For purposes of “driving while intoxicated” in Texas, you can be charged if you have lost the normal use of your mental or physical faculties because of the introduction of alcohol, a controlled substance, drug, dangerous drug, combination of any of those two, or any other substance. In short, this covers everything from intoxication due to alcohol, to Xanax (lawfully or unlawfully possessed), to Benadryl, to weed.
With alcohol, there is specific legal limit in Texas, at which point you are legally “intoxicated,” regardless of how drunk or sober you look or feel—0.08 BAC (blood alcohol concentration.) With the legalization of marijuana for recreational use in a growing number of states (but not in Texas), lawmakers in other states and cops have turned their attention towards attempting to establish a legal threshold for marijuana. Put another way, what is the “0.08” of driving while high on marijuana? That is, if a person lives in Massachusetts, and legally smokes some pot, at what threshold level are they too high to drive and how could the cop tell?
The same way a cop in Texas would determine if you are DWI due to marijuana. So, how do cops in Texas determine if you are too high to drive? The same way that cops determine if you have lost the normal use of your mental or physical faculties due to, say, Ambien that you have taken under a valid prescription: via tests you are asked to perform by a Drug Recognition Expert (DRE). DRE’s are just cops who have been trained to identify driver’s whose ability is impaired due to drugs. The tests are like the standard field sobriety tests, only tailored towards drug impairment.
On November 20, 2016, the Boston Globe ran a story about a University of Massachusetts professor who created an app to test for marijuana impairment. There is nothing novel about the app (well, apart from a way to make some money by selling the app to police departments)—it simply has the user perform DRE-style tests to determine if marijuana has caused the person to lose the use of her normal mental or physical faculties. Because Massachusetts does not yet have a threshold limit for intoxication due to marijuana (Washington and Colorado do—5 or more nanograms of THC per milliliter of blood), DRE is the next best thing, I suppose.
If you are pulled over under suspicion of driving while intoxicated due to marijuana or drugs, know that you have the right to refuse to perform any DRE of field sobriety tests, and have the right to refuse a blood test (even on so-called “no-refusal weekends.”)