A major concern of police about state laws legalizing marijuana has been how to detect when motorists may be driving under the influence of pot — or other drugs. Now, San Diego police are joining a growing number of cities relying on mouth swabs for chemical tests to detect drivers who are on drugs.

The swabs and two portable testing machines hit the field Friday night in San Diego, reported CBS-8 TV. It’s the second city in the state after Los Angeles to use the new detection method.

The machines test for the presence of marijuana and six other drugs: cocaine, methamphetamine, amphetamine, methadone, opiates and benzodiazepines. The level of intoxication, however, is not determined in the tests.

Law enforcement officials believe the number of impaired drivers are bound to increase with California’s legalization of marijuana. A study by the state’s Office of Traffic Safety found that 38 percent of drivers killed in car crashes in California in 2014 tested positive for drugs, legal or illegal. That was up 6 percent from the previous year.

San Diego began using the Dräger 5000 test after officials met with authorities in Colorado, which legalized marijuana for recreational use in 2014, the San Diego Tribune reported.

Canadian authorities began using similar testing swabs and machines from a different company in a pilot program in late 2016 in the city of YellowKnife, the capital of the country’s Northwest Territories.

“The goal of any of these initiatives is to save people’s lives,” said Royal Canadian Mounted Police Corporal Todd Scaplen. “And if we have further tools to be able to do that, I think it’s very important.”

Swabs with portable testing kits are also being used in New York, Arizona and Nevada, among several other states, as well as in Germany and Belgium, and Australia, notes the Tribune. Other strategies, including breathalyzers that test for drugs and apps to help police measure impairment are also on the market.

San Diego police will first observe a driver to make an initial determination about possible impairment. The motorist may then be asked to run a swab inside his or her mouth for as long as four minutes, then the swab is tested in the portable machine and results are available in up to eight minutes. For pot, the machine only tests for the active THC compound that causes a high, which usually stays in the body for a few hours. A driver can refuse the test, but then an officer can order a blood test.

The next problem states face is determining the level at which drugs in a person’s system could result in dangerous driving. Once defined, levels can only be determined with a blood test.