California residents may drive with certain levels of THC in their bodies after using marijuana, but experts say there is no scientific consensus on how much is enough to make a person impaired. While there are concerns about the safety of drivers who have been smoking marijuana, there are also concerns about drivers being prosecuted when they are not impaired.
Companies have been racing to develop the equivalent of the Breathalyzer device for testing marijuana users. However, the Automobile Association of America recommends that instead of setting legal limits, judging whether a person is too impaired to drive should involve a combination of establishing when a person last smoked marijuana and how a person is behaving. Most existing drug tests measure drugs in the system that may be weeks old, and there is no way to ascertain how much THC in the system leads to impairment. While alcohol affects bodies in a predictable way, THC works differently in different users.
One company says that it has developed a device that can measure THC in a person's breath although it has not yet been tested. However, the use of such a device would rest on the assumption that safe and unsafe levels of THC could be accurately set.
People who are charged with driving while impaired by drugs will in general face the same types of serious penalties if a conviction is obtained as those who have been charged with drunk driving. Accordingly, it might be advisable for people in this position to meet with an attorney to determine how best to fight those allegations. One path could be to question the initial traffic stop itself as being made without the required reasonable suspicion.