There was widespread concern: would cannabis partial legalization in April 2024 increase the number of impaired drivers in Germany? A study published in January 2026 in The Lancet Regional Health – Europe now provides the first reliable answers. Researchers from the Centre for Interdisciplinary Addiction Research (ZIS) at the University Medical Centre Hamburg-Eppendorf systematically compare the situation in Germany with Austria, where cannabis remains prohibited. The results are surprisingly clear for traffic policy.
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Eight Months After Legalization: No Increase in Cannabis-Impaired Driving
The Hamburg research team led by first author Anna Schranz employed a quasi-experimental difference-in-differences design. Two representative cross-sections were surveyed before and after the reform: November to December 2023 and November 2024 to January 2025. In total, approximately 16,300 people participated, including 982 monthly cannabis users in Germany and 178 in Austria. This sample size makes the study the most robust data foundation to date for examining how the reform affects driving behavior.
The proportion of those who drove under cannabis influence slightly decreased in Germany from 28.5 to 26.8 percent. In Austria, which served as a control, the same value rose from 12.8 to 16.3 percent over the comparable period. In the difference-in-differences comparison, there is no statistically significant effect of legalization on driving under cannabis influence. Cannabis consumption prevalence in the general population in Germany increased only moderately from 12.1 to 14.4 percent, while Austria remained virtually unchanged at 9.4 to 9.6 percent.
Polysubstance Use Remains the Greatest Risk
The study becomes particularly interesting where it examines the quality of individual instances of driving under cannabis influence. 78.5 percent of these driving instances involved cannabis alone. In the remaining 21.5 percent, those affected combined cannabis with alcohol or other substances. Precisely this polysubstance use is considered particularly dangerous from a traffic medicine perspective, as the psychoactive effects overlap and reaction capability declines disproportionately. The frequency of polysubstance use did not change noticeably due to legalization, but was particularly evident among weekly users.
Also noteworthy is what the study does not show: a dramatic effect from removing the consumption ban. The concern that lower inhibition thresholds could lead to significantly more instances of driving under cannabis influence cannot be supported by data from the first reform year. This places the UKE analysis in line with international research, which found similar findings following legalizations in Canada, Uruguay, and individual U.S. states.
Political Impact on the THC Limit Debate
For the ongoing debate surrounding the 3.5 nanogram per milliliter blood serum THC limit introduced in 2024, the study carries significant political weight. Traffic safety organizations like the German Road Safety Council (Deutsche Verkehrswacht) have repeatedly demanded stricter limits in recent months, arguing that legalization would bring more impaired drivers onto the road. More details are available in the background on the Road Safety Council’s demands. This expectation is precisely what the Hamburg data does not support.
The results align with the picture drawn by the second interim report of the legally mandated accompanying research project EKOCAN. There too, no evidence was found for relevant consumption increases, but rather indications of declining significance of the black market. An analysis of these developments is available in the report on the CanG interim report. Those wishing to review the current limits and legal situation surrounding cannabis and driving will find the key points in the overview of the 2026 THC limit.
What the Study Cannot Answer
Despite the clarity of the findings, the authors also mark the limits of their investigation. The survey covers only the first eight months after the Cannabis Consumption Act came into force. Whether consumption and driving behavior shift in the medium term—for example, when the first cultivation associations provide supply across the board—cannot be inferred from this data. Self-reported information on sensitive topics such as drug use while driving is also a known limitation. Further survey rounds are already planned as part of the accompanying research.
What is clear, however: the Hamburg study provides political discussion with a data foundation that was previously lacking. It replaces speculation with statistics and thus challenges arguments based on an assumed, but not empirically proven, increase in cannabis-impaired driving. For the upcoming consultations on medicinal cannabis legislation and possible adjustments to driving license regulations, this is a signal that cannot be easily ignored.
Frequently Asked Questions
Who conducted the study on cannabis legalization and driving under the influence?
The investigation comes from the Centre for Interdisciplinary Addiction Research at the University Medical Centre Hamburg-Eppendorf under the leadership of Anna Schranz. It was published in January 2026 in the journal The Lancet Regional Health – Europe and is part of the accompanying research for the German cannabis reform.
Did cannabis legalization in Germany lead to more impaired driving?
No. Eight months after the Cannabis Consumption Act came into force, the rate of driving under cannabis influence among monthly users in Germany was even slightly lower than before. Compared to the control group Austria, there is no statistically significant effect of legalization.
What method was used?
Researchers employed a quasi-experimental difference-in-differences design. Two representative population surveys in Germany and Austria were compared before the reform and approximately eight months after. Austria served as a control group because the legal situation there remained unchanged.
How significant is the risk from polysubstance use?
Approximately one in five instances of driving under cannabis influence occurred under additional influence from alcohol or other substances. This polysubstance use scenario is considered particularly risky from a traffic medicine standpoint and primarily affects regular, weekly users.
What do the results mean for the THC limit?
The 3.5 nanogram per milliliter blood serum limit in effect since 2024 was recently discussed because traffic safety groups demanded stricter limits. The new data provides no empirical justification for stricter limits, as no increase in cannabis-impaired driving is evident. A political decision on this will likely occur within the framework of further reform discussions.
Sollte der THC-Grenzwert im Straßenverkehr angehoben werden?
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