Marijuana plants at the Ataraxia cultivation center in Albion Credit: AP Photo/Seth Perlman

When Governor Bruce Rauner signed a new law decriminalizing marijuana possession Friday, Illinois became the 17th state to consider small possession a civil matter subject to a citation and a fine rather than a criminal offense.

If you’re caught carrying less than ten grams of marijuana (which, it seems in some photos, is a lot of marijuana), cops may issue tickets for $100 to $200. Chicago has had an ordinance decriminalizing pot possession of up to 15 grams since 2012.

“I’m obviously thrilled that the state is taking this approach,” says Amy Campanelli, who heads the Cook County Public Defender’s office. “It’s a progressive, fair criminal justice reform.”

Campanelli’s office represents overwhelmingly low-income, minority clients in Cook County courts, and she hopes the new law will lead to fewer arrests for possession. Even with the Chicago decriminalization ordinance, Campanelli says that her lawyers continued representing dozens of clients charged with small possession every month.

“These arrests are stopping people from getting jobs and getting school loans,” Campanelli says. “It stops opportunity.” Importantly, the law also stipulates that possession citations will be automatically expunged after six months.

The new state statute also adds an important wrinkle to the legalization landscape by defining what it means to drive under the influence of marijuana. Drivers will be subject to DUI charges only if they have five or more nanograms of THC in their blood, or ten or more nanograms of THC in their saliva.

Defense attorneys in Chicago are also celebrating this aspect of the new law. Before, the state could bring DUI charges against anyone with even trace amounts of THC in their system. This meant that someone who used pot weeks ago could still test positive and wind up facing misdemeanor charges and steep fines.

The DUI stipulation is just as important as decriminalizing small possession statewide, Campanelli argues, because it will protect drivers who were not in fact driving under the influence from DUI charges. The fact that people could be slapped with DUIs for trace THC in their systems “was making a crime without any criminal intent,” she says.

So what do these new DUI thresholds mean for recreational smokers? Can you have a postdinner joint and still be fine to drive?

As with blood alcohol levels, the answer to this question varies depending on a person’s size, weight, and the potency of the bud. However, research has shown that red blood cells do not absorb THC well and that the blood content of THC spikes right after use but drops off fairly quickly. Similarly, saliva does not absorb THC well and testing has not proven reliable.

In any case, waiting a couple of hours to drive after getting high is a good idea, especially since tests have shown that even the mildest buzz can correlate to blood THC levels far above the legal limit.

Whether police departments in Illinois will be able to test drivers effectively is another question. Practically speaking, private defense attorney Michael O’Meara, who was also a Cook County prosecutor for several years, is skeptical that testing for THC in blood and saliva could be done in any systematic way, especially during traffic stops. “It is going to be much more difficult for law enforcement to successfully prosecute DUI cases for marijuana,” he says.

When it comes to blood testing for THC, there is no equivalent to portable alcohol breathalyzers, and experts say portable saliva tests are unreliable. It’s not clear whether the Chicago Police Department even owns any portable saliva-testing equipment—the department responded to a request for comment with its protocol for alcohol testing.

If cops don’t have a warrant, O’Meara notes, drivers can refuse to submit to blood, urine, or saliva testing—just as they can refuse breathalyzers and field sobriety tests after being pulled over for suspected drunk driving.

“A portable breathalyzer is not considered as reliable as a breathalyzer at the station,” O’Meara explains. Cops use portable breathalyzers to establish probable cause for arrest, but the results from these tests are not admissible during trial, O’Meara says. Arrestees can even refuse breathalyzer tests at police stations. O’Meara predicts that, given the invasiveness of blood tests, more people will be refusing them than do breathalyzers.

Without evidence from these tests, it is very difficult for the state to win DUI cases, O’Meara says. He also notes that refusing to submit to field sobriety tests or bodily fluid testing can earn a driver’s license suspension.

The public, he says, will have to weigh that consequence alongside all the other bad potential consequences of driving while stoned.

Correction: An earlier version of this story mistakenly stated the amount of weed covered by the new law. Possession of up to ten grams was decriminalized in the law signed July 29.