What does it mean to be driving while impaired?

There are many skills/capacities that are important for driving.  Reaction time is important, as is the capacity to do divided attention tasks.  If a person consumes a substance like alcohol, cannabis, opioids or one of many over the counter drugs that interfere with these skills, that person is impaired will likely be impaired.

If a person has a blood alcohol level of .08 or higher, every U.S. state has laws that allow police and courts to “infer” or “presume” that this person is “impaired,” and thus is legally culpable if they drive.  Because of the chemistry of cannabis, there is no cannabis “breathalyzer.” So, all police judgments of a person’s cannabis impairment are necessarily subjective.

How does marijuana or other drugs affect the ability to drive?

Driving is a complex skill.  There are many capacities that are important for driving, such as reaction time, decision-making, hand-eye coordination and the ability to do divided attention tasks.  Police use the single-leg stand from the Standard Field Sobriety Test to test for balance, because about one-third of the neurons in the brain are in the cerebellum that controls balance.  If you have disrupted balance, that is an indication that there is brain-wide impairment.

What else can affect the ability to drive?

In addition to alcohol and other drugs, there are many different sources of impaired driving.  Lack of sleep can cause significant impairment in driving. Looking at one’s cell phone is also dangerous since studies of attention and concentration show that it takes at least 20 seconds for a person to regain their focus and normal reaction time after looking at their cell phone.  This is obviously even worse if a person is using their phone while driving, e.g., texting.

Can a police officer charge me with impaired driving without a test?

The details of what is necessary to prove impaired driving varies by state.  Police officers can base an arrest on their own personal observations that a person is driving dangerously, but this may not hold up in court if proper procedures were not followed, confirming evidence not obtained, etc.

How dangerous is it to drive after using marijuana?

It will be the most dangerous to drive within an hour or two of consuming cannabis, but here the evidence presents a mixed picture.  Research shows that cannabis consumption disrupts a person’s reaction time, hand-eye coordination, balance, time estimation, and their ability to perform divided attention tasks.  At the same time, studies of people actually on the road indicate that cannabis users do not show a higher risk of accidents. Some researchers have speculated that regular cannabis users compensate for their impairment by driving more slowly.  This will likely reduce their chance of an accident, but if they have not developed tolerance, they will be at higher risk of responding too slowly to an emergency.

But there is no one-size-fits-all answer to this question for a number of reasons. How high a person gets is dependent upon how much cannabis they have consumed, how high the THC level of that cannabis is, when they consumed it, how frequent a consumer of cannabis the person is, and how recently a person has used cannabis prior to the immediate use we are talking about.  Frequent, daily users of cannabis (e.g., medical cannabis patients) can develop a tolerance that reduces their reactivity. But, there just has not been enough research on medical cannabis patients yet to understand the process of tolerance to cannabis and how it develops.