The United States has a robust body of laws, standards, and tools for netting drunk drivers.
But as many states continue to loosen restrictions on marijuana, a tangle of legal questions could still pose problems for users and governments alike, particularly on the road.
Nine more U.S. states will vote Tuesday on laws that ease statewide marijuana restrictions, with four voting to legalize medicinal use, and five deciding on whether they will follow states such as Colorado and Washington in allowing full recreational use.
At the same time, concerns grow about public health and safety implications — such as how potentially wider use of the drug will affect road safety. A study earlier this year from the Automobile Association of America's Foundation for Traffic Safety found that fatal crashes involving drivers who recently used marijuana doubled in Washington after the state legalized the drug, for example.
But rising along with safety concerns are doubts that laws prescribing legal limits of the drug in drivers are rooted in science, and whether testing methods are adequate.
For instance, that same report from AAA also said there is no science showing that drivers "reliably become impaired at a specific level of marijuana in the blood." In other words, a certain level of marijuana in the body can affect different people in different ways. Marijuana also interacts with the body in ways that are different from alcohol.
Many states, including some with legalized marijuana laws, have set minimum levels of THC that critics say are plucked out of the air and could end up wrongly imprisoning people for DUI offenses.
"Legal limits, also known as per se limits, for marijuana and driving are arbitrary and unsupported by science," the report said.
The AAA report called for abandoning legal limits in favor of other testing systems, including one test that established recent marijuana use, and a second test that evaluated "behavioral and physiological evidence of driver impairment," according to a press release attached to the study.
Most common "drug tests," typically measure for the presence of metabolites in the blood or urine — essentially chemical traces of different drugs that, in marijuana's case, can be detected weeks after use.
These are not necessarily indicators that a person is impaired, but they could still land users in legal trouble, even if they weren't under the influence while they were driving.
Some partial field tests can detect the presence of THC on the spot, but not the amount in the body. Furthermore, some say there is no reliable way to establish whether a certain level of the drug corresponds to a certain level of impairment. Officers often cobble together a variety of tests to establish whether someone was using the drug prior to an accident, according to Scientific American.
Currently, there is no field test for marijuana akin to the "breathalyzer"-type device, where exhaling into the device produces readings that can be used to identify drivers who have consumed too much alcohol.
But there is something of an arms race underway to develop field tests that would highlight unsafe drivers, according to Nick Morrow, a retired Los Angeles County Sheriff deputy and drug detection expert.
"The good thing about the U.S. is if there's a way to make a dollar, you can bet people will be lined up around the block to make the next best mousetrap," Morrow told Scientific American.
"That's what's going on with cannabis testing. Breath, oral, fluid, saliva — all of them are competing to come out with the easiest, greatest devices," Morrow said. "Maybe those devices aren't ready for prime time yet, but everybody wants to get the patents. As far as having something that really works, defense and prosecutors can agree — the science is not there yet."
At least one company says it will have a breathalyzer-type device ready for sale soon.
Hound Labs, based in Oakland California, has a "breathalyzer" type device the company says can measure the presence and amount of THC in a person's breath. The company still needs to test the product, but the CEO estimated the devices could sell for anywhere between $500 to $1,000.
Of course that assumes some kind of legal standard for an unsafe level of THC can even be agreed on.