Nearly 80% of Spanish drug-positive drivers tested positive for THC, with half also positive for other drugs | Research

The takeaway

In Spain's national roadside testing program, THC was found in 79.5% of all drug-positive tests, with 50.8% of THC-positive drivers also testing positive for cocaine or amphetamines, with polysubstance use most common at lower THC levels.

Policymakers, road safety researchers, and anyone interested in the real-world patterns of drug-positive driving.

80% of drug-positive Spanish drivers had THC; 50.8% had THC plus cocaine or amphetamines

What the researchers found

Of 65,244 confirmed drug-positive tests (2011-2016), 51,869 (79.5%) were THC-positive. In 50.8% of THC-positive tests, cocaine and amphetamines were also detected. Polysubstance use predominated at lower THC levels (58.6% of those under 25 ng/mL). Mean age was 29.6 years; men accounted for 96.3% of THC-positive drivers.

Why it matters

The massive scale of this dataset (nearly 180,000 tests) reveals that cannabis-positive driving rarely occurs in isolation. Half of THC-positive drivers also had cocaine or amphetamines, complicating attribution of impairment risk to cannabis alone.

The numbers in context

179,645 roadside tests. 65,244 drug-positive. 51,869 THC-positive (79.5%). 50.8% of THC-positive also had cocaine/amphetamines. THC-only decreased with age (OR 0.948). Polysubstance increased with age (OR 1.021). Men: 96.3% of THC-positive. Men had higher THC concentrations (OR 1.394).

How the study worked

Analysis of national administrative data from Spain's roadside drug testing program, 179,645 oral fluid tests between 2011 and 2016, with confirmation analysis for driving-impairing substances.

What this study cannot tell us

Administrative testing data, not a random sample of all drivers. Only drug-positive tests analyzed, not the full 179,645. Spanish driving culture and drug use patterns may not generalize. Cannot determine impairment level from substance detection alone.

How to read the evidence

Strong: massive national dataset (179,645 tests) over 6 years with confirmed laboratory analysis.

When this study was published

Published in 2019, covering 2011-2016.

The bigger picture

Studies attributing crash risk to cannabis may be overestimating its independent effect, since so many THC-positive drivers also have other drugs on board. Enforcement and policy focused solely on cannabis may miss the bigger polysubstance picture.

Questions still open

How much of the cannabis-crash risk association is actually driven by polysubstance use? Would targeting polydrug combinations be more effective for road safety than targeting THC alone? Why were older THC-positive drivers more likely to use multiple substances?

Common questions

How common is THC-positive driving?

In Spain's roadside testing program, 79.5% of all drug-positive drivers had THC in their system. However, over half of those also had cocaine or amphetamines, meaning pure cannabis-impaired driving was less common than polysubstance-impaired driving.

Who are the typical THC-positive drivers?

Mean age was 29.6 years. Men accounted for 96.3% of THC-positive drivers and tended to have higher THC concentrations. Younger drivers were more likely to test positive for THC alone, while older drivers more often had multiple drugs.

Read the original research

Drivers who tested positive for cannabis in oral fluid: a longitudinal analysis of administrative data for Spain between 2011 and 2016.

BMJ open, 9(8), e026648

View on PubMed ↗Publisher’s paper ↗

Citation

Herrera-Gómez, Francisco; García-Mingo, Mercedes; Colás, Mónica; González-Luque, Juan Carlos; Alvarez, F Javier. (2019). Drivers who tested positive for cannabis in oral fluid: a longitudinal analysis of administrative data for Spain between 2011 and 2016.. BMJ open, 9(8), e026648. https://doi.org/10.1136/bmjopen-2018-026648