As New Jersey moves to legalize weed, the state may face a new challenge: how to handle driving while stoned.

Critics of a bill to make cannabis legal warn it could lead to more drugged-driving cases and unsafe highways. But lawmakers say heightened police training and drug enforcement will keep the problem in check. Others say arresting people for driving while high and prosecuting could wreak havoc in the courts.

Consider the case of a South Jersey man who was charged with two counts of vehicular homicide after his pickup truck struck and killed a jogger, hit a tree, flipped over, and then killed his passenger. A blood test revealed that he had a small amount of THC — the psychoactive compound in cannabis — in his blood after the 2015 accident. Last year, a jury acquitted him.

That case and several others highlight an often-overlooked problem that could be exacerbated if marijuana is legalized in New Jersey. What about people who test positive for cannabis — due to previous use — even though they are not actually intoxicated at the time of an arrest or accident?

“I’m concerned that a lot of innocent people will get ensnared and they will be convicting people who are not impaired,” said John S. Sitzler, a Hainesport lawyer who has specialized in DUI cases for decades. His client, Patrick Miller, was a 28-year-old handyman with no criminal record when he lost control of his 1996 Chevy pickup in Southampton Township and caused the death of jogger Allison McGinnis and his passenger, David Eldridge.

Sitzler argued that a tie rod in the front of the truck’s suspension system broke, disabling the steering and causing Miller to abruptly veer off the highway. A patrol sergeant who was driving behind Miller minutes before the crash testified he noticed no signs of erratic driving before the crash. Twenty minutes earlier, Miller was finishing building a children’s swing set for a police officer who testified that he too saw no impairment when Miller left his house.

Prosecutors blamed the accident on intoxication and pointed to the 4 nanograms per milliliter of THC in Miller’s blood sample. Miller admitted he had smoked marijuana that week, but testified he had not consumed cannabis that day and was not impaired when he got behind the wheel.

Joel Bewley, a spokesperson for the Burlington County Prosecutor’s Office, did not respond to requests for comment on the case. The indictment said Miller drove “a vehicle recklessly while intoxicated” and caused two deaths.

If Miller had not gone to trial, he could have faced 20 years in state prison. Prosecutors offered to recommend a sentence of 12 years if he pleaded guilty, Sitzler said.

In a similar case tried in Burlington County last year, State Superior Court Judge Charles Delehey noted the difficulty of relying on blood tests. The case against Santos Morales fell into an “undefined black hole,” Delehey said in a pretrial ruling, because state law is silent on whether blood tests should be used to determine intoxication and what THC concentration should be the standard. Delehey said conflicting studies call into question the validity of blood-test results.

Tamika McKoy, a Camden attorney who represented Morales, said the trial became a “battle of the experts,” and her client was found not guilty of vehicular homicide despite the THC detected in his blood after a fatal collision with a motorcyclist in Bass River Township in 2012.

“Troopers are trained to recognize if someone is under the influence of CDS, and based on the officer’s observations at the scene, no one suspected marijuana might be the cause until the lab results came back,” said McKoy. Morales was indicted two years after the accident, but a toxicology expert for the defense testified that the findings and handling of the blood sample were flawed.

State Sen. Nicholas Scutari, a key sponsor of a bill to legalize recreational marijuana in New Jersey and a municipal prosecutor in Linden, said blood tests are rarely used in DUI cases. He said field sobriety tests and police officers' observations are more accurate. Impaired drivers “convict themselves with respect to videotape taken by body cams,” he said.

The bill calls for towns to receive funding for advanced police training to help officers determine if a driver is high based on an array of sobriety tests.

States and law enforcement agencies have discretion on whether to use blood tests and what standards to use. In New Jersey, blood tests are allowed as evidence, but there are no standards for drugged driving, leaving prosecutors and defense lawyers with the option to call in experts to analyze the test results.

In the Miller case, the THC reading was below the standard used by many of the nine states that have legal weed — 5 nanograms of THC concentration. But even that is arbitrary, Sitzler said. "The National Highway Traffic Safety Administration flat-out cautions against using blood tests” to determine cannabis impairment, he said.

A 2015 Highway Traffic Safety report found that THC could remain in the blood for days — or even weeks — after ingestion. The report also said there is no statistical increase in the number of accidents caused by cannabis users compared with sober drivers, but warned: “These results do not indicate that drug use by drivers is risk-free.”

In Pennsylvania, one of the few “zero tolerance” states in the country, a trace of THC is deemed evidence of impairment and considered sufficient to charge motorists with DUI.

Unlike alcohol impairment, which can be quantified in blood, urine, and breath tests, marijuana use has no comparable test, said Fran M. Gengo, the neuropharmacological director for the cannabis clinic at the DENT Neurological Institute in Buffalo.

Gengo testified as an expert at Miller’s trial and said cannabis is stored in fat cells and slowly released.

Someone who smokes marijuana three times or more a week “may always have it in their blood or urine in measurable concentrations,” he said. The data show that a person who smokes marijuana may be high up to three or four hours later, but after that likely would not be impaired, he said.

Since Miller admitted he had been a regular user of marijuana, THC would have been in his blood even when he wasn’t intoxicated, Gengo said. “The results of the blood test provided no information on whether or not he was impaired at the time of the accident,” he said.

To fill the void, many states are training police to do a series of sobriety tests and certifying them as Drug Recognition Experts. Christopher Dudzik, the president of the New Jersey Association of Drug Recognition Experts, said DREs are called in to assist arresting patrol officers with drugged-driving stops.

“We do comprehensive testing — one-legged stand, walk and turn, blood pressure, pulses, pupil size, urine samples ... ,” Dudzik said. “We’re looking for signs of impairment.”

Such tests are a beginning, but their effectiveness is in question, Sitzler said. “These sobriety tests were never validated for marijuana," he said. “The DRE program was never peer-reviewed or validated by studies.”

Sitzler said that marijuana legalization could lead to more arrests for drugged driving and that challenges to their validity could “jam the courts.” As a contributor to the Garden State Continuing Legal Education program, he plans to highlight the Miller trial in a lecture for lawyers on how to defend clients arrested for drugged driving.

“Though I favor legalization, I’m concerned that they do this right,” he said. “They are working on a breath test and saliva test that some say is promising. But until then, they need to get a better grip on enforcement. ... They have alcohol impairment down to a science, but not marijuana."