For decades the same test has been used to convict drunk drivers.
Police ask a driver to stand on one leg, walk a straight line and recite the alphabet. If the driver fails, the officer will testify in court to help make a case for driving under the influence.
But defense lawyers argue, science has yet to prove that flunking the standard field sobriety test actually means that a person is high, the way it's been proven to measure drunkenness.
So, as attorney Rebecca Jacobstein argued to the Massachusetts high court, the tests shouldn't be allowed in evidence.
"If there's reliable science, reliable science gets to come in," Jacobstein argued. "It's just that unreliable science does not."
Prosecutors like attorney Michelle King don't agree. They argue that rapidly advancing science does prove field tests' reliability.
"Three investigations have come to light and those are the most important for your honors to look at at this point," King said in court.
What makes the stakes so high here, is that police have few alternatives; they do not yet have reliable roadside toxicology tests that can say for sure if someone's too high to drive in the way a breathalyzer or blood test can show if someone's too drunk.
Margaret Haney, a professor of neurobiology at the Columbia University Medical Center says testing a person for alcohol intoxication is a breeze in comparison to testing a person to determine if they are high. As she explains, marijuana is fat soluble, so traces of its main ingredient tetrahydrocannabinol, or THC, can show up in blood long after a person has sobered up.
"That just tells you somebody has smoked," Haney says. "But you don't know if they smoked an hour ago or if they smoked a week before or two weeks before."
It used to be that police could always fall back on arresting a driver for possession, but now that marijuana is legal in many states, including Massachusetts, officers worry they'll be faced with more stoned drivers and fewer ways to stop them.
John Carmichael is the police chief in Walpole, Mass. and says the legalization of marijuana "couldn't be at a worse time."
"It's really gonna cause a problem out on the street," Carmichael says. "I mean, police officers know when something is off. It's usually quite obvious. So if they take away the ability to do a field sobriety test, I don't know what the police officer on the street is supposed to do."
As studies continue on standard field sobriety tests, efforts are also underway to design new ones to better weed out drivers who may be high on weed.
University of Massachusetts psychology professor Michael Milburn has invented an iPad app that he calls Druid, that specifically measures symptoms of marijuana intoxication — like slow reaction time, misperception of time passing and inability to handle divided attention tasks.
For example, one test on his app asks the driver to keep track of different shapes. "When the circle flashes on the screen, you hit the screen where you saw the circle appear," he explains. "If a square appears, [you] hit the white oval on the top of the screen."
The test is not meant to be easy.
"I figure someone who's stoned is gonna go 'Alright, was it the circle or the square?' " Milburn says.
And it's definitely not your grandfather's old "count backwards from ten" test. But, Millburn says, appropriately so.
"If you're going to be driving a car, you should be able to perform at a fairly high level," he says.
The app includes 4 different tests, including one that asks drivers to balance on one foot, while holding the iPad that records every wobble.
In the end, instead of a police officer's subjective judgement of how a driver did, the iPad calculates a total impairment score, that Millburn says could be compared against a standard, just like the .08 blood alcohol limit for alcohol.
While Milburn says research is just beginning on the reliability of his app, experts says it won't be long before science validates a whole new generation of impairment tests. But they say, they'll only stand up to court challenges when used in conjunction with new and better biological tests that can also prove that the person who was impaired, also recently used marijuana.
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