SACRAMENTO, Calif. (AP) — Joseph Brenan, a 40-year-old father of four, was changing a flat tire along Interstate 80 near Sacramento when he was struck and killed by a passing motorist who had drifted onto the shoulder of the highway.

The California Highway Patrol arrested Brandon Rotolo, 24, on suspicion of driving under the influence of marijuana and vehicular manslaughter.

Last week's tragedy is the sort law enforcement officials fear may become more common after California voters approved Proposition 64 legalizing recreational pot in November. The state will become the world's largest cannabis market.

Police across California are scrambling to keep up by increasing training as they prepare to spot drug-impaired drivers. Their task is made more difficult because there is no presumed level of intoxication in California, unlike the 0.08 percent blood level for alcohol, and drugs affect everyone differently.

But driving impaired remains illegal, no matter the substance.

Police were set to demonstrate how they currently conduct roadside drugged-driving tests near the state Capitol on Wednesday.

Assemblyman Tom Lackey, R-Palmdale, and the California Police Chiefs Association also planned to demonstrate a portable saliva test that proponents said could one day be widely used to screen for the recent use of six drugs.

Lackey, a former California Highway Patrol member, carried a bill two years ago that would have allowed police to use the cheek-swab oral fluid device to test for drugs in much the same way as officers currently use breathalyzers to test drivers' blood alcohol level, though his bill died in its first committee.

The devices are being tested in Kern, Los Angeles and Sacramento counties along with states such as Colorado, which also allows recreational marijuana. Michigan and Vermont recently authorized use of the tests, according to Lackey's office.

More California police departments are using the saliva tests after a Kern County judge last year accepted the results as evidence in a drugged driving case, said Lauren Michaels, the police chiefs association's marijuana and drunken driving policy expert.

"It's an additional tool to be used, but at the end of the day in an arrest you'd get a blood sample to tell the actual levels of each substance," she said. It currently is a voluntary test in California that drivers can refuse.

Other law enforcement and academic experts said there are too many variables in how marijuana and other drugs are consumed and metabolized to rely on a saliva or breath test. They said the best current method is to train law enforcement officers to spot the signs of impairment.

"The science is still developing," California Highway Patrol Sgt. Glen Glaser said. "The mere presence of a drug should not make a person feel like they're subject to arrest if they're not impaired."

The highway patrol plans to have every road officer trained in advanced roadside drug detection techniques before Jan. 1, said Glaser, state coordinator of the patrol's drug recognition expert program. The CHP is busy training officers from other law enforcement agencies as well.

The CHP and other agencies also are cooperating with the Center for Medicinal Cannabis Research at the University of California, San Diego. The center is analyzing an d trying to improve both the human drug-recognition experts and the saliva testing as part of a two-year, $1.8 million study.

Researchers are giving 180 volunteers marijuana with varying levels of potency, then measuring both their performance in a driving simulator and ways of spotting any impairment.

They also are trying to learn if there is a particular presumptive level of marijuana intoxication that impairs driving, said Thomas Marcotte, the study's chief investigator and co-director of the research center.