month for driving under the influence—the most common reason an adult in San Diego County is arrested.

• The three adult arrest categories with the greatest one-year decreases were: marijuana possession (98%), prostitution (40%), and traffic violations (35%).

• The three juvenile arrest categories with the greatest one-year decreases were: marijuana possession (57%), narcotics (38%), and larceny (35%).

• The most common cause for juvenile arrests was curfew violation.

• Truancy arrests went down by 51 percent in 2011, but arrests for runaway behavior and curfew violations were up slightly (1% and 2%, respectively).