Marijuana and driving

Over the last five years, there has been a gradual rolling back of the laws on marijuana in the United States. Although still illegal at a federal level, several states (and an ever-increasing number) have legalized the drug, meaning that you can now buy it over-the-counter in stores from Seattle to Denver. As with any drug, marijuana affects brain function, and, as a result, inhibits driving ability. This has led to concerns about the impact of marijuana legalization on road safety.

As the graph below shows, legalization has been driven by (or has driven) growing marijuana usage in the United States.

Number of people in the U.S. who used marijuana in the past month from 2009 to 2017 (in thousands)



of marijuana

As of October 2018, 30 states have some legislation that decriminalizes (or legalizes) marijuana. From 1996 to 2012, most states allowed the use of medical marijuana, although there was a back-and-forth process as the federal government challenged states’ rights to do so.

After 2012, however, the debate was shifted as a result of Colorado and Washington State both legalizing the use of recreational marijuana. In 2014, Alaska, D.C., and Oregon all legalized recreational marijuana, followed in 2016 by California, Massachusetts, Nevada, and in 2018 by Vermont.

With more states set to legalize recreational marijuana (usually through the use of ballot initiatives) and the federal government increasingly less willing to intervene, it seems as if the United States has reached a critical mass of legalization.

It is extremely unlikely now that we will return to the pre-2012 period whereby marijuana possession becomes a crime, and as such, many state legislatures are focused instead on regulation of marijuana, taxation of marijuana trade, and mitigation of any harm caused by cannabis usage.


Impact of marijuana
on the body

One of the biggest changes that has taken place in marijuana usage in the past decades has been the rising THC content in a given amount of cannabis. Further, the growing popularity of edibles (which release THC slower than inhaling) means that people often consume more THC than they intended, because it takes them longer to feel the effects.

Regardless of the amount consumed, the impact of THC on the body has been studied and classified and generally centers on the psychological and physiological responses over both the short and long terms.

The Brain

The most noticeable and most important effects of THC on the body comes in the brain. As with alcohol, THC is absorbed into the bloodstream, where it is carried around the body, including to the brain. Depending on the amount of food the individual has consumed, THC usually can be felt within 30 minutes to one hour.


In the short term, the marijuana acts to overstimulate brain receptors that usually function on a chemical very similar to THC. This is what makes you feel ‘high’, but also causes the following:

  • Altered sense of vision and of time

  • Mood changes

  • Poor coordination

  • Difficulty with thinking, problem-solving, and memory

  • Delusions, hallucinations, and psychosis (when taken in high doses)


The long-term effects of marijuana usage on the brain effectively result in the short-term effects lasting longer, and eventually becoming permanent. For those who start smoking in their teens, marijuana may inhibit full development, and may result in a decline in IQ of an average of eight points, according to a study by Duke University.

Physical effects

Along with the mental impacts of smoking or consuming marijuana, there is also the physical impact it has on the body.


Like any form of smoking, inhaling marijuana irritates the lungs. Extensive smoking can cause coughing and lung infections, culminating in a higher rate of lung cancer.

Nausea & Vomiting

Although less serious than breathing problems or heart attacks, nausea and vomiting is one of the most visceral reactions to long-term marijuana usage. In some cases, this can require medical attention, and is certainly something that can be deleterious when driving.

Increased heart rate

Marijuana causes an increased heart rate for up to three hours after you have smoked or consumed it. For those who are are unhealthy or older, this may lead to undue stress on the heart, and can cause heart attacks.

Mental effects

Although the long-term physical effects of marijuana smoking are little different than tobacco smoking, the long-term mental effects are much different. They include damage to the sensory perception aspect of brain function, leading to the following in some cases:

  • Hallucinations

  • Paranoia

  • Disorganized


on driving

Naturally, because of the impact of marijuana on the brain, driving functions are also inhibited by marijuana usage. Although studies into this are relatively limited, particularly when it comes to longitudinal studies, legislation has been based on the following assumptions,

which are extrapolated from studies into the general usage of THC as well as expanding existing legislation pertaining to the use of alcohol when driving. For example, driving under the influence of cannabis increases the likelihood of crashing by 300%.

The causes of this are the physiological and psychological effects of the THC on the brain and body, with specific consideration given to the following factors:

Attention span

One of the main effects of consuming marijuana is the diminishment of the attention span. This means that drivers are unable to stay focused on the road for any meaningful length of time.

When traveling at speed, that can result in a great distance being traveled before the driver’s attention returns to the road.


Even if the driver is paying attention to the road, the ability to concentrate is also diminished after having consumed marijuana.

