A number of media reports have highlighted the trend of accidents involving drugged drivers. This may prompt law enforcement to seek out more of these drivers to take them off the road. However, many portable testing kits may not accurately determine if a driver is driving while impaired by THC (the psychotropic element of marijuana).
In essence, since marijuana does not metabolize in the same manner as alcohol, smoking before getting behind the wheel may not mean that a person is too "high" to drive. Also, the saliva tests that are used may indicate that a person has marijuana in their system (as it tends to remain in a person's system for a few weeks). But this, by itself, also does not mean that a person was impaired while behind the wheel.
However, this may change in the near future.
According to a VICE news report, a Canadian company is working on a cannabis breathalyzer that would be similar to those used to test a driver's breath to measure alcohol levels. In what is being called the "Cannabix Breathalyzer," this device would test for the presence of THC in a person's lungs, thereby indicating whether the person smoked marijuana within the last two hours. This ostensibly would remedy the problem created by traditional saliva tests.
However, the discovery of THC still doesn't indicate whether a person was impaired. So if you are charged with driving while impaired by marijuana, it may be worth consulting with an experienced criminal defense attorney to learn about your rights and options.