On Nov. 8, Massachusetts residents voted to legalize the use of recreational marijuana. Now that people in the state are able to smoke pot without risk of criminal prosecution, there are concerns that there will be more car accidents caused by drivers who are impaired by the drug. However, a scientific testing process to measure marijuana impairment has not yet been created.

Though studies have shown that marijuana field tests are far from perfect, there are many companies working to develop a cannabis test that could be used by police officers. A California-based company called Hound Labs has even developed a marijuana breath test that can detect the amount of THC in a breath sample. Other companies are working on tests that measure THC levels in saliva.

Right now, the drug tests that law enforcement officers use to test drivers for marijuana are conducted with blood and urine samples. These tests cannot reliably measure a driver’s impairment from marijuana because chemical traces of THC can appear weeks after the test subject last smoked pot. A study by the Automobile Association of America’s Foundation for Traffic Safety showed that the same level of THC affects drivers differently, so police officers should look for behavioral and physiological signs of impairment rather than a specific level of THC in the blood.

If THC is detected in a drug test after a traffic stop, the driver could be charged with driving under the influence. A criminal defense attorney could try to help a DUI defendant to combat the allegation on the basis that the test used was unscientific and did not conclusively prove that the driver was impaired at the time of the stop.