Cannabis and Driving

How Cannabis Affects Driving

THC, the primary psychoactive compound in cannabis, affects the brain regions responsible for attention, reaction time, tracking, and divided attention — all of which are critical for safe driving. Research consistently shows that cannabis impairs driving performance, particularly at higher doses.

Specific effects documented in driving studies include:

• Slower reaction time to unexpected events
• Reduced ability to track moving objects
• Lane-keeping difficulty and increased weaving
• Impaired divided attention (monitoring multiple things simultaneously)
• Underestimation of time elapsed (which affects gap judgment in traffic)

Some studies show cannabis-impaired drivers partially compensate by slowing down and increasing following distance — unlike alcohol-impaired drivers, who tend to speed and take more risks. However, this compensation is incomplete and unreliable, and does not offset impairment in emergency or unexpected situations.

The Impairment Window Is Longer Than People Assume

One of the most important and underappreciated facts about cannabis and driving is that subjective impairment — how "high" you feel — does not reliably track with functional impairment. Research shows that driving performance can remain impaired for two to four hours after consumption of inhaled cannabis even as subjective intoxication fades.

For edibles, this window is longer still. Edible THC can produce functional impairment lasting four to eight or more hours, depending on dose and individual metabolism.

The practical implication: feeling "fine to drive" is not the same as being safe to drive. Regular cannabis consumers are particularly susceptible to underestimating their own impairment because tolerance reduces the subjective "high" while driving-relevant impairment persists.

Maryland Law

Maryland law generally prohibits driving while impaired by any drug, including cannabis. Maryland transportation law recognizes two related offenses, often described as:

• Driving While Impaired by a Drug (DWID): Driving while impaired to any extent by a drug or combination of drugs. Generally treated as a criminal offense under Maryland law.
• Driving Under the Influence of a Drug (DUID): A more serious level of drug-related driving offense, generally requiring proof of more substantial impairment.

Unlike alcohol, Maryland does not have a numerical per se THC limit. This is because blood THC concentration does not reliably correlate with driving impairment the way blood alcohol content does — a person can have detectable THC in their blood hours or days after consumption with no meaningful functional impairment, while another person can be significantly impaired at lower concentrations. Impairment is established through officer observation and standardized field sobriety testing.

Why There Is No Reliable Roadside THC Test

Police cannot reliably determine cannabis impairment at roadside the way they can for alcohol (breathalyzer). Current blood, saliva, and urine tests can detect THC and its metabolites, but these tests measure presence and concentration — not impairment.

THC metabolites persist in the body long after impairment ends — days to weeks in regular consumers. Conversely, someone acutely impaired immediately after smoking may show different blood THC levels than someone who consumed the same amount hours earlier. Active research into impairment-based testing continues, but no validated per se THC test has been widely adopted for roadside use in Maryland.

Maryland law enforcement uses Drug Recognition Experts (DREs) trained in a 12-step evaluation protocol to assess drug impairment at roadside.

Transporting Cannabis in Maryland

Even when you are not impaired, Maryland law places conditions on how cannabis may be transported in a vehicle. The following reflects general guidance based on publicly available Maryland regulations as of mid-2025 — confirm specifics with legal counsel if you have questions about compliance:

• Cannabis should generally be transported in a sealed, unopened container. An opened or accessible container in the passenger area may constitute a violation.
• The lowest-risk location is the trunk or a locked compartment where it is not accessible to occupants.
• Consuming cannabis in a vehicle, whether moving or parked in a public place, is generally prohibited under Maryland law.

The Safest Approach

There is no reliably safe amount of cannabis to consume and then drive. The safest approach is:

1. Plan transportation before you consume. Decide how you are getting home before you visit the dispensary, not after.
2. Use rideshare or a designated driver if you plan to consume cannabis the same day.
3. Wait significantly longer than you feel you need to if you do decide to drive after cannabis consumption. The general guidance from driving researchers is a minimum of 4–6 hours after inhaled cannabis for most people, and significantly longer for edibles — recognizing that individual variation is high and no universal wait time is guaranteed to be safe.
4. If in doubt, do not drive.

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