Quick answer
No US state permits driving while impaired by cannabis. A medical card does not change that. Some states add a numeric THC limit. Some prohibit any detectable amount of THC. Others rely on officer observation. Penalties range from a fine to jail time, license suspension, and a criminal record.
Three ways states define cannabis-impaired driving
Impairment-based laws
The prosecutor must show the driver was impaired. Officer testimony, field sobriety tests, and drug recognition expert evaluations carry the case. No THC number is required.
Per se laws
A set amount of Delta-9-THC in blood or urine equals a violation. The prosecutor does not need to prove impairment. Colorado, Washington, Montana, Nevada, Illinois, and Ohio use this model.
Zero-tolerance laws
Any detectable amount counts. Arizona, Utah, Idaho, and Pennsylvania are examples. Courts have limited some of these laws. In Arizona, a registered patient can be convicted on THC metabolites alone only with evidence of impairment, under the 2015 state supreme court ruling in Dobson v. McClennen.
Per se THC limits by state
- Colorado: 5 ng/mL Delta-9-THC in whole blood. The figure is a permissible inference, so a driver can rebut it.
- Washington: 5 ng/mL Delta-9-THC in whole blood.
- Montana: 5 ng/mL Delta-9-THC in blood.
- Nevada: 2 ng/mL Delta-9-THC in blood.
- Illinois: 5 ng/mL Delta-9-THC in whole blood, and 10 ng/mL in other bodily fluids.
- Ohio: limits set in statute. Values differ by sample type.
Pennsylvania law lists Schedule I drugs and their metabolites, a standard that has reached medical cannabis patients. Laws change each session. Check your state motor vehicle code.
What happens during a traffic stop
Field sobriety tests such as the walk and turn and one leg stand were validated for alcohol, not THC. Some states use oral fluid tests at the roadside. A blood draw follows. Some states require a warrant. Others treat a driver's license as implied consent to the test. Refusal can bring its own license penalty.
Detection windows
Smoked THC peaks in blood within minutes and falls to 1 to 2 ng/mL within 2 to 3 hours in occasional users. Impairment can outlast the blood peak. Chronic users hold low blood levels for days. Urine tests detect carboxy-THC for days to weeks. A positive urine test does not show that a person was impaired behind the wheel.
Cannabis inside a vehicle
Container rules come from state law, not federal law. Many states require the product in a sealed, unopened package or in a locked glove box or trunk. An open container of cannabis in the cabin counts as a separate violation in several states. Consuming while driving is illegal everywhere. Passenger rules differ by state.
Curbside pickup and drive-thru sales grew after 2020, which puts more sealed containers in cars. Those sales do not change transport rules.
Federal and interstate points
Cannabis remains illegal under federal law. Driving on federal land, including national parks, falls under federal authority. Crossing a state line with cannabis is illegal even between two states that allow adult use.
Steps to reduce risk
- Do not drive for at least 4 to 6 hours after smoking or vaping. Wait longer after edibles, which have delayed effects.
- Read your state statute. THC numbers and container rules differ.
- Do not mix alcohol and cannabis. The combination raises impairment.
- Do not treat a medical card as a defense. Only a few states limit how the law applies to patients.
What is not known
No US state publishes a safe-driving limit for THC. Researchers have not found a blood THC level that separates impaired drivers from unimpaired ones. The relationship between THC concentration and crash risk stays unclear.