Studies have failed to show that field sobriety tests (FSTs) are unreliable measures of whether a motorist is driving under the influence of marijuana (DUI). Nevertheless, law enforcement continues using them to prosecute these cases, often in tandem with other tests, like blood, urine, or saliva. A good defense lawyer can raise reasonable doubts about the results of any of these tests.
What do studies have to say about field sobriety tests for drugged driving?
Several studies have shown that field sobriety tests are unreliable when detecting marijuana impairment in a driver.
One study involved participants who ingested delta-9-tetrahydrocannabinol (THC). Their THC levels were monitored as they performed FSTs. The study concluded that the FSTs were ineffective in determining the participant’s THC level or whether they were impaired.[1]
Another study gave 184 marijuana users either a placebo or a dose of THC. Police officers then gave the participants FSTs to determine whether they were impaired. The law enforcement officers’ findings were that:
- 81 percent of the participants who were given THC were impaired, and
- 2 percent of the placebo recipients were also impaired.
The high rate of FST failure by the placebo group led to the conclusion that, without other indicators, FSTs were insufficient for determining marijuana impairment.[2]
Do police in California still use FSTs for marijuana DUI cases?
Yes, police in California and elsewhere still use FSTs when they have a reasonable suspicion that you are driving while under the influence of marijuana.
The DUI attorneys at our law firm have found that, in many cases, police use FSTs because they do not have many other methods to turn to. It is well understood that THC can impair the brain and inhibit your motor functions. This impairs your ability to drive a motor vehicle. However, the level of THC required to produce impairment is unsettled. Furthermore, no devices accurately detect THC in your body at the moment of testing, rather than latent THC from days or weeks before. This leaves police who suspect that you are driving while under the influence of marijuana with few other options than FSTs.
What other standardized field sobriety tests are used to detect impairment?
When using FSTs to detect marijuana impairment, police lean heavily on those that test your:
- balance,
- movement,
- coordination, and
Our criminal defense attorneys and DUI lawyers have found that police tend to use the following 2 FSTs most often:
- the one-leg stand, and
- the walk-and-turn.
Both are standardized by the National Highway Traffic Safety Administration (NHTSA).
Failing either of these can prove that you are too impaired to drive. They are often used to establish probable cause to make an arrest. Unfortunately, many FST failures are due to other, unrelated medical conditions.
One-leg stand test
In the one-leg stand test, you have to:
- raise your foot about 6 inches from the ground,
- hold still in that position,
- look down at your foot, and
- count for 30 seconds, often from 1,001 through 1,030.
The officer will look for the following 4 signs of impairment:
- swaying,
- using your arms to keep your balance,
- hopping, and/or
- putting your foot down before time is up.
Walk-and-turn test
During the walk-and-turn test, you have to:
- take 9 steps along a straight line, putting your heel to your toe with each step,
- pivot at the end of the line, and
- take 9 heel-to-toe steps back to the starting point.
There are 8 signs of impairment that the officer will be looking for:
- you struggle to keep your balance while the officer gives you the instructions for the test,
- you start too soon,
- you stop walking after starting the test,
- the steps you take are not heel-to-toe,
- you step off the line,
- you use your arms to keep your balance,
- you take the wrong number of steps, and/or
- you fail to turn correctly.
What other tests do police use?
When police suspect that you are impaired by marijuana, they may also turn to other tests. Common ones include:
- blood tests,
- urine tests, and
- saliva swabs.
Typically, blood and urine tests can only be done after making a DUI arrest during a traffic stop. These test your bodily fluids for THC.
Saliva swabs have become increasingly common because of their portability. They also claim to be able to detect other controlled substances in addition to THC, such as:
- methadone,
- cocaine,
- crystal meth, and
- some prescription medications.
However, THC is known to remain in your saliva for up to 3 days after you have ingested it. Stale THC can still show up on test results, putting you in legal jeopardy even though you are not impaired by it.
There are also so-called “marijuana breathalyzers” that claim to detect THC on your breath. These are supposed to act similarly to how breath tests measure your blood alcohol concentration or blood alcohol content (BAC) in drunk driving cases. However, unlike for breath tests that measure the influence of alcohol against the state’s legal limit, marijuana breathalyzers have struggled to produce reliable results.[3]
Do implied consent laws require me to take FSTs when requested?
Generally, no. Most states, including California, have implied consent laws covering only chemical tests. FSTs are not chemical in nature. If a police officer requests that you get out of your car for a field sobriety test, you can refuse to do so without getting an automatic driver’s license suspension from the DMV.
How can a DUI defense attorney help me?
A DUI defense lawyer will understand the numerous difficulties that law enforcement faces when investigating marijuana DUIs. There are many ways to raise reasonable doubts about whether you were too impaired to drive. This can weaken the prosecutor’s case and may even lead to your DUI charge getting dropped. You can also move for it to be dismissed.
At the very least, establishing an attorney-client relationship and getting the legal advice of a DWI defense lawyer from a reputable law office can help you understand your rights and protect your interests, often by reducing the length of a potential prison term or jail time.
Legal References
[1] National Institute of Justice, “Field Sobriety Tests and THC Levels Unreliable Indicators of Marijuana Intoxication,” (April 5, 2021).
[2] T Marcotte, A Umlauf, D Grelotti, “Evaluation of Field Sobriety Tests for Identifying Drivers Under the Influence of Cannabis: A Randomized Clinical Trial,” JAMA Psychiatry 2023;80(9):914-923 (August 2, 2023).
[3] Chris Roberts, “Study: Sure Looks Like ‘Marijuana Breathalyzers’ Don’t Work – And May Never Work,” Forbes (December 31, 2021).
