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Drug Driving, Limits and Legislation

Written by: Alan Baker 27th October, 2023

There are limitations associated with all types of drug driving legislation, and different jurisdictions have assessed the advantages and disadvantages in their own ways.

In most jurisdictions across Europe and the USA, legal limits for drugs are not established, with the exception of alcohol. Nevertheless, some places have implemented zero-tolerance laws for illicit drugs without specifying concentration limits in their legislation.

Research [1] identified 19 jurisdictions with legal limits for driving under the influence (DUI) of illicit drugs. These limits varied in terms of the types of drugs included, the biological matrices tested, and the concentration levels considered.

The European DRUID project recommended two types of limits:

  1. “Lower effect limits,” which are akin to the impact of a Blood Alcohol Concentration (BAC) of 0.02%.
  2. “Risk thresholds,” comparable to the effect of a BAC of 0.05%.

However, specific concentrations for drugs were not suggested, except for THC. It appears that most European countries have implemented “lower effect limits” or zero-tolerance laws, often without attempts to standardise them.

Notably, the primary distinction is that concentration limits in jurisdictions with zero tolerance laws may be altered without changing the underlying legislation.

Many European countries and New Zealand have established limits for amphetamines, MDMA, cocaine, THC, and morphine. Some countries have also included limits for certain hallucinogens like ketamine, LSD, and phencyclidine. In contrast, most American jurisdictions have limits for a smaller number of drugs, with some only having limits for THC.

In some cases, the legal limits for drugs in US jurisdictions are higher than those in Europe and New Zealand. Additionally, except for Utah, all US jurisdictions have a BAC limit of 0.08%, suggesting that a higher level of impairment is tolerated compared to regions with lower BAC limits. In Europe, only England, Wales, and Northern Ireland have a BAC limit of 0.08%, while all other jurisdictions maintain limits of 0.05% or lower.

Drugs like amphetamine, THC, and morphine have both recreational and medical uses. These substances may be obtained illicitly or through medical prescriptions. In most countries, the legal limits only apply when the drugs are acquired without a prescription. In the UK, the limits for amphetamine and morphine correspond to high therapeutic concentrations, which are higher than limits set in other countries.

The relationship between Blood Alcohol Concentration (BAC) and impairment or crash risk is widely accepted for alcohol. For some drugs, studies with drug-naïve individuals or occasional drug users have shown that higher concentrations of depressant drugs, such as benzodiazepines, opioids, and gamma-hydroxybutyrate (GHB), lead to increased impairment.

Consequently, “risk threshold” concentrations, which entail graduated or enhanced sanction limits, have been implemented for various drugs in Norway, New Zealand, Canada (specifically for THC), and Denmark.

Canada also employs a graduated sanction limit for GHB. However, for drugs like cannabis, stimulants, and hallucinogens, the relationship between drug concentration, impairment, and crash risk remains less clear. This issue has been a subject of extensive discussion, especially following the legalisation of cannabis products in the USA and Canada.

The use of zero-tolerance legislation and low legal limits is a straightforward and practical approach. Nevertheless, it can result in the conviction of some drug users who are not clinically impaired and do not pose a risk to road traffic safety. This occurs because traces of drugs can persist in the blood or other biological samples for several hours after the impairing effects have dissipated. Such outcomes may clash with people’s sense of justice. Furthermore, applying legal limits to inactive drug metabolites, like THC-COOH and benzoylecgonine, can also lead to the conviction of drivers who were impaired by cannabis or cocaine long before driving but were no longer impaired while driving, unless the driver also tests positive for the active drug or there is evidence of drug use shortly before driving, along with documented clinical impairment at the time of arrest or blood sample collection.

In contrast, impairment-based legislation, which necessitates a positive assessment of clinical impairment for a DUI conviction, may not identify all drivers who are incapable of driving safely.