That means that you are unable to perceive hazards in the same way, and are likely to make a critical mistake when it comes to driving, particularly if you are on a busy street where you are required to interact with other drivers and pedestrians.

Hand-eye coordination

Hand-eye coordination is the ability of your body to respond to the signals given by the brain when it comes to movement. Marijuana negatively impacts this across the board.

What this means for drivers is that, if they are required to turn the steering wheel, or place their foot on the break, their body will not do exactly what the brain wants. This makes it extremely difficult to control a car, particularly when combined with the other inhibiting factors.

Reaction time

A driver’s reaction time can be extremely important in avoiding an accident, or determining how damaging an accident can be. At high speeds, a small change in reaction time corresponds to great distances, meaning that drivers who have dulled their reaction times with marijuana will be unable to respond to hazardous situations in the appropriate amount of time.

Again, when combined with the reduced attention span (meaning they are less likely to be paying attention to the road) and the inhibited hand-eye coordination (meaning they will be unable to control their body when they do decide to respond), marijuana can make driving extremely dangerous for both those inside of, and those outside of, the vehicle.

Compared with alcohol

Many of the responses to marijuana when driving are comparable to the impact of alcohol on the body. Alcohol has the additional impact of diminishing inhibitions, meaning that drivers who are drunk are more likely to speed, which is an additional factor in making it an extremely dangerous drug to consume when driving.

However, the commonalities of marijuana and alcohol when it comes to the impact on driving means that many states have crafted their ‘high driving’ laws in the same way as drunk driving laws.



Because of the inhibitory impact of marijuana on the mental and physical capacities of drivers, states have created robust laws to attempt to outlaw its use among drivers. Each state has different provisions, and each has different technology it uses, both of which have consequences for drivers.

State prohibitions

  • In every state of the U.S., being caught driving while under the influence of marijuana is a bad idea. However, because of the different legal status of marijuana in each state, there are slightly different regulations for each state.

  • In Washington, Montana, Nevada, Ohio, and Colorado, the states have set a limit for the quantity of THC in your bloodstream. This operates on the same principle as alcohol, meaning that with a low level of either in your blood, you are actually safe to drive. However, these states are in the minority.

  • The other two groups of states are those that recognize THC as a compound, and totally prohibit its use (by name) when driving, and other that have a broader, more vague ban on driving while incapacitated. The full list of states, and which category they fit into, is below:

  • Ban on THC in the blood when driving
  • Legal cut off limits
  • Ban on driving ‘while incapacitated’

Mobile roadside testing

Although it varies from state to state, generally a stop for DUI (or DWI) will follow the same procedures. The officers will hold a field sobriety test, involving mental and physical challenges. The officers may also run a Horizontal Gaze Nystagmus Test (running a finger from side to side to check your vision). If the officer determines that you are impaired, they will do one of the following:

Blood, breath, or urine test

In each of these tests, it is extremely difficult to determine THC levels, as all were designed to test for alcohol.

Saliva Swab

In California, and some other states, police are using swab tests to check for THC as well as drugs like crystal meth, cocaine, and prescription drugs. These tests involve a swab of the inner mouth, followed by an 8-minute wait while the machine reads your swab. This is an increasingly cost-effective way for officers to check for THC in drivers.

Of course, in states with a small
permissible level of THC in the bloodstream,
the swab may be calibrated to check for
certain levels, although additional testing
(at a police facility) may be required.

What constitutes driving

A key fact to remember is that ‘driving’ is a broad term for being in control of a vehicle. Generally, if you are in the driver’s seat and the key is in the ignition, you can be classified as driving. In some cases, even sleeping in a car has led to an arrest.


If you are found guilty of driving while high, you will be charged under DUI laws, carrying with it the full force of a DUI punishment. This can include any of the following (amongst other things):

  • Fines

  • Jail time

  • Revocation of license

  • Installation of an
    interlock device

  • House arrest

  • Enforced counseling

On top of this, your car insurance premiums will rise as a result of your conviction and often remain high for a period of several years.

Marijuana is increasingly accepted within contemporary society. Like alcohol and tobacco, it is legalized (or soon will be) throughout the United States.

Where both alcohol and tobacco are seen as unhealthy vices, and treated accordingly, much of the legislation and technology relating to marijuana is still playing catch-up.

States that were the vanguard of marijuana legalization are increasingly concerned about the impact of legalization on road safety, and most have taken active steps to address driving under the influence of THC.

The growing use of roadside testing kits, as well as public safety announcements demonstrate that states are taking this issue very seriously. To a certain extent, the problem is relatively new, since legalization has normalized marijuana usage.

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