When it comes to marijuana-impaired driving, everyone agrees that a fairer test is needed. When marijuana was illegal nationwide, it made some sense to have a so-called "per se" rule. If a properly performed test found marijuana in your system, you were considered guilty.
Even then, people were prosecuted who had not been impaired. Marijuana can stay in your system for weeks even though the intoxicating effect dissipates within a couple of hours. People were found guilty regardless of their actual impairment, simply because marijuana was illegal in the first place.
The problem is that there has been no scientific test for marijuana impairment -- only for the presence of marijuana in the suspect's system. Detecting THC in the suspect's breath would probably indicate actual impairment, but THC in breath is extremely hard to detect because it is perhaps a billion times less concentrated than alcohol would be in the breath of a drunk person.
A company called Hound Labs claims to have invented a breathalyzer-like device that can detect THC in a person's breath. It can accurately measure THC concentrations in parts per trillion. (Alcohol is measured in parts per thousand.) Although the device doesn't solve every problem, several U.S. cities have agreed to put it to the test.
One problem with the test is that it can't determine the amount of THC the suspect has consumed. It could give the same reading for someone who inhaled a single puff of marijuana smoke as someone who had taken numerous hits. The person inhaling a single puff, however, might not be impaired.
Even if the device could measure different concentrations of THC, scientists haven't yet determined a baseline "safe" level of THC to have in one's system. Some states have set an impairment level by law, but many scientists are skeptical of those because they're not backed by evidence.
Another issue is technical. The weed breathalyzer requires a consistent temperature to work correctly. It comes with a laptop-sized base station meant to protect against extreme heat or cold, but the device may not be reliable in field conditions.
With new technology comes new evidentiary issues. In order for these test results to be admissible as evidence, Hound Labs will need to show that the tests are reliable when performed as expected. The results will also need to be based on generally accepted scientific principles. Will the weed breathalyzer meet those criteria? We will just have to wait and see. Hound Labs' device enters field testing this fall, and several other manufacturers are working on devices of their own.