A cop pulls over a motorist who’s driving like he’s drunk, but the officer suspects the driver may be high on marijuana. Unlike with alcohol, there’s no objective way for police to detect whether somebody is impaired while driving under the influence of marijuana. There’s no breath test for THC, pot’s active ingredient, and no 0.08 limit to drive like California law sets for alcohol in the blood.

Now, as Californians consider making the state the first in the country to legalize the recreational use of marijuana, police officials are concerned that Proposition 19 could put an extremely difficult burden on police: How will they determine whether a driver is too drugged to drive?

The blood alcohol concentration in one’s breath pretty closely tracks how much booze is in a person’s blood. Not so for THC, which is absorbed by the fat in a person’s body and stays there for days or weeks after the marijuana is consumed.

That’s because pot isn’t metabolized by the body the same way alcohol is, said Steven Gust, an official at the National Institute on Drug Abuse. So just having THC in your system doesn’t mean you’re too intoxicated to drive.

That means officers base arrests only on observations, field sobriety tests and, ultimately, their judgment. To improve that judgment, the California Highway Patrol uses the Drug Recognition Expert program to train police throughout the state. More than 1,200 current officers have received the training and 55,000 arrests have been made using the program’s evaluation techniques since 1992.

While there is no simple roadside THC test being used in California, police in Australia and parts of Europe have started using saliva tests during stops. Those are likely years away from general use and have raised some legal questions, Gust said.

But even if California police start using the saliva tests, the problem of no legal standard for THC intoxication remains.

Having a legal THC limit, for driving or public intoxication, hasn’t been necessary because marijuana is illegal, said San Mateo Police Chief Susan Manheimer, who also heads the California Police Chiefs Association.

“It’s a lot of science and years of work (to set a limit). Who’s to say that any level is OK?” she asked.

The effect alcohol has on drivers has undergone decades of testing. The thousands of deaths attributed to drunken driving each year also pushed lawmakers and researchers to focus on coming up with a standard as well as increasingly stringent ways to enforce it, officials said.

Currently, prosecutors who want to prove drugged driving need to show the driver was under the influence and, as result, couldn’t safely operate a vehicle. Prosecutors also need a positive drug test.

California has taken a less hard-line approach than that of 15 other states, including Arizona, Nevada and Utah, which have adopted the stance that any detectable level of THC is illegal. Manheimer said lack of a “per se” law, which makes illegal any detectable amount, and the unavailability of a simple roadside test have made it difficult to convict stoned drivers.

She predicted the situation will only get worse if voters pass Proposition 19 in November.

But Joshua Dale, head of the California DUI Lawyers Association, had this to say about Manheimer’s concerns: “It’s a bunch of hooey.”

He said if an officer has made a valid arrest, the jury is good and so is the prosecutor, getting a drugged driving conviction is no easier or harder than in alcohol-related cases. If police want another layer of proof, he said, they should videotape and audio record stops for suspected driving under the influence. The evidence, if shown to a jury, will be enough to get a conviction as long as the person was impaired.

It’s hard to say how big of a problem drugged driving already is in California because the Department of Motor Vehicles doesn’t track the issue. But California arrested 214,800 drivers and convicted 162,000 of them of DUI, which includes drugs and alcohol, in 2008 — the most recent year available.

A 2007 National Highway Transportation Safety Administration study does give an idea of drug use nationally among drivers. The survey found 16.3 percent of nighttime weekend drivers tested positive for drugs, including 8.6 percent who were using ﻿marijuana. The survey also found cocaine in 3.9 percent and over-the-counter and prescription drugs in 3.9 percent.

But Dale Gieringer, head of the California wing of the National Organization for the Reform of Marijuana Laws, asks, “So what?”

He said marijuana use was twice what it is now in 1979 and there wasn’t an epidemic then of stoned drivers running over people or smashing into each other.

“We’ve been there before,” he said. “There is no difference. Just enforce the law.”

Contact Joshua Melvin at 650-345-4335.