You meander. They notice.

Then, if you’re one toke over the line, the decision to get behind the wheel and drive can quickly turn into a full-fledged medical exam — and an arrest.

“If you feel impaired, please get someone else to drive,” said California Highway Patrol Sgt. Glen Glaser. “That’ll be a whole lot cheaper than a $10,000 DUI ticket.”

Related: New California bill would ban cannabis use while driving • Ask an attorney: Can I smoke pot on probation? And what’s a closed container?

Here are the rules about stoned driving: Any driver suspected of cannabis use can be ordered to take a sobriety test — and, depending on results, brought into a police station for a more extensive Drug Recognition Evaluation, or DRE.

Driving while high isn’t new. But since Prop. 64 legalized cannabis last November, CHP officers are seeing — and testing — more impaired drivers, although there are no numbers yet to confirm how many more.

If you’re pulled over, what can you expect? We asked Glaser, the state’s DRE expert. He has trained the 1,520 officers who test California drivers for signs of drug impairment.

Fullerton Police Officer Jae Song conducts a field sobriety test on a driver suspected of driving while impaired by marijuana. Officer Lenny Ledbetter assisted on the stop. (Bill Alkofer, Orange County Register/SCNG)

Officers will first look for signs such as slurred speech, fumbling to find your license and registration and bloodshot, glassy eyes, he said. They’re also attentive to the smell of cannabis or signs of recent use — say, that joint sitting in your ashtray. (If you’re driving with weed, the law says you have to keep it in a closed container.)

You’ll likely be asked to get out of the car and take the longstanding tried-and-true tests of sobriety: walking a straight line and turning; balancing on one leg; and following simple instructions, such as putting your finger to your nose.

This might include the tricky “Modified Romberg Balance Test,” which checks your internal clock, balance and ability to follow directions. You’ll tilt your head back slightly, close your eyes and estimate 30 seconds — then tilt your head forward, open your eyes and say “Stop.”

Having trouble? Expect a breathalyzer test. Once alcohol is ruled out, the officer will then probe for a possible medical problem, looking for symptoms of things like diabetes, stroke, epilepsy or head injury.

If you’re not drunk or sick, CHP will assume you’re stoned, and will bring you into a police station. They might do an “inventory” of your car, but not a deep search into the trunk and glove box, he said.

Plan to spend 45 minutes to an hour on the 12-step DRE evaluation.

“We do a thorough exam to try to get to the root of the issue…There are no shortcuts,” he said. They’re looking not just for cannabis, but other drugs, as well.

They’ll test for:

  • Vital signs, including blood pressure, temperature and pulse.  Certain drugs elevate these vital signs; others depress them.  They’ll take your pulse three different times; if a racing pulse is caused by stress, they want to let it return to normal.
  • Vision. They’ll test pupil dilation; saucer-sized pupils are a dead giveaway. How is this done?  In three different room conditions — very bright, average conditions and complete darkness — a narrow beam of light is directed into your eyes. They watch how your pupils respond. They’ll also test the ability of your eyes to “converge,” following an officer’s finger as it moves towards the bridge of your nose. They’ll want to see your eyes stay converged for at least one or two seconds.
  • Muscle tone.  Hard drugs, such as narcotic analgesics, cause flaccid muscles and even “nodding out.”
  • Signs of ingestion, such as sticky residue around the mouth or on hands, or smoking-related heat bumps on the tongue. If they suspect intravenous use of other drugs, they’ll look for injection sites.
  • Background information, in an interview.  They’ll ask about your drug history — when you first started and what you’re using, Glaser said. This can help the CHP identify who is a good candidate for treatment. “We are arresting people but we also try to get them services, if they want help,” he said.

They may also ask to draw blood, using a trained phlebotomist.  Lab results take three to six weeks for results.

If arrested, you’ll be charged with a “DUI-Drugs,” 23.152 (E). Penalties vary by county, but plan for your bank balance to go up in smoke.

Officers are so well trained that they can accurately identify the category of drug used — cannabis, hallucinogens, stimulants, depressants, narcotic analgesics or inhalants, like model glue — 90 to 92 percent of the time, said Glaser.

But cannabis presents a unique enforcement challenge, because its effects vary based on strain, potency and individual tolerance, he said. Some people are easily impaired; others are not.  A recent U.S. Department of Transportation study echoed this point, concluding that “specific drug concentration levels cannot be reliably equated with a specific degree of driver impairment.”

“Cannabis is a weird beast,” said Glaser. “The sativa strain can mimic a stimulant. Indica makes people really sleepy. Tolerance, the amount of THC and the type of ingestion also affects people differently. ”

And, importantly, positive results don’t necessarily demonstrate impairment, especially because cannabinoids stay in the system much longer than other drugs.

“Impairment is the key issue. We are not interested in arresting someone who is not impaired, just because a drug is present in their system,” Glaser said.

By comparison, alcohol testing is easy, according to a recent report by the Public Policy Institute of California. Blood alcohol content has long been used to measure driving impairment.  Technology allows accurate roadside testing. And the California Department of Motor Vehicles has issued guidelines that let drivers estimate their blood alcohol content based on body weight and quantity of alcohol consumed.

But no equivalent standard exists for marijuana.

Unsure if you’re safe? Request a ride from Uber, Lyft or a friend and and you’ll be on your way, safely, in minutes.

“When we remove an impaired driver,” Glaser said, “we can prevent a major collision or even fatality.”