On the Radar: Roadside Saliva Testing for Marijuana in Illinois DUI Cases.

As technology advances, Illinois law enforcement may soon employ new tools to detect THC during a roadside stop in marijuana-related DUI cases. At present, Illinois is authorized to use validated roadside THC testing under 625 ILCS 5/11-501.2 (a-5), but does not employ such tests in practice. Rather, Illinois law enforcement often performs blood or urine tests for THC after a marijuana-related DUI arrest, generally at a hospital or police station.

The legal landscape for roadside THC testing is beginning to change across the country. Currently, four states have roadside saliva testing programs in use to detect THC: Alabama, Indiana, Wisconsin and Michigan. Like Illinois, many other states have laws authorizing some form of oral specimen use, but do not collect roadside samples in practice.

In the future, Illinois may employ saliva testing during a roadside stop for a suspected marijuana-related DUI. In the following sections, we discuss Illinois law for marijuana-related DUI offenses, the national trends for roadside saliva testing relating to THC, and the potential pitfalls of using THC testing in DUI cases.

What Constitutes a Marijuana DUI in Illinois?

Under the Illinois law, a person may be charged with a marijuana-related DUI if they show signs of impairment from THC, or if chemical testing discloses a THC content that exceeds the Illinois nanogram limit within two hours of driving or being in actual physical control of a vehicle.

The legal limit for THC varies by the type of sample provided by a subject. The law sets the following per se limits for THC in Illinois:

• Blood: Five nanograms or more of delta-9 tetrahydrocannabinol per whole milliliter of blood; or

• Other Bodily Substances: Ten nanograms or more of delta-9 tetrahydrocannabinol per milliliter of other bodily substance, such as urine.

The THC concentration derived from testing gives rise to certain legal presumptions under Illinois law, subject to certain exceptions for medical marijuana users. Under the law, a person’s whole blood or bodily substance test result shall be construed as follows:

• It is presumed that a person is under the influence of cannabis if chemical testing reveals 5 nanograms or more in a whole blood sample, or 10 nanograms in another bodily substance.

• There is no presumption that a person was under the influence of cannabis if a test results in a concentration of less than 5 nanograms in a whole blood sample, or 10 nanograms in another bodily substance. However, the test result may be considered at trial with other competent evidence in determining whether a person was under the influence of cannabis.

• If a person refuses chemical testing, their refusal may be admissible in any criminal or civil proceeding for DUI.

It should be noted that exceptions exist for medical cannabis users in Illinois. Medical cannabis users do not face a presumption of impairment if they are lawfully consuming cannabis and hold a valid registry card in Illinois. In such cases, the State must provide additional evidence of impairment aside from relying solely on the THC concentration disclosed through testing.

Are Roadside Tests for THC Currently Being Used in Illinois?

Unlike portable breath testing (PBT) for alcohol, Illinois is not currently using roadside testing for THC in practice as of April 2026. This is likely to change in the future. As technology develops, Illinois law enforcement may find a roadside THC test that receives general scientific acceptance and they may begin using roadside THC testing at any point during marijuana-related DUI encounters, subject to legal challenges in court.

It should be noted that Illinois law already includes a provision authorizing the use of validated roadside chemical tests during a DUI stop where a driver is suspected of being under the influence of cannabis. The law states, in relevant part:

“Law enforcement officials may use validated roadside chemical tests or standardized field sobriety tests approved by the National Highway Traffic Safety Administration when conducting investigations of a violation of Section 11-501, or a similar local ordinance, by drivers suspected of driving under the influence of cannabis.” 625 ILCS 5/11-501.2 (a-5)

However, unlike roadside tests for breath alcohol content, the law goes further for cannabis impairment. A BAC result for a roadside alcohol test is generally used for probable cause only – the result of a roadside BAC score is not admissible at trial in Illinois.

In contrast, Section 5/11-501.2 (a-5) states that, if properly performed, the results of a validated roadside chemical test for cannabis would be admissible at trial in a marijuana-related DUI proceeding. Given this differential treatment, a roadside chemical test for THC could impact marijuana-related DUI defense in both the probable cause and trial stages of a case.  

Emerging Trends in Roadside Tests for THC.

Nationally, the law on roadside saliva testing in marijuana-related DUI cases is fragmented. Like Illinois, many states authorize roadside chemical testing for THC, but have no active roadside screening programs. As of April 2026, the four states with active roadside THC chemical testing programs are as follows:

• Alabama. Alabama uses a two-part program consisting of roadside screening and laboratory confirmation.
• Indiana. Indiana has an oral fluid program to help build probable cause for arrest.
• Michigan. Michigan has a pilot program for roadside cannabis testing.
• Wisconsin. Wisconsin recently enacted a law establishing oral roadside testing for THC in March of 2026. According to a Wisconsin Public Radio article, the Wisconsin law allows roadside THC testing to establish probable cause, but not to show guilt at trial.

Common Problems with Chemical Testing for THC

The greatest challenge with THC roadside testing is that chemical presence of THC does not always equal impairment, even if an individual has a THC concentration that gives rise to a presumption of impairment in Illinois. There are several reasons why THC testing may not be a reliable indicator that a person is driving under the influence of cannabis, which include the following points below.

THC Does Not Track Impairment like Alcohol

Alcohol concentration has a relatively predictable relationship to diminished driving ability. THC does not. Blood THC levels rise and fall rapidly, and a measured level may not accurately reflect the person’s condition at the actual time of driving. A driver may have a low blood level while still experiencing impairment, or may have a detectable level after the impairing effects have passed.

Frequent cannabis users may test positive long after impairment ends

THC is fat-soluble and can remain in the body well after recent use. Frequent cannabis users can carry measurable levels even when not impaired. This creates a serious fairness problem in per se states where the prosecution leans too heavily on the number itself without connecting it to actual driving behavior.

Saliva tests show presence, not necessarily legal impairment

Oral fluid tests are useful because they can detect recent use quickly. But presence is not the same as proof of legal intoxication. A saliva swab may support probable cause, yet still leave major unanswered questions about dose, timing, tolerance, and actual impairment.

Timing problems can distort results

The longer the delay between the stop and the official sample collection, the less certain the chemical result becomes as a measure of driving time impairment. This is especially true for THC, which changes rapidly after use. Delays can therefore create both prosecution and defense problems.

Conclusion

The use of oral roadside chemical testing for THC is an emerging trend in marijuana-related DUI cases. At present, technology is in its early stages and only a handful of states have roadside THC tests in use during marijuana-related DUI stops. However, as technology improves, we expect roadside testing for THC to become more common throughout the United States, including Illinois. We anticipate many legal challenges regarding the reliability of roadside THC chemical testing as case law emerges with broader use of such tests.

About Our Office

Wirth Law focuses exclusively on Illinois driver’s license reinstatement and DUI defense. With nearly 25 years of DUI defense experience, our firm is committed to helping clients navigate the complexities of the Illinois DUI law, with a focus on license reinstatement after a DUI revocation. We offer flat-rate pricing, flexible scheduling, and virtual meetings with clients.

Our office provides exceptional communication with clients and regularly wins driver’s license hearings with the Illinois Secretary of State. Our firm is top-rated by many ranking agencies, including the following:   

• Recognized as a top-rated firm on Avvo and Justia
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For those navigating Illinois driver’s license reinstatement issues, Wirth Law provides free consultations. To schedule, call (312) 761-8290 or visit our website to complete the contact form.