The LAPD's increased DUI patrols tonight will also include a beefed-up effort to find stoned drivers, authorities say.

The department will expand its use of an instant drug test to include those pulled over in traffic stops. Previously the test was used only at DUI checkpoints and at three LAPD jails, the City Attorney's office says.

City Attorney Mike Feuer indicated the tests will come in handy when cops encounter those suspected of being under the influence of marijuana. There is good news, however:

The test is purely optional. You don't have to take it.

You have to submit to a blood test if police tell you to do so (or face an instant, one-year suspension of your drivers license). But so far there's no law that says you have to take this mouth swab.

Nonetheless, the LAPD and the City Attorney plan on being aggressive about it.

Feuer told this to reporters:

It is the case that as medical marijuana dispensaries are prevelant here in Los Angeles, for example, we have as an anecdotal matter seen evidence that there are those who are driving under the influence of medical marijuana.

Though the swabs can test for seven drugs, weed seems to be officers' big catch here. LAPD Cmdr. Andrew Smith:

We want to make sure everybody gets the message loud and clear that there will be zero tolerance ... for people driving under the influence of narcotics or drugs.

A new $520,790 grant by the California Office of Traffic Safety and the National Highway Traffic Safety Administration will help fund the tests.

The cash will also help employ two deputy city attorneys who will specifically be prosecuting alleged drug drivers, the City Attorney's office says.

About 534 DUI drug cases and 64 "combination alcohol and drug cases" were taken to court in 2013, the office says.

By Dennis Romero Tue., Dec. 31 2013 | LA Weekly

The big question here is how judges and juries will measure marijuana intoxication.

Besides the instant, roadside results, the LAPD will seek results from a second sample of a suspect's saliva that will be sent to a lab. Those might include some quantification of blood content. Or maybe not.

The science on cannabis intoxication is murky at this point. It can be hard to determine with accuracy whether someone has smoked within hours or within weeks.

See also: Smoking Marijuana a Month Ago Could Get You a DUI Today Under Propo...

And for that reason medical marijuana advocates have been concerned about efforts to crack down on stoned driving. They say it criminalizes medical users who aren't necessarily high but who have cannabis in their blood from past use.

For now you have a choice: When a cop asks you to do the swab, you can politely say no.

Send feedback and tips to the author. Follow Dennis Romero on Twitter at@dennisjromero. Follow LA Weekly News on Twitter at @laweeklynews.