Everyone knows that the "legal limit" for drinking and driving in NH - and just about everywhere else - is .08% blood alcohol concentration. At or over that limit, means an administrative license suspension, and the State potentially prove guilt without proof of impairment, based solely on the BAC. These are called "per se" prosecutions - because evidence of a .08 or higher BAC is "per se" (Latin for "in itself") evidence of guilt.
Frequently Asked Question: How Much is too Much?
We hear a frequently asked question about marijuana DWIs - is there a legal limit for THC concentration? The answer, as is often the case when it comes to DWI defense, is different depending on whether we are interested in actual science, or just looking at the law.
No Scientific Consensus Regarding Blood THC Concentrations and Impairment
In terms of science, there is no generally recognized scientific consensus as to how much "Delta-9 THC" (the psychoactive component of THC that causes impairment) will cause impairment for most or all individuals. And that's not my opinion - that is the opinion of the National Highway Traffic Safety Administration, which published a study paper that says: "It is difficult to establish a relationship between a person's THC blood or plasma concentration and performance impairing effects.
Accordingly, NHTSA tells us: "It is inadvisable to try and predict effects based on blood THC concentrations alone, and currently impossible to predict specific effects based on THC-COOH concentrations."
Some States Set Legal Limits for THC, NH does not
In terms of the law, some States have a "zero tolerance" law - any level of THC in the blood is evidence of guilt -- and other States set the legal limit at 5 nanograms per milliliter. These laws, especially the zero tolerance laws, are becoming increasingly controversial as some of these same States legalize medical marijuana. As commentators point out, a zero tolerance law means that a person can be convicted of DWI based on smoking marijuana days or even weeks before driving, because marijuana metabolites stay in the system for a long time.
In NH, There is No Legal Limit for THC and Driving
The NH legislature, however, has wisely declined to set any "legal limit" for THC. That means a positive blood test for Delta-9 THC ordinarily will not bring about an administrative license suspension. It also means that the State cannot "play a numbers game" when the case goes to trial. The State has to prove that THC ingestion caused impairment of the person's ability to drive safely, and that's the way it should be.
Treat THC the Same as Other Medicines
No one should read this as encouragement to get high and go for a drive. In my experience, most police officers, believing that a driver has recently smoked marijuana, will charge a DWI, even if the evidence of impairment is weak - and let the courts sort it out.
The point, however, is that THC DWIs are addressed in NH no differently than other prescription drug DWIs - and this is the way it should be, particularly since NH will soon (hopefully) implement our medical marijuana law. For example: if a person is believed to be driving under the influence of Alprazolam (a benzodiazepam, the family of drugs that includes valium), the State has to prove that the benzo drug caused impairment. There is no "legal limit" for benzodiazepam concentrations in the bloodstream - but its against the law in NH to drive if the ability to drive safely is impaired by any substance